EP3137688A1

EP3137688A1 - Magnetic grout detection method and system

Info

Publication number: EP3137688A1
Application number: EP15720771.3A
Authority: EP
Inventors: Rebecca J LUNN; Grainne El MOUNTASSIR; Simon L HARLEY
Original assignee: University of Strathclyde
Current assignee: University of Strathclyde
Priority date: 2014-05-02
Filing date: 2015-05-01
Publication date: 2017-03-08
Anticipated expiration: 2035-05-01
Also published as: GB201407796D0; EP3137688B1; WO2015166281A1

Abstract

A method comprising detecting injected magnetic grout to provide an indication of penetration into an injection region of the injected magnetic grout.

Description

Magnetic Grout Detection Method and System

Introduction The present invention relates to a system and method for detecting the extent of penetration of grout, for example during or after injection into a subsurface region.

Background Grouting is commonly used for ground strengthening, for example to support surface construction (e.g. buildings, roads and bridges) or to strengthen ground for subsurface tunnelling/mining. Grouting is also used for the creation of hydraulic barriers. Hydraulic barriers may include, for example, barriers for reservoir dams and barriers for the prevention of contaminant migration in groundwater.

The subsurface is made up of soil and rock, which may contain voids in the form of pores and/or fractures. In many engineering contexts it is necessary to inject grout into these voids in order to stabilise the ground or to reduce its permeability. Grout may be injected into boreholes that extend into the subsurface. For example, grout may be injected into a line of boreholes that are drilled into the subsurface at appropriate intervals such that a grout curtain (continuous grout barrier) may be formed.

Grout may comprise a combination of water and cement or a cement-alternative material such as colloidal silica. Grout may additionally comprise further materials, for example superplasticisers, stabilising agents, accelerators, setting/hardening agents or fly ash.

The construction and mining industries spend many millions of pounds per year on grouting rocks and soils via borehole injection. For example, £4.5M was spent injecting 41 ,430 tonnes of grout to stabilise old mine workings before construction of the Emirates Arena and the Sir Chris Hoy Velodrome in Glasgow.

One of the greatest difficulties with grouting is that it may not be possible to detect the extent of the grout penetration in the subsurface. Conventional practice in grouting fractured rock masses uses the Grout Intensity Number (GIN) method to control the injection energy with which grout is pushed into the subsurface (see, for example, Lombardi, G. and D. Deere, (1993) Grouting design and control using the GIN principle, Water Power and Dam Construction, pp. 15-22; Lombardi, G. (1996) Selecting the grouting intensity, Hydropower and Dams, Issue 4 pp. 62-66). Injected volumes and pressures are measured at the mouth of the injection borehole and grouting intensity is calculated by multiplying the final grout pressure by the final grout take per metre of injection (i.e. volume of grout injected per length of injection interval in borehole). An approximately radial spread of grout is assumed as the location of the injected grout is not known.

A controlling GIN (also called a limiting GIN) may be determined for a particular grout injection. The controlling GIN may be a GIN that should not be exceeded in operation. The GIN method may prevent an injection having a combination of high pressure and high volume. By restricting the use of injections having both high volume and high pressure, the risk of ground heave may be reduced.

The controlling GIN for a specific zone of a site can be determined by the experimental method (Lombardi, 1996). However, the experimental method requires the ability to determine the penetration distance of the grout in a trial injection, which is not routinely achievable in practice. As a result, GIN curves (which may be referred to as standard GIN curves) may often be used along with the observational method (Lombardi, 1996).

At present, it may not be possible to detect or infer the movement of grout in the ground, and therefore it may not be possible to determine the extent of grout present after a grouting operation. The lack of information about the movement and extent of grout may result in grout wastage, drilling of unnecessary boreholes and a lack of data for design optimisation. In some circumstances, gaps in grouting may not be known. Therefore gaps may remain in grouting. This is of particular concern where hydraulic containment is critical.

Figures 1 a to 1 d are schematic diagrams presenting an example of a grouting campaign comprising the injection of grout into multiple boreholes. The grouting campaign of Figure 1 is a grouting campaign (carried out by Bam Ritchies) intended to hydraulically isolate a vertical shaft containing nuclear waste at Dounreay, UK. Grouting proceeded via a series of concentric borehole injection stages. To ensure integrity of the resulting hydraulic barrier, grout injection was designed with significant overlaps between the regions of grout intended to be produced from adjacent boreholes (the split-spacing borehole technique). Figures 1 a to 1 d show the borehole drilling and injection sequence in plan view. Desired grout penetration circles 2, 4 represent desired grout penetration.

Figure 1 a shows the first stage of the grouting campaign. Vertical shaft PW2 is the vertical shaft containing nuclear waste. Vertical shaft PW2 is surrounded by eight first- stage injection boreholes DBB1 to DBB8, which are equally spaced around a first ring of boreholes centred on vertical shaft PW2. Each first-stage borehole has an associated desired grout penetration distance, which is represented for each first-stage borehole by a desired grout penetration circle 2.

At the first stage of the grouting campaign as represented in Figure 1 a, grout was injected into 3 m vertical intervals in each first-stage borehole DBB1 to DBB8, starting with the deepest 3 m section and sweeping upwards section-by-section in each borehole.

Figure 1 b represents the second stage of the grouting campaign. In the second stage of the grouting campaign, grout was injected into 3 m vertical intervals in each of eight second-stage injection boreholes DB101 to DB108. The second-stage boreholes DB101 to DB108 are equally spaced around a second, larger ring of boreholes centred on vertical shaft PW2. Each of the second-stage boreholes DB101 to DB108 has an associated desired grout penetration distance which is represented for each second injection borehole by a desired grout penetration circle 4. The desired grout penetration circle 4 for each second-stage borehole DB101 to DB108 is larger than the desired grout penetration circle 2 for each first-stage borehole DBB1 to DBB8.

Figure 1 c represents the third stage of the grouting campaign. At the third stage, third- stage injection boreholes DB201 to DB208 were drilled on the second ring at intermediate positions between the second-stage boreholes DB101 to DB108. Therefore the third-stage boreholes are at the same distance from the vertical shaft PW2 as the second-stage boreholes. Grout was injected into 3 m vertical intervals in each of the eight third-stage boreholes DB201 to DB208. Each third-stage borehole has a desired grout penetration circle 4 that is the same size as the desired grout penetration circle for each of the second-stage boreholes DB101 to DB108.

Figure ^"I d represents the fourth stage of the grouting campaign. At the fourth stage, sixteen fourth-stage injection boreholes DB301 to DB316 were drilled at positions on the second ring such that each fourth-stage borehole is positioned between a second- stage borehole and its neighbouring third-stage borehole. Each fourth-stage borehole DB301 to DB316 has a desired grout penetration circle 4 that is the same size as the desired grout penetration circle for each of the second-stage boreholes DB101 to DB108 and third-stage boreholes DB201 to DB208.

In the grout campaign described with reference to Figures 1 a to 1 d, a barrier of 10 m thickness around the vertical shaft and extending 20 m below the shaft was required. It was not possible to monitor penetration distances during grout injection. Therefore, penetration distances were estimated in a precursory trial.

Figures 2a and 2b are taken from the precursory trial. In the precursory trial, pH and temperature were investigated as an indication of grout breakthrough, as described in Henderson, A. E., Robertson, I. A., Whitfield, J. M., Garrard, G. F. G., Swannell, N. G. and Fisch, H. (2008) A new method for real-time monitoring of grout spread through fractured rocks, MRS Proceedings, Vol. 1 107.

Figure 2a shows the layout of six observation boreholes (Obs. 1 to Obs. 6) and a central injection borehole (Injection). Observation boreholes Obs. 1 to Obs. 3 are placed at a 4 m distance from the injection borehole. Observation boreholes Obs. 4 to Obs. 6 are placed at a 6 m distance from the injection borehole.

Grout was injected into the injection borehole within a 3 m borehole interval between 15 m and 18 m below ground surface. Temperature and pH were measured at each of the observation boreholes during a time range starting before the start of the grout injection and finishing after the end of the grout injection.

Figure 2b shows the temporal evolution of temperature and pH at three of the observation boreholes, Obs. 1 to Obs. 3, each of which is placed at a 4 m distance from the injection borehole. Each of lines P01 1 pH, P012 pH, P013 pH represents the measured pH against time at a respective one of observation boreholes Obs. 1 to Obs. 3. Each of lines P01 1 °C, P012 °C, P013 °C represents the temperature against time at a respective one of observation boreholes Obs. 1 to Obs. 3. Figure 2b also shows the injected grout volume expressed in litres per metre length of the injection interval against time (indicated by the Volume line).

In the precursory trial, it was found that temperature appeared to be insensitive to grout breakthrough. Groundwater pH was seen to rise in all three observation boreholes Obs. 1 to Obs. 3 at a 4 m distance from the injection borehole.

The initial rise in pH appears to correspond to the expected pH for grout dilutions. The grout dilutions may be bleed water and grout diluted by groundwater. Bleed water is water that accumulates at the surface of freshly mixed grout. The peak pH readings appear to correspond to the expected values for neat grout. Physical grout arrival was confirmed by the presence of grout coating some observation instruments.

The precursory trial using pH has possible drawbacks. Firstly, it may not be possible to locate the penetration front of the grout itself. From Figure 2b, it appears that the penetration front of the grout may be non-radially symmetric (contrasting with the common assumption of radial penetration), since the recorded rise in pH occurs at different times for each of the three observation boreholes Obs. 1 to Obs. 3. Secondly, the observation boreholes are located within the grouted rock volume. Location of observation boreholes within the grouted rock volume may be suitable for a precursory field trial (which may take place in an adjacent and similar rock volume to the rock volume in which a final grouting campaign will take place). However, location of observation boreholes within the grouted rock volume may not be suitable for a final grouting campaign.

Several proposed methods for detecting grout penetration have been previously trialled within the research literature. These are summarised below.

Chen et al. (2000) developed a fluorescent approach to attempt to visualise grout spread. This approach is described in Chen, Y., Nishiyama, T., Terada, M. and Iwamoto, Y. (2000) A fluorescent approach to the identification of grout injected into fissures and pore spaces. Engineering Geology, 56, 395-401 . Grout was mixed with a fluorescent substance, to help distinguish between the rock and the grouting material. The injected grout could then be viewed by a borehole television system using ultraviolet light and microscopic observation. The fluorescent particles added to the grout were less than 1 μιη and did not react readily with the other components within the cement mixture. The grouting process was split into three different stages and for each stage a different coloured fluorescent substance was mixed with the grout. After the grouting was completed additional boreholes were drilled at distances of 0.2 to 2 metres away from the injection boreholes. Core samples were taken at the additional boreholes and the distribution of the grout was imaged using a borehole television system in the additional boreholes. The approach was deemed useful in the analysis of grouting mechanisms. However, the requirement to drill through the grout curtain, and hence potentially compromise the integrity of the grout curtain may mean that the method of Chen et al may have limited application. Further, the method of Chen et al only gives data on the grout composition at each additionally-drilled observation hole. The method of Chen et al may not provide information on the overall location and integrity of the whole grout curtain.

High frequency seismic monitoring has also been trialled to detect the location of grout (Majer, E.L. (1989) The Application of High Frequency Seismic Monitoring Methods for the Mapping of Grout Injections. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 26, 249-256). This work, undertaken by the Lawrence Berkeley Laboratory, developed seismological techniques for mapping the location of grout injections in real time. Seismic signals may be generated by hydrofracturing during grout injection. Seismic signals that may have been generated by hydrofracturing during grout injection were monitored. In field experiments, seismic events were detected due to shear failure at some locations in the rock, but the largest events occurred after pumping had ceased and only these later stage large events were found to be locatable. Hence, it may not be possible to use the described seismic method to map the extent of grout penetration in real-time.

A further method trialled to detect grout after injection is ground penetrating radar (GPR). GPR is a geophysical method that sends short bursts of electromagnetic energy into the ground and detects the reflected signals (see, for example, Milsom, J. (2003) Field Geophysics - The Geological Field Guide Series. Third Edition, Wiley). Zhang et al. used GPR as a non-destructive testing method to attempt to determine the thickness of grout behind lining segments of metro lines in Shanghai, China (Zhang, F., Xie, X. and Huang, H. (2010) Application of ground penetrating radar in grouting evaluation for shield tunnel construction. Tunnelling and Underground Space Technology, 25, 99-107) . The GPR was found to be successful in this scenario. In the study of Zhang et al, the concrete segments, grout and soil provided a good material contrast and were only 1 meter in depth. Such a GPR approach may not be appropriate to deploy in many grouting campaigns, since the material property contrast between fractured rock and grout may be much lower than the material contrast in Zhang et al. Furthermore, the penetration of GPR in rock is limited and thus it may not be possible to use GPR where required detection distances are greater.

Summary of invention

In a first aspect of the invention there is provided a method comprising detecting injected magnetic grout to provide an indication of penetration into an injection region of the injected magnetic grout.

By detecting injected magnetic grout, an indication of penetration of the injected magnetic grout into an injection region may be provided. For example, the extent to which the magnetic grout has penetrated in one, two, or three dimensions may be determined. It may thereby be possible to determine whether the grout has penetrated up to a predicted penetration distance, or whether the grout has penetrated to a greater or lesser distance than the predicted penetration distance in any given direction. The method may further comprise injecting the magnetic grout into the injection region.

An injection region may be any region into which grout may be injected. For example, an injection region may comprise a subsurface region, a region which is to be strengthened, a ground region for which ground strengthening is required, a region associated with tunnelling or mining, or a region in which a hydraulic barrier is to be formed. Grout may be injected into the injection region in order to strengthen at least part of the injection region or to reduce the permeability of at least part of the injection region. Knowledge of the extent of penetration may enable assessment of the effectiveness of the grouting, for example whether the grouting is likely to have an adequate strengthening effect or form an adequate hydraulic barrier. Grout may be considered to have penetrated into an injection region if the grout extends into at least part of the injection region. Grout may extend into an injection region if the grout fills at least some pores, voids, fractures or cracks in the injection region.

Injecting the magnetic grout into the injection region may comprise injecting the magnetic grout into a borehole that extends into the injection region. Injecting the magnetic grout into the injection region may comprise injecting the magnetic grout into a plurality of injection boreholes, each of which extends into the injection region. On injection, grout may spread from the or each injection borehole into at least part of the injection region. Magnetic grout may be injected into the injection region and then non-magnetic grout may be injected into the injection region.

By injecting magnetic grout first and then non-magnetic grout, the furthest extent of the injected grout may be detected without requiring all the injected grout to be magnetic. Using magnetic grout only for the first part of a given injection may reduce cost in some cases.

The magnetic grout may be injected into the injection region for a selected time before the non-magnetic grout is injected. A selected volume of magnetic grout may be injected into the injection region before the non-magnetic grout is injected.

The magnetic grout may be injected for a predetermined time, or a predetermined volume of magnetic grout may be injected. By selecting a time for which to inject magnetic grout, or a volume of magnetic grout to inject, a suitable quantity of magnetic grout may be injected. The quantity of magnetic grout may be enough for the magnetic grout to be detectible. The quantity of magnetic grout may be restricted to limit cost in some cases.

Detecting the injected magnetic grout may comprise detecting at least one magnetic property associated with the magnetic grout. A magnetic property may comprise at least one of magnetic field, magnetic susceptibility, magnetisation, or a parameter associated with any of magnetic field, magnetic susceptibility or magnetisation.

The method may comprise obtaining detection data by the detection of the injected magnetic grout and processing the detection data to determine the indication of penetration into the injection region of the injected magnetic grout.

Detecting the injected magnetic grout may comprise detecting a magnetic field associated with the magnetic grout.

The detecting of the magnetic field associated with the magnetic grout may comprise detecting a change in magnetic field, the change in the magnetic field being caused by or associated with the magnetic grout. The magnetic field associated with the magnetic grout may, in some cases, comprise an alteration in a background magnetic field (for example comprising the Earth's magnetic field) that is caused by the presence of the magnetic grout.

Detecting the magnetic field associated with the magnetic grout may comprise detecting a background magnetic field before injecting the magnetic grout into the injection region (which may be the Earth's magnetic field and/or an initial applied magnetic field) and detecting a further magnetic field during or after injecting the magnetic grout into the injection region, thereby to determine a change in magnetic field due to the injected magnetic grout.

The method may comprise applying a magnetic field to the injection region.

The magnetic field associated with the magnetic grout may, in some cases, comprise an alteration to an applied magnetic field (for example a magnetic field applied to the injection region) that is caused by the presence of the magnetic grout.

Detecting the magnetic field associated with the magnetic grout may comprise detecting a background magnetic field before injecting the magnetic grout into the injection region and detecting a further magnetic field during or after injecting the magnetic grout into the injection region, thereby to determine a change in the applied magnetic field due to the injected magnetic grout.

Detecting the magnetic field associated with the magnetic grout may comprise detecting a first magnetic field before injecting the magnetic grout into the injection region and detecting a second magnetic field during or after injecting the magnetic grout into the injection region.

The first magnetic field may be a background magnetic field (which may comprise the Earth's magnetic field and/or an applied magnetic field). By measuring the magnetic field before and after (or during) the injection of the magnetic grout, a change in field due to the injected magnetic grout may be determined. In an injection procedure having multiple stages, by measuring the magnetic field before and after (or during) each injection stage, a change in field due to the magnetic grout injected during that stage may be determined.

Detecting the magnetic field associated with the magnetic grout may further comprise subtracting the first magnetic field and the second magnetic field to obtain a subtracted magnetic field.

Either of the first and second magnetic field may be subtracted from the other of the first and second magnetic field, thereby obtaining a signal that is the difference between the first and second magnetic fields. The difference signal may represent a magnetic field anomaly that is caused by the injected magnetic grout.

Providing an indication of penetration of the injected magnetic grout may comprise using the change in magnetic field to determine an extent of penetration of the injected magnetic grout. Providing an indication of penetration of the injected magnetic grout may comprise inverting the subtracted magnetic field. The subtracted magnetic field may be inverted to determine a shape and position of the injected magnetic grout. Detecting the magnetic field associated with the magnetic grout may comprise using a magnetometer to detect the magnetic field. Detecting the magnetic field associated with the magnetic grout may comprise using any suitable magnetic field detector. The magnetometer may be positioned in a further borehole adjacent the injection borehole. The further borehole may be a monitoring borehole. The further borehole may be a borehole into which grout will later be injected.

The magnetometer may be placed in the injection borehole vertically above the injection interval. The magnetometer may be placed at the ground surface. A magnetometer placed at the ground surface may be used to detect the upper boundary of the injected magnetic grout volume. The magnetometer may be placed in a subsurface tunnel or shaft. The magnetometer may be positioned adjacent to the injection borehole. The magnetometer, or other magnetic field detector, may be placed at a distance from the injection borehole such that the magnetic grout may be detected. The magnetometer, or other magnetic field detector, may be placed at a distance such that the injected grout does not spread as far as the magnetometer.

The distance from the injection borehole to the magnetometer may be between 1 m and 50 m, optionally between 5 m and 15 m, optionally between 5 m and 10 m. The distance from the injection borehole to the magnetometer may be greater than 1 m, optionally greater than 5 m, optionally greater than 10 m. The distance from the injection borehole to the magnetometer may be less than 50 m, optionally less than 15 m, optionally less than 10 m.

A plurality of magnetometers, or other magnetic field detectors, may be positioned adjacent to the injection borehole. Each of the plurality of magnetometers may be positioned in a respective monitoring borehole. Detecting the magnetic field associated with the magnetic grout may comprise combining signals from the plurality of magnetometers.

Detecting the magnetic field associated with the magnetic grout may comprise, for each magnetometer, performing a series of magnetic field measurements at different positions, for example at different heights in the further borehole. Detecting the magnetic field associated with the magnetic grout may comprise performing magnetic field measurements in the injection borehole. Detecting the magnetic field associated with the magnetic grout may comprise performing magnetic field measurements at the ground surface.

Detecting the injected magnetic grout may comprise non-intrusively detecting the injected magnetic grout. The magnetic field of the injected magnetic grout may be detected without making contact with the magnetic grout, for example without bringing a magnetometer in contact with the magnetic grout. The injected magnetic grout may be detected without drilling a borehole into the injected magnetic grout. The injected magnetic grout may be detected without drilling a borehole into the injected non-magnetic grout. By avoiding drilling into the grout, the injected grout may be detected without compromising the integrity of the injected grout, for example without compromising the low permeability function of the injected grout. The method may be non-intrusive in the sense that it is not intrusive to the grouted volume (or grout curtain). Providing an indication of penetration of the injected magnetic grout may comprise determining a shape representative of the extent of penetration of the injected magnetic grout.

Providing an indication of penetration of the injected magnetic grout may comprise determining a three-dimensional shape representative of the extent of penetration of the injected magnetic grout.

Determining a shape representative of the extent of penetration of the injected magnetic grout may comprise determining a shape representative of a leading edge of the injected magnetic grout.

Providing an indication of penetration of the injected magnetic grout may comprise determining a three-dimensional shape of the leading edge of the injected magnetic grout. The shape of the leading edge of the injected magnetic grout may represent the furthest extent to which any injected grout (magnetic or non-magnetic) has penetrated from the injection borehole.

Providing an indication of penetration of the injected magnetic grout may comprise forming an image of an extent of penetration of the injected magnetic grout into the injection region.

By forming an image, the extent of penetration of the injected grout may be displayed to a user. The user may take further action in response to the displayed extent of penetration, for example in planning further grouting.

The method may further comprise determining a location for a further injection borehole in dependence on the indication of penetration of the injected magnetic grout. The determination of the location for the further injection borehole may be performed automatically or by a user. By knowing the extent of the penetration from a first injection borehole, a user or program may determine a suitable distance from the first injection borehole to place a further injection borehole, such that the extent of the grout injected at the further injection borehole may be expected to join up or overlap with the grout injected at the first injection borehole. For example, the injection boreholes may be placed so as to create a grout curtain. An automatic or semi-automatic method, for example a computer program, may be used to determine the distance from the first injection borehole to the further injection borehole. The injection region may comprise a subsurface region.

The injection borehole may extend from the surface to the subsurface region.

The injection region may comprise at least one of a subsurface region, a foundation, a mining region, a tunnelling region, a structural region, a region required to perform a hydraulic function.

The method may further comprise adding a magnetic additive to grout to form the magnetic grout. The magnetic grout may comprise a magnetic material. The magnetic material may comprise any material giving a suitable signal in a magnetic field. The magnetic grout may comprise at least one of: a ferromagnetic material, a ferrimagnetic material, a super-paramagnetic material. The magnetic material may be present in the magnetic grout in suitable proportions to create a suitable signal in a magnetic field. The magnetic material may be any material having a suitable magnetic susceptibility or magnetic moment. The magnetic material and the proportion of magnetic material in the magnetic grout may be selected such that the magnetic grout has a suitable magnetic susceptibility or magnetic moment. The magnetic grout may have a suitable magnetic susceptibility for measurement with a magnetometer at a given distance. The magnetic grout may have a suitable magnetic susceptibility such that the induced magnetic field is measurable using a magnetometer at a given distance.

The proportion of magnetic material in the magnetic grout may be, by weight or volume, between 0.1 and 25%, optionally between 1 and 10%, optionally between 2 and 5%, optionally between 5 and 10%, optionally less than 5%, optionally less than 10%, optionally less than 25%, optionally more than 0.1 %, optionally more than 0.5%, optionally more than 1 %, optionally more than 5% or more than 10%. The magnetic material may comprise, for example, at least one of: magnetite, maghemite, iron filings, neodymium (and neodymium compounds), ilmenite (iron titanium oxide, FeTi0₃), jacobsite (manganese iron oxide), magnesioferrite (manganese iron oxide), gregite (iron sulphide), samarium-cobalt magnets (Sm₂(Co, Fe, Cu, Zr)₁₇ in either standard or high temperature magnet variants), NEO magnets (Nd₂Fe₁₄B and variants with some Al, Nb, Dy and including N42 grade NEO magnet that is Ni-Cu-Ni coated), AINiCo magnets (for example AINiCo₅ grade LNG37), ferrite magnets (ceramic magnets made up of complex oxides with compositions XOFe₂0₃ and Fe₂0₃ as the main component, for example ferrite grade C5 or Y30 magnets), or super-paramagnetic nanoparticles.

Each of the magnetic grout and the non-magnetic grout may comprise a cement or cement-alternative material. The cement or alternative to cement may comprise, for example, an Ordinary Portland Cement (for example, Lafarge blue circle, 42.5 CEM1 , 52.5 CEM1 ), a microfine or ultrafine cement (for example, Ultrafin from Cementa, Rheocem 650/900 from BASF, Spinor from Holcim, TamCrete MFC from TAM UK, Microcem 550, 650, 650 SR, 900 all produced by Lafarge), a colloidal silica/silica sol (for example MP320 colloidal silica, sold by MEYCO BASF), a silicate grout (for example sodium silicate grout such as N(R) sodium silicate sold by PQR Corporation). The grout may further comprise at least one of a superplasticiser (for example Glenium 51 from BASF), a stabilising agent (for example Grout Aid from Elken Multigrout), an accelerator for silica sol (for example NaCI or CaCI₂), a setting or hardening agent for silicate grout, or fly ash. The injection region may comprise a construction site, for example a construction volume beneath a surface construction area, and the method may further comprise injecting the magnetic grout into a borehole that extends into the construction site. The injection region may comprise a region in which construction is taking place. The injection region may comprise a region in which hydraulic barriers are being formed (for example, for drinking water protection). The injection region may comprise a region of ground improvement (for example for improving ground stability by, for example, filling in sinkholes or remediating differential settlements).

The injection borehole may extend from above ground to a subsurface construction site. The injection borehole may extend upwards from a subsurface construction site, for example, wherein the subsurface construction site comprises a volume of ground surrounding a tunnel.

In a further aspect of the invention, there is provided a system for detecting grout, the system comprising a detector for detecting injected magnetic grout to provide an indication of penetration into an injection region of the injected magnetic grout. The detector may comprise a magnetic field detector. The detector may comprise a detector configured to detect at least one magnetic property associated with the magnetic grout.

The system may further comprise an injector for injecting the magnetic grout into a borehole that extends into the injection region.

The injector may be operable to inject magnetic grout into the borehole and then inject non-magnetic grout into the borehole. The injector may be configured to inject the magnetic grout into the borehole for a selected time before injecting the non-magnetic grout into the borehole. The injector may be configured to inject a selected volume of the magnetic grout into the borehole before injecting the non-magnetic grout into the borehole.

The detecting of the magnetic grout may comprise detecting at least one magnetic property of the magnetic grout. The system may be operable in use to apply a magnetic field to the injection region. The system may comprise a magnetic field source, for example an electromagnet.

Detecting the magnetic grout may comprise detecting a background magnetic field before injecting the magnetic grout into the injection region and a further magnetic field during or after injecting the magnetic grout into the injection region, thereby to determine a change in magnetic field due to the injected magnetic grout.

Providing an indication of penetration of the injected magnetic grout may comprise using the change in magnetic field to determine an extent of the injected magnetic grout.

The detector may comprise a magnetometer. The injection region may comprise a construction site. The injection region may comprise a subsurface region. The injector may be operable to inject the magnetic grout into a borehole that extends into the construction site.

The magnetic field detector may be positioned in use in a further borehole adjacent the borehole into which grout is injected. The detector may be configured to be positioned in use in the borehole into which grout is injected. The detector may be configured to be positioned in use at the ground surface adjacent to the borehole into which grout is injected.

The detector may be configured to be positioned in use in a tunnel or shaft adjacent to the borehole into which grout is injected.

Detecting the injected magnetic grout may comprise non-intrusively detecting the injected magnetic grout.

The system may comprise a processing resource configured to receive detection data and to process the detection data to provide the indication of penetration into the injection region.

The system, optionally the processing resource, may be configured to use the detected magnetic signal from the injected magnetic grout to determine a shape representative of the extent of penetration of the injected magnetic grout. Determining a shape representative of the extent of penetration of the injected magnetic grout may comprise determining a shape representative of the leading edge of the injected magnetic grout.

Providing an indication of penetration of the injected magnetic grout may comprise forming an image of an extent of penetration of the injected magnetic grout into the injection region. The system, for example the processing resource, may be configured to use the detected magnetic signal from the injected magnetic grout to form an image of an extent of penetration of the injected magnetic grout into the construction site. The system, for example the processing resource, may be further configured to determine a location for a further injection borehole in dependence on the indication of penetration of the injected magnetic grout.

The magnetic grout may comprise at least one of: a ferromagnetic material, a ferrimagnetic material, a super-paramagnetic material. In a further independent aspect of the invention there is provided a computer program product comprising computer-readable instructions executable to process detection data obtained by detection of injected magnetic grout to determine an indication of penetration into an injection region of the injected magnetic grout.

The computer-readable instructions may be executable to determine a shape representative of the extent of penetration of injected magnetic grout by processing magnetic field data and further magnetic field data to determine a change in magnetic field due to the injected magnetic grout, wherein the magnetic field data is representative of a magnetic field before injection of the magnetic grout, and the further magnetic field data may be representative of a magnetic field during or after the injection of the magnetic grout.

There may be provided a method or system substantially as described herein with reference to the accompanying drawings.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. For example, apparatus features may be applied to method features and vice versa.

Specific description Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings of which:

Figure 1 a, 1 b, 1 c and 1d are schematic diagrams in plan view of successive stages of a grout campaign;

Figure 2a is a schematic diagram in plan view of an injection borehole and monitoring wells and Figure 2b is a plot showing temperature and pH at the monitoring wells of Figure 2a;

Figure 3 is an example of a deployment array for monitoring a grout curtain campaign;

Figure 4 is a flow diagram illustrating in overview a method according to an embodiment; Figure 5 is a plot of increasing magnetic susceptibility with % mass of magnetic mineral for OPC and Ultrafin cement;

Figure 6 is a plot showing results of bleed tests carried out on OPC and Ultrafin cements with different percentages of magnetic mineral additives;

Figures 7a, 7b and 7c are contour maps of magnetic field, in which Figure 7a is a map of magnetic field prior to burial of a magnetic grout object, Figure 7b is a map of magnetic field including the buried magnetic grout object, and Figure 7c shows processed data once the earth's magnetic field has been removed;

Figure 8 is a schematic diagram showing a configuration of magnetometer and samples;

Figure 9 is a plot from field trials, showing magnetic field data against distance of a grouted object from the magnetometer, along with a model of the decay of the magnetic field. A grout penetration detection system according to an embodiment is illustrated in Figure 3.

A first injection borehole 10 (which may also be called an injection well) extends into a subsurface, which may comprise soil and/or rock. Further injection boreholes 40 may be subsequently drilled into the subsurface. Several monitoring boreholes 12 (which may also be called monitoring wells, observation wells or observation boreholes) also extend into the subsurface. The distance between each monitoring borehole 12 and the first injection borehole 10 is known. A magnetometer 14 is placed in each monitoring borehole 12. In the present embodiment, the magnetometer 14 is a G-882 magnetometer adapted for downhole application. In other embodiments, any appropriate magnetometer may be used. In further embodiments, any magnetic field detector suitable for measuring magnetic field or other magnetic parameter may be used in place of or in addition to magnetometer 14.

Each magnetometer 14 may have an integrated GPS for accurate positioning in 3D. Each magnetometer 14 may be capable of easy motion up and down the borehole and capable of real-time data acquisition. A data acquisition unit 5 is connected to each of the magnetometers 14. In the present embodiment, the connection between the data acquisition unit 5 and the magnetometers 14 is a wired connection. In other embodiments, the connection between the data acquisition unit 5 and the magnetometers 14 may be a wireless connection. The data acquisition unit 5 is connected to a processing resource in the form of computing apparatus 6. The connection between the data acquisition unit 5 and computing apparatus 6 may be permanent or temporary and may be wireless or wired. Computing apparatus 6 is configured to run analysis software 8. In the present embodiment, the computing apparatus 6 is a general purpose personal computer (PC). In other embodiments, computing apparatus 6 may be any suitable apparatus capable of running analysis software 8, for example a laptop or tablet, or a high-performance computer. In the present embodiment, the computing apparatus 6 is present at the grouting site. In other embodiments, the computing apparatus 6 may be remote from the grouting site. While in the present embodiment, the magnetic field functionality is implemented in analysis software 8, in other embodiments it may be implemented in any hardware, software, or combination of hardware and software. Although in the present embodiment, a separate data acquisition unit 5 and computing apparatus 6 are used, in other embodiments the data acquisition unit 5 and computing apparatus 6 may be replaced by a single unit having the functionality of both the data acquisition unit 5 and the computing apparatus 6. In further embodiments, the functionality of the data acquisition unit 5 and/or computing apparatus 6 may be split over multiple units. In operation, magnetic grout 16 is injected into a 3 m interval of the injection borehole 10 for a period of time. The magnetic grout 16 spreads into the subsurface. The injection of magnetic grout 16 is followed by injection of normal (non-magnetic) grout 18 which also spreads into the subsurface. The injection may then be repeated for further intervals of the injection borehole, moving upwards.

Figure 4 is a flowchart illustrating in outline a method according to an embodiment, in which magnetic grout is injected into the first injection borehole 10 and (optionally) into further injection boreholes 40 and the penetration of the grout is monitored using magnetometers 14 in monitoring boreholes 12. The magnetic grout may be injected by an injector. At stage 20, before any injection of grout is performed, a magnetic grout 16 is selected or designed. The magnetic grout 16 may also be described as a detectible grout. The magnetic grout 16 is selected or designed to have sufficiently high magnetic susceptibility or other magnetic property that, once injected, it can be detected using the magnetometers 14. In particular, the magnetic grout 16 is designed to have sufficiently high magnetic susceptibility or other magnetic property that it can be detected with a desired accuracy, from the distance of the monitoring boreholes 12. In the embodiment of Figure 3, the monitoring boreholes are placed around 10 m from the injection borehole 10. In other embodiments, any suitable distance may be used, such that the monitoring boreholes are far enough away from the injection borehole not to interfere with the grouting, and close enough to the injection borehole to be capable of detecting the magnetic grout 16. In some embodiments, the distance between an injection borehole and a monitoring borehole may be between 5 and 15 m. In other embodiments, the distance between an injection borehole and a monitoring borehole may be between 5 and 10 m. In some embodiments, the distance between an injection borehole and a monitoring borehole may be up to 50 m. Such embodiments may use magnetic materials with a high magnetic susceptibility. In other embodiments, the distance between an injection borehole and a monitoring borehole may be less than 5 m, for example in embodiments for which the desired grout penetration is in the range of 1 to 2 m.

In the present embodiment, a magnetic grout 16 is selected which comprises cement, water, and a magnetic additive (which may be described as a magnetic mineral additive). The magnetic additive has an appropriate magnetic susceptibility and the magnetic grout 16 contains a given percentage of the magnetic additive. The choice of magnetic grout 16 (for example, the percentage of magnetic additive and/or the choice of magnetic additive) may be dependent on a length of time for which grout containing the magnetic additive is intended to be injected into the injection borehole. Therefore, a length of time for which magnetic grout 16 will be injected is also determined at stage 20.

A change in the properties of a magnetic grout 16 may be achieved by (i) altering the magnetic susceptibility of the additive, (ii) increasing the percentage of magnetic mineral additive or (iii) by injecting the magnetic grout 16 for a longer time period (thereby increasing the mass of magnetic material).

Design or selection considerations for the magnetic grout 16 may also include consideration of the magnetic grout performance (e.g. flowability and bleed) to ensure that the grout remains workable, and the cost of the magnetic additive.

In the present embodiment, the magnetic grout 16 is selected from a range of potential magnetic grouts. For each of the potential magnetic grouts, data on various properties of the potential magnetic grout is available. The properties may include percentage of magnetic additive, magnetic susceptibility, flowability, bleed, and cost.

Magnetic additives used in one or more of the magnetic grouts may comprise at least one of magnetite, maghemite, iron filings, neodymium (and neodymium compounds), ilmenite (iron titanium oxide, FeTi0₃), jacobsite (manganese iron oxide), magnesioferrite (manganese iron oxide), gregite (iron sulphide), samarium-cobolt magnets (Sm₂(Co, Fe, Cu, Zr)₁₇) in either standard or high temperature magnet variants), NEO magnets (Nd₂Fe₁₄B and variants with some Al, Nb, Dy and including N42 grade NEO magnet that is Ni-Cu-Ni coated), AINiCo magnets (for example AINiCo₅ grade LNG37), ferrite magnets (ceramic magnets made up of complex oxides with compositions XOFe₂0₃ and Fe₂0₃ as the main component, for example ferrite grade C5 or Y30 magnets), or super-paramagnetic nanoparticles. In embodiments, any appropriate ferromagnetic, ferrimagnetic or super-paramagnetic material may be used. Any material having appropriate magnetic properties such that it may be detected using a magnetometer or other magnetic field detector may be used. Any material having an appropriate magnetic susceptibility or magnetic moment may be used. In some embodiments, the material may be a material that only shows magnetisation when an external magnetic field is applied.

In the present example, the magnetic grout 16 comprises Procem 52.5N + 5% magnetite. Procem is an Ordinary Portland Cement (OPC), which is produced by Lafarge and has a grain size between 5 and 30μιη. The cement is mixed with water at a ratio of 1 part water to 2 parts of cement/. Magnetite is then added at 5% by mass of cement. Therefore the magnetic grout has a ratio of 1 part water to 2.1 parts cement/magnetite mix. In further embodiments, any suitable amount of magnetic material and any suitable ratio of water to cement may be used.

In alternative embodiments, any suitable cement or alternative to cement may be used, for example, an Ordinary Portland Cement (for example, Lafarge blue circle, 42.5 CEM1 , 52.5 CEM1 ), a microfine or ultrafine cement (for example, Ultrafin from Cementa, Rheocem 650/900 from BASF, Spinor from Holcim, TamCrete MFC from TAM UK, Microcem 550, 650, 650 SR, 900 all produced by Lafarge), a colloidal silica/silica sol (for example MP320 colloidal silica, sold by MEYCO BASF), a silicate grout (for example sodium silicate grout such as N(R) sodium silicate sold by PQR Corporation).

Further materials may be added to the grout mix, for example at least one of a superplasticiser (for example Glenium 51 from BASF), a stabilising agent (for example Grout Aid from Elken Multigrout), an accelerator for silica sol (for example NaCI or CaCI₂), a setting or hardening agent for silicate grout, or fly ash.

Although in the present embodiment, the magnetic grout 16 is selected from a range of potential magnetic grouts, in other embodiments, the magnetic grout 16 may be designed by performing experiments as described below with reference to Figures 5 and 6. For example, the grout may be designed by determining a magnetic additive, a percentage of magnetic additive, and a cement or cement-alternative. A magnetic grout additive (mass and susceptibility) may be designed such that the grout has the desired detection accuracy, grout properties (such as flowability and bleed) and can be used at an appropriate distance from the monitoring boreholes 12.

In the present embodiment, the selected magnetic material has a susceptibility that does not change much with time. In other embodiments, magnetic materials may be chosen that produce a change in magnetic signal with time (which may be described as an evolving magnetic field) corresponding to changes in the cement due to grouting.

In some embodiments, a look-up table of magnetic grouts and/or of magnetic additives may be provided, and the magnetic grout 16 or a magnetic additive to form the magnetic grout 16 may be selected at stage 20. In alternative embodiments, the magnetic grout 16 is pre-selected and stage 20 is omitted from the process of Figure 4. For example, a magnetic grout 16 may be used that has been previously selected or designed for a different grouting campaign. At stage 22, prior to grout injection, a background measurement of the magnetic field is taken using the magnetometers 14. Each magnetometer 14 is swept throughout the length of its respective monitoring borehole 12. In the present embodiment, the magnetometer 14 starts at the bottom of the borehole and is moved through the borehole from bottom to top, taking magnetic field measurements as it is moved up the borehole. The magnetometer records continuously while it is swept up the borehole. A GPS is used to enable accurate location determination during the measurement. In other embodiments, the magnetometer 14 is moved from top to bottom while measurements are taken, or measurements can be taken in either direction of movement. Measurements may be taken continuously or at any appropriate time increment or distance increment.

Although in the current embodiments the magnetometer 14 moves through the entire length of the monitoring borehole 12 while taking magnetic field measurements, in other embodiments the magnetometer 14 may be moved through a portion of the monitoring borehole 12 or may remain stationary.

Although In the present embodiment magnetic field measurements are taken in the monitoring boreholes, in further embodiments, measurements may alternatively or additionally be made in the injection borehole (for example, at a position vertically above the interval in which grout is being injected) and/or in one or more boreholes in which grout is subsequently to be injected. In additional embodiments, measurements may alternatively or additionally be made within a subsurface tunnel or shaft.

In alternative embodiments measurements of the magnetic field may alternatively or additionally be made at the ground surface. For example, although in some circumstances cement grouts not be used in the upper 15 m of the subsurface due to possible ground heave, alternative grouts such as silica sol may be used in the 15 m nearest the surface. For grouts used near the surface, measurements made from magnetometers placed on the ground surface may be appropriate. Magnetometers placed at the ground surface may be used to detect the distance to the upper boundary of the injected magnetic grout volume and may also be used to estimate the shape of the injected magnetic grout volume.

Magnetic field data is transmitted from each magnetometer 14 to data acquisition unit 5 and from data acquisition unit 5 to computing apparatus 6. In the present embodiment, the result of each measurement taken by each magnetometer 14 is transmitted to the data acquisition unit 5 and computing apparatus 6 in real time or near-real time. In alternative embodiments, data may be stored at the magnetometer 14 for a time before being transmitted to the data acquisition unit 5, or may be stored at the data acquisition unit 5 for a time before being transmitted to the computing apparatus 6. For example, measurements may be taken for the entire sweep of the monitoring borehole 12 and then transmitted as a batch.

At stage 24, grout is injected into the bottom 3 m interval of first injection borehole 10. Although in the present embodiment, 3 m intervals are used, in other embodiments any suitable interval size may be used.

For a fixed interval of time (designed at stage 20 above to achieve the desired susceptibility), magnetic grout 16 is injected. For example, in the present embodiment the magnetic grout may be injected for a time between 10 and 30 minutes. In other embodiments, any appropriate time period may be used. After that fixed period of time, injection of normal (non-magnetic) grout 18 is performed. In the present embodiment, the injection of normal grout 18 is stopped when a maximum pressure is reached or when a maximum volume of grout is injected. Therefore the injection of the bottom 3 m interval of injection borehole 10 comprises an injection of magnetic grout 16 and subsequent injection of normal grout 18 into the same interval. In the present embodiment, the total grout injection (magnetic grout plus normal grout) may take, for example, between half an hour and two hours. In other embodiments, any suitable injection duration may be used. In the present embodiment, the normal (non-magnetic) grout 18 is made from the same cement as the magnetic grout 16 but without the magnetic additive. In the present embodiment, the normal grout comprises 1 part of water mixed with 2 parts of Procem 52.5N. Normal or non-magnetic grout may refer to grout to which no magnetic additive has been added. Normal or non-magnetic grout may itself have an inherent susceptibility, but the susceptibility of the normal or non-magnetic grout is lower than that of the magnetic grout 16. In some embodiments, the magnetic grout 16 has a susceptibility such that the magnetic grout 16 may be detected by the magnetometer 14, and the non-magnetic grout 18 has a susceptibility such that the non-magnetic grout may not be detected by the magnetometer 14 (or may not easily be detected by the magnetometer 14).

The magnetic grout 16 spreads through the subsurface, forming a grouted volume of which the leading edge may be described as a grout front. The normal grout 18 follows behind the magnetic grout 16. The furthest grout from the injection borehole 10 is expected to be magnetic grout 16, and therefore detection of the magnetic grout 16 may be used to determine the extent of grout penetration into the subsurface.

Although in the present embodiment, both magnetic grout 16 and non-magnetic grout 18 are injected into the injection interval, in some embodiments only magnetic grout may be injected without subsequent injection of non-magnetic grout 18 into that injection interval. For example, if the desired penetration distance is relatively small and/or the grout volume is small, then only magnetic grout may be used.

The subsurface region into which the borehole is drilled, and into which the grout is injected, may be referred to as an injection region. The grout may be said to extend into or penetrate the injection region if the grout extends into at least part of the injection region. The grout may be said to extend into or penetrate the injection region if the grout fills at least some pores, voids, cracks or fractures in the injection region. The grout may not fill the whole injection region.

At stage 26, a further measurement of magnetic field is made by sweeping each magnetometer 14 through its respective monitoring borehole 12. In the present embodiment, each magnetometer 14 is swept throughout the entire length of its monitoring borehole 12. In other embodiments, each magnetometer is swept through only a portion of the length of the observation borehole at stage 26. For example, the magnetometer 14 may be swept through a length that extends above and below the interval in which the magnetic grout is being injected into the injection borehole 10. For example, if the grout is injected into the 15 to 18 m interval, the magnetometer 14 may be swept between 10 and 23 m in its monitoring borehole 12. Magnetic field data is transmitted from each magnetometer 14 to data acquisition unit 5 and from data acquisition unit 5 to computing apparatus 6. The transmission of data may be in real time or as a batch. The measurements of magnetic field recorded after the grout injection, with the magnetic grout present, are expected to be different to the background measurements of magnetic field that were collected at stage 22.

At stage 28, the background measurements of magnetic field from stage 22 are subtracted from the post-injection magnetic field measurements taken at stage 26. The resulting magnetic field data should represent the magnetic signal produced by the injected grout volume. Although in the present embodiment, the background magnetic field measurements from stage 22 are subtracted from the magnetic field measurements from stage 26, in other embodiments the magnetic field measurements from stage 26 are subtracted from the background magnetic field measurements from stage 22.

At stage 30, the resulting magnetic field measurements that were determined in stage 28 are used to estimate the 3D shape of the leading edge of the volume of magnetic grout 16 that has been injected into the subsurface. The measurements of the magnetic field thereby provide a non-intrusive method of detecting the injected magnetic grout, since no contact with the injected magnetic grout is required. Although in the present embodiment monitoring boreholes are drilled into the subsurface, there is no need to drill into the grouted volume (or grout curtain) and the method is therefore non-intrusive to the grouted volume. In the present embodiment, the analysis software 8 performs a 3D inversion of the resulting magnetic signal determined in stage 28 to estimate the position of the grout front for the grouted interval. The analysis software 8 performs a method using known equations for 3D inversion of magnetic anomalies (see, for example, Reid, Allsop, Granser, Millet and Somerton, 1990, Geophysics, Vol. 55, No. 1 (January 1990) pp 80- 91 ; Li and Oldenburg, 1996. Geophysics, Vol. 61 , No. 2 (March-April 1996); P. 394- 408). The analysis software 8 makes use of the fact that the magnetic susceptibility of the injected magnetic grout 16 is known from stage 20. The background (ambient) magnetic field prior to the injection is also known, having been detected at stage 22. The magnetic susceptibility and ambient magnetic field give known parameters for the inversion process. The analysis software 8 allows the information gained from multiple data sets to be maximised.

In further embodiments, any suitable method for determining a shape representative of the injected magnetic grout from the magnetic field data may be used. The method may comprise an inversion of the magnetic field to determine what 3D shape of magnetic grout would lead to the observed resulting magnetic field. The method may comprise forward modelling of possible shapes representative of the injected magnetic grout, and fitting those shapes to the observed magnetic field data.

The 3D shape of the leading edge of the magnetic grout is an indication of the penetration of the magnetic grout into the subsurface. The extent to which the magnetic grout has penetrated is represented by the 3D shape. In other embodiments, a different indication of penetration may be used. For example, 2D distances may be provided, such as the distance from each magnetometer or monitoring borehole to the leading edge of the magnetic grout. In some embodiments, the indication of penetration may be an indication of whether or not the magnetic grout has reached a desired point (for example, a message indicating whether grouting should be terminated or not). The indication of penetration may or may not be communicated with a user at stage 30. For example, in some embodiments an image is formed of the determined 3D shape of the leading edge and the image is displayed to a user.

Although in the present embodiment, the analysis software 8 performs both the determination of the magnetic field due to the injected magnetic grout and the inversion of the magnetic field to determine the 3D shape of the magnetic grout, in other embodiments two or more programs may be used.

At stage 32, the shape of the leading edge of the magnetic grout that was determined at stage 30 is added to an overall penetration shape estimate. The overall penetration shape is the combined shape of penetration determined for each injection interval along the length of the borehole. For the first injection (the bottom interval of the first injection borehole 10) the overall penetration shape estimate is the shape of the leading edge that was determined at stage 30. For subsequent injections, the overall penetration shape estimate may combine the shape estimated for the first injection with the shape estimated for a second and subsequent injections. In the present embodiment, an image representing the overall penetration shape estimate is displayed on a display screen where it may be viewed by a user. In other embodiments, the overall penetration shape estimate may not be displayed. In the present embodiment, images representing the background magnetic field and magnetic field after injection are also available to the user. For example, images of the background magnetic field and magnetic field after injection may be displayed alongside images of the grout penetration shape estimate for each injection interval or the overall penetration shape estimate for the length of the borehole, or on different screens or frames.

Stage 34 is a decision point in the flow chart which depends on whether more grout is being injected in the borehole. If more grout is to be injected into the borehole, then the process of the flow chart returns to stage 24.

After the first injection into the bottom 3 m interval of the first injection borehole 10 as described above, a second injection into the second-to-bottom 3 m interval of the first injection borehole 10 is performed. The injection once again comprises an injection of magnetic grout 16 for a predetermined time followed by non-magnetic grout 18. In the present embodiment, the injection time for the magnetic grout 16 for the second interval is the same as the injection time for the magnetic grout 16 in the first interval. In other embodiments, different times may be used. Although in the present embodiment, magnetic grout 16 is injected for a given time, in alternative embodiments a given volume of magnetic grout 16 may be injected.

At the second iteration of stage 26, each magnetometer 14 acquires magnetic field data over depth for its respective monitoring borehole 12 and transmits the data to data acquisition unit 5 and computing apparatus 6, in the same manner as in the first iteration of stage 26. In the present embodiment, the magnetometer is swept over the full length of the monitoring borehole 12, but in other embodiments the magnetometer may be swept over only a portion of the monitoring borehole 12.

At the second iteration of stage 28, the analysis software 8 subtracts the magnetic field data from the first iteration of stage 26 (the data for the first injection interval) from the magnetic field data from the second iteration of stage 26 (the data for the second injection interval). Therefore the magnetic field measured after the previous injection is now considered to be the background (ambient) magnetic field.

At the second iteration of stage 30, the computing apparatus 6 estimates the 3D shape of the magnetic grout that has just been injected (the magnetic grout that was injected into the second interval) by inverting the magnetic field data from the second iteration of stage 28 using analysis software 8.

At the second iteration of stage 32, the computing apparatus 6 adds the 3D shape of the leading edge that was determined for the second interval to the 3D shape of the leading edge that was determined for the first interval, thereby obtaining an overall penetration shape estimate comprising grout from both injections. The overall penetration shape estimate is displayed on a display screen for viewing by a user. In further embodiments, the previous penetration shape estimate, from before the most recent injection, is also available to the user.

At the second iteration of stage 34, more grout is still required in the first injection borehole 10, and stages 24 to 32 are repeated until the grouting of the first injection borehole 10 has been completed. Each time the magnetic field data is acquired, the previously-detected magnetic field is removed from newly-detected magnetic field at stage 28 so that just the signal from the new injection interval remains. This can then be used to estimate the shape of the leading grout edge for that interval alone.

Once all the intervals in the first injection borehole 10 are complete, then the decision point at stage 34 directs the flow chart to stage 36.

In the present embodiment, a location has previously been proposed for a second injection borehole 40, which has not yet been drilled. At stage 36, the overall penetration shape estimate (which may also be called a 3D grouted volume estimate) is used to review the location of the second injection borehole 40.

The shape of the final penetrated volume for the first injection borehole 10 can be used to determine the optimal location for the second injection borehole 40. For example, if the penetration of the grout (as determined by the detection of the magnetic grout front) from the first injection borehole 40 has been greater than expected, the second injection borehole 40 may be drilled further away from the first injection borehole 10 than was originally planned. Conversely, if the penetration of the grout has been less than expected, the second injection borehole 40 may be drilled closer to the first injection borehole 40 than was originally planned. The second injection borehole 40 may be drilled in a position such that the extent of the grout injected at the further injection borehole may be expected to join up or overlap with the grout injected at the first injection borehole. For example, the injection boreholes may be placed so as to create a grout curtain. In the present embodiment, analysis software 8 reviews the proposed location for the second injection borehole 40 and, if necessary, determines a revised location for the second injection borehole 40 based on the determined 3D grout penetration. In other embodiments, different software may be used to determine the location of the second borehole location than is used to determine the 3D grout penetration. In further embodiments, the position of the second injection borehole 40 may be determined by a user, for example by viewing a 3D image of the extent of penetration of the grout injected into the first injection borehole 10. In other embodiments, any manual, automatic, or semi-automatic method of determining the position of the second injection borehole 40 may be used.

In alternative embodiments, the location of the second injection borehole 40 may be predetermined and may not be altered at stage 36. In further embodiments, the second injection borehole 40 may already have been drilled. In some such embodiments, stage 36 may be omitted.

In the present embodiment, the same monitoring boreholes 12 containing the magnetometers 14 are used to measure magnetic field for the injection of grout into the second injection borehole 40 as were used to measure magnetic field for the injection of grout into the first injection borehole 10. In other embodiments, additional monitoring boreholes 12 are also drilled along with the second injection borehole 40. In some embodiments, for injection boreholes close to the first injection borehole, some of the same monitoring boreholes are used as were used for the first injection borehole. As grouting progresses and subsequent injection boreholes are placed further away from the first injection boreholes, it may be necessary to drill further monitoring boreholes. In some embodiments, boreholes that will later be injected into may be used as monitoring boreholes before they are used as injection boreholes.

Stages 24 to 32 are repeated for all the intervals at the second injection borehole 40 until the second injection borehole 40 is complete. When the second injection borehole 40 has had grout injection performed at all intervals, then a third borehole and subsequent boreholes may be drilled and grout injected. The penetration shape estimate is updated for each injection at each borehole, until the grout curtain is complete.

In the present embodiment, the same set of monitoring boreholes 12 are used to monitor injections for each of the injection boreholes 10, 40. However, in other embodiments, additional monitoring boreholes 12 may be drilled when a further injection borehole 40 is drilled. In some embodiments, only some of the monitoring boreholes 12 have magnetometers 14 present for any given injection, for example the monitoring boreholes 12 within a given distance (for example, 10 m) of the borehole that is currently being injected. In further embodiments, all the monitoring boreholes 12 contain magnetometers 14, but only data from the magnetometers 14 nearest to the borehole that is currently being injected are used.

After a final injection is performed on a final borehole 40, a final magnetic field measurement is taken in a final iteration of stage 26. The penultimate magnetic field data is subtracted from the final magnetic field data at stage 28, and the results are used to determine the extent of the grout at stage 30. At stage 32 the determined extent of the grout is added to the overall grout penetration estimate. The overall grout penetration estimate is displayed to the user, and the flowchart terminates at stage 38 with a completed grouting campaign.

In the present embodiment, each set of magnetic field data is stored to a data store, and each 3D estimate of the grout penetration volume due to each injection and overall penetration estimate is also stored to a data store.

In the embodiment of Figure 3, the magnetic field is measured after each grout injection, and the magnetic field prior to a given injection is subtracted from the magnetic field after the injection. In the case of the first injection, the background magnetic field is subtracted from the magnetic field after the first injection. For each subsequent injection, the magnetic field after the injection immediately previous to the current injection is subtracted from the magnetic field after the current injection. Although in the embodiment of Figure 3, the magnetic field is measured after each injection, in other embodiments, the magnetic field may be repeatedly or continuously measured during an injection. Several magnetic field measurements are taken during the injection, with the magnetometers 14 fixed in one location (at a depth corresponding to the injection interval) within each monitoring borehole. The magnetic field prior to injection is subtracted from the magnetic field measured during injection. The resulting magnetic field may be used to determine the extent of the magnetic grout in real time while the injection is being performed. The determined extent of the magnetic grout may be used to control the injection, for example to determine a time at which to stop the injection. The time at which to stop the injection may be the time at which the desired grout penetration has been achieved, as indicated by the magnetic field measurement.

In further embodiments, the magnetic field for the first (or other) injection stage, or multiple stages, may be monitored in real time to monitor the grout penetration during that stage as described above. The results of the real-time monitoring of the grout penetration during the first (or other) injection stage(s) may be used to determine an appropriate GIN for the rock volume.

Real time measurement may further be of use where the magnetic material is a material that produces a change in magnetic signal with time. The change in magnetic signal with time may correspond to changes in the cement due to setting or curing. The magnetic signal may therefore be monitored in real time to monitor changes in the cement. Although in the present embodiment, magnetic grout 16 is injected at the start of the injection for each interval, followed by normal grout 18, in other embodiments, magnetic grout may be repeatedly introduced during the injection. In further embodiment, magnetic grout 16 is used for the entire injection and no normal grout 18 is used. In alternative embodiments, magnetic grout 16 is only used in certain selected injections, and other injections are carried out using only normal grout 18. Although in some embodiments the magnetic field of the magnetic grout 16 is determined using the method of Figure 3 without an external magnetic field being applied to the magnetic grout 16 (other than the Earth's magnetic field), in other embodiments an external magnetic field is applied to the magnetic grout 16, for example an external magnetic field is applied to the injection region. In such embodiments, the magnetic field associated with the magnetic grout may comprise an alteration to the applied magnetic field that is caused by the presence of the magnetic grout 16. The apparatus may comprise a magnetic field source, for example an electromagnet, for applying the magnetic field. Any suitable magnetic field source may be used.

The method described above with reference to Figure 4 may be used in a variety of civil engineering applications. For example, grouting with magnetic grout 16 may be used in ground strengthening, for example to support surface construction or to strengthen ground for subsurface tunnelling or mining. The injection region may comprise a region of ground improvement (for example for improving ground stability by, for example, filling in sinkholes or remediating differential settlements). Grouting with magnetic grout 16 may be used for the creation of hydraulic barriers, for example barriers for reservoir dams and barriers for the prevention of contaminant migration in groundwater. Grouting with magnetic grout may be used in any appropriate application, for example in any appropriate ground engineering application. Experiments have been performed to test the concept of the method of Figure 4. The experiments are described with reference to Figures 5 to 10.

For the experiments discussed below two different cements, Procem 52.5N and Ultrafin 16 have been used. Procem 52.5N and Ultrafin 16 are common in the grouting industry. Procem is an Ordinary Portland Cement (OPC), which is produced by Lafarge and has a grain size between 5 and 30μιη. Ultrafin 16 is a microcement produced by Cementa where 95% of the cement material has a grain size less than 16μιη.

The volume magnetic susceptibilities of the detectible grouts were determined using a Bartington MS2B Dual Frequency Sensor. Volume magnetic susceptibility (χ_ν) indicates how responsive a material is to an applied magnetic field and is calculated as χ^Μ/Η, where M is the magnetisation of the material (A/m) and H is the applied magnetic field strength (A/m). Samples were created by mixing cement with water in the ratios of 1 part water to 2 parts cement for the OPC and 1 part water to 1 part cement for the Ultrafin Cement. Magnetic minerals were then added to the cement. A number of different materials were trialled at different percentages (by solid mass). Results presented below primarily focus on magnetite, with 95% purity and a particle size of less than 5μιη. Mixtures of cement, magnetic minerals and water were tested for magnetic susceptibility at 7, 14, 21 and 28 day intervals and then, to check for long-term drift, after a further 12 weeks.

The volume magnetic susceptibilities of the pure Ordinary Portland Cement and pure Ultrafin Cement mixed with water in the ratios listed are 5.7460 x 10^"4 and 2.2307 x 10^"4 respectively. For these materials and for the additives listed, it was found that susceptibility did not vary much with time. However, the variation of susceptibility with time may depend on the magnetic mineral and cement used. Some magnetic materials may produce a change in the magnetic signal with time corresponding to changes in the cement during grouting.

Figure 5 is a plot of volume magnetic susceptibility against percentage of magnetic material (expressed as a percentage of the mass of cement). The magnetic susceptibilities of OPC and Ultrafin with 1 %, 2.5%, 5% and 10% quantities of magnetite are plotted in Figure 5. The susceptibilities of OPC with 5% maghemite and OPC with 5% iron filings are also plotted.

Figure 5 shows that the magnetic susceptibility of the grouting mixture increases in a predictable way for the magnetic additives shown: susceptibility increases linearly with increasing mass of the magnetic additive. This predictability may indicates that the grout mixture may be designed to achieve a desired volume magnetic susceptibility by either changing the type of magnetic mineral added, or the mass of magnetic mineral added. Cement properties of the detectable grout were established to ensure usability. Cement properties of the cements include bleed and time of efflux through the flow cone. The time of efflux of a specified volume of grout (1 .725 I) through a standardised flow cone allow the flowability of the cement to be established. For grouting fracture rock, the time of efflux may be required to be in the range of 31 to 35 s. Other grouting applications may require the time of efflux to be within alternative ranges to ensure usability.

The bleed of the cement refers to the accumulation of water at the surface of freshly mixed grout. For grouting fractured rock, a stable grout with less than 2% bleed after 2 hours is usually required. Other grouting applications may have alternative requirements for bleed.

Flowability tests were carried out in accordance with ASTM International standard C939. Grout mixtures were prepared for the OPC cement with a water to cement ratio of 1 :2. OPC cement (with no magnetic material) in a ratio of 1 :2 (water to cement) was found to have a time of efflux of 33 s. OPC with 5% magnetite in a ratio of 1 :2 (water to cement/magnetite mix) was found to have a time of efflux of 35 s. The results of the flow cone tests show that the behaviour is within the acceptable range for workability of the new grout mixture, and is similar to those values achieved for the cement/water mixture containing no magnetic additives.

Bleed tests were carried out in accordance with ASTM International standard C940. Bleed is the volume of accumulated surface water divided by the initial grout volume. Bleed was measured for 800 ml samples measured in a 1000 ml graduated cylinder. Grout mixtures were prepared for the OPC and Ultrafin cements in the following water to cement ratios: 1 :2 and 1 :1 .5 respectively. Figure 6 presents the bleed test results for the magnetic grout mixtures (OPC plus 5% magnetite, Ultrafin plus 5% magnetite) and for the corresponding normal grouts (without any magnetite present). The bleed for the detectible grout was found to be similar to that of the pure cement grouts containing no magnetic additives and was within the acceptable range, having less than 2% bleed after 2 hours.

Samples of grout mixtures were cast into a variety of shapes, each with a known volume and mass. The grout mixture samples were buried at a range of depths. To undertake field trials, samples of cement and magnetite mix were cast for burial. The two cements used were again Procem 52.5N and Ultrafin 16. The magnetic grout mixture deployed used the addition of Magnetite. The Magnetite was in powder form with a particle size of less than 5μιη and was 95% pure. Samples of detectible grout cement were produced by mixing the cement and Magnetite with water in the ratios of

1 part water to 2 parts cement for the Ordinary Portland Cement and 1 .5 parts water to

2 parts cement for the Ultrafin Cement. The samples used contained either 5% or 10% Magnetite. After thorough mixing, the detectible grout mixture was poured into cylindrical and rectangular shaped moulds and left for several days to cure. The volumes of the final samples for burial, and subsequent detection, were between 760 cm³ and 1200 cm³.

A G-858 MagMapper Magnetometer was used for the field trials. The G-858 is a high performance caesium vapour magnetometer, which has a high sensitivity of 0.01 nT. The G-858 has two magnetometers, which can be deployed vertically, one above the other.

Field trials were conducted at Troon, UK, on the beach. Trials were undertaken within a 5m by 5m square. Magnetic field readings (measured in Tesla) were collected along lines within the 5m by 5m square. The lines along which magnetic field readings were taken were spaced at 0.5m apart. A 5m by 5m map of magnetic field was created from the magnetic field readings. All of the trials were unidirectional (i.e. all of the lines were walked in the same direction each time) and were undertaken walking towards North. An example of magnetic field maps collected from the field trial is shown in Figure 7. To determine the location and depth of a grout sample, the magnetic field was mapped before and after burial of the sample.

Figure 7a is a magnetic field map showing the ambient magnetic field over the 25 m² area (the 5m by 5m square) prior to burial of the grout sample. The results of Figure 7a may be considered to be a measurement of the earth's magnetic field. Figure 7b shows a map of magnetic field which was taken after the burial of the grout sample, and includes the influence of the buried grout sample. The grout sample in this trial was three bricks of magnetic grout prepared using Ultrafin cement with 10% magnetite. All bricks were buried at the same depth. The bricks were buried centred around point (2.75 m, 2.75 m) at a depth of 15 cm below the ground surface.

The magnetic field that was measured before the grout sample was buried (corresponding to the magnetic field map of Figure 7a) was subtracted from the magnetic field that was measured after the burial of the grout sample (corresponding to the magnetic field map of Figure 7b). Figure 7c shows the resulting subtracted data, which may be described as the processed data once the earth's magnetic field has been removed. The presence of the buried grout sample beneath the ground is visible in Figure 7c in the centre of the survey area.

A number of field trials were carried out for samples at vertical distances between 0.5m and 1 m from the magnetometer. The configuration of the magnetometer and the samples is shown in Figure 8. An operator 60 holds a magnetometer 62 having two sensors, upper sensor 64 and lower sensor 66. In each trial, the magnetometer was held steady at 0.45m above the ground surface 68 and the sample 70 was buried at a range of depths. (Although samples 70 are shown in Figure 8 at different depths, this is to represent the various sample positions. In the field trials of Figure 7a, 7b and 7c, samples 70 were only buried at one specific depth.)

Figure 9 plots the magnetic field of the grouted objects 70 against their distance from the magnetometer (for samples buried at different depths as shown in Figure 8). In this case the magnetometer used is the lower magnetometer 66. Magnetic field data (shown as points) is plotted alongside a model of the decay of the magnetic field (shown as a solid line) Figure 9 demonstrates that the magnetic field appears to be inversely proportional to the cube of the distance between the grouted object 70 and the magnetometer 66. This is as expected based on theoretical predictions for the decay of a magnetic dipole point anomaly. This may demonstrate that the magnetic field due to the presence of a magnetic grout 16 may be used to predict the distance of the magnetic grout 16 from a magnetometer 66.

The main source of error in these walk-over field trials may be a lack of precise knowledge of where the sensor itself is, both laterally and vertically, since it is handheld by a person walking across the site. The embodiment described above with relation to Figures 3 and 4 may be expected to have smaller positioning errors, as the sensor will be within a monitoring borehole and GPS technology may be able to determine its 3D coordinates with mm-scale precision.

In certain embodiments described above, magnetic field data is processed to determine penetration into an injection region of injected magnetic grout. In alternative embodiments detection data other than magnetic field data may be obtained, for example one or more of magnetic susceptibility data, magnetisation data, or further data associated with any of magnetic field, magnetic susceptibility or magnetisation may be obtained, and that detection data may be processed to determine the penetration into the injection region of the injected magnetic grout.

It will be well understood by persons of ordinary skill in the art that embodiments may implement certain functionality by way of a computer program or computer programs having computer-readable instructions that are executable to perform the method of the embodiments. The computer program functionality could be implemented in hardware (for example by means of CPU). The embodiments may also be implemented by one or more ASICs (application specific integrated circuit) or by a mix of hardware or software. It may be understood that the present invention has been described above purely by way of example, and that modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claims

CLAIMS:

1. A method comprising detecting injected magnetic grout to provide an indication of penetration into an injection region of the injected magnetic grout.

2. A method according to Claim 1 , further comprising injecting the magnetic grout into the injection region.

3. A method according to Claim 2, wherein injecting the magnetic grout comprises injecting the magnetic grout into a borehole that extends into the injection region.

4. A method according to Claim 2 or 3 wherein the magnetic grout is injected into the injection region and then non-magnetic grout is injected into the injection region.

5. A method according to Claim 4 wherein at least one of a) and b):- a) the magnetic grout is injected into the injection region for a selected time before the non-magnetic grout is injected;

b) a selected volume of magnetic grout is injected into the injection region before the non-magnetic grout is injected.

6. A method according to any preceding claim wherein detecting the injected magnetic grout comprises detecting at least one magnetic property of the magnetic grout.

7. A method according to any preceding claim wherein detecting the injected magnetic grout comprises detecting a magnetic field associated with the magnetic grout.

8. A method according to Claim 7 wherein detecting the magnetic field associated with the magnetic grout comprises detecting a background magnetic field before injecting the magnetic grout into the injection region and detecting a further magnetic field during or after injecting the magnetic grout into the injection region, thereby to determine a change in magnetic field due to the injected magnetic grout.

9. A method according to Claim 7, the method further comprising applying a magnetic field to the injection region, wherein detecting the magnetic field associated with the magnetic grout comprises detecting a background magnetic field comprising the applied magnetic field before injecting the magnetic grout into the injection region and detecting a further magnetic field during or after injecting the magnetic grout into the injection region, thereby to determine a change in the background magnetic field due to the injected magnetic grout.

10. A method according to Claim 8 or 9, wherein providing an indication of penetration of the injected magnetic grout comprises using the change in magnetic field to determine an extent of the injected magnetic grout

1 1. A method according to any of Claims 7 to 10 wherein detecting the magnetic field associated with the magnetic grout comprises using a magnetometer to detect the magnetic field.

12. A method according to Claim 1 1 as dependent on Claim 3, wherein the magnetometer is positioned in a further borehole adjacent the borehole.

13. A method according to Claim 1 1 as dependent on Claim 3, wherein at least one of a), b) and c):- a) the magnetometer is positioned in the injection borehole;

b) the magnetometer is positioned at the ground surface;

c) the magnetometer is positioned in a subsurface tunnel or shaft.

14. A method according to any preceding claim, wherein detecting the injected magnetic grout comprises non-intrusively detecting the injected magnetic grout.

15. A method according to any preceding claim, wherein providing an indication of penetration of the injected magnetic grout comprises determining a shape representative of the extent of penetration of the injected magnetic grout.

16. A method according to Claim 15, wherein determining a shape representative of the extent of penetration of the injected magnetic grout comprises determining a shape representative of a leading edge of the injected magnetic grout.

17. A method according to any preceding claim, wherein providing an indication of penetration of the injected magnetic grout comprises forming an image of an extent of penetration of the injected magnetic grout into the injection region.

18. A method according to any preceding claim, further comprising determining a location for a further injection borehole in dependence on the indication of penetration of the injected magnetic grout.

19. A method according to any preceding claim, wherein the injection region comprises a subsurface region.

20. A method according to any preceding claim, the method further comprising adding a magnetic additive to grout to form the magnetic grout.

21 . A method according to any preceding claim, wherein the magnetic grout comprises at least one of: a ferromagnetic material, a ferrimagnetic material, a superparamagnetic material.

22. A method according to any preceding claim, wherein the injection region comprises a construction site and the method further comprises injecting the magnetic grout into a borehole that extends into the construction site.

23. A system for detecting grout, the system comprising a detector for detecting injected magnetic grout to provide an indication of penetration into an injection region of the injected magnetic grout, wherein optionally the detector comprises a magnetic field detector.

24. A system according to Claim 23, the system further comprising an injector for injecting the magnetic grout into a borehole that extends into the injection region.

25. A system according to Claim 24, wherein the injector is configured to inject magnetic grout into the borehole and then inject non-magnetic grout into the borehole.

26. A system according to Claim 25, where at least one of a) and b):- a) the injector is configured to inject the magnetic grout into the borehole for a selected time before injecting the non-magnetic grout into the borehole;

b) the injector is configured to inject a selected volume of the magnetic grout into the borehole before injecting the non-magnetic grout into the borehole.

27. A system according to any of Claims 23 to 26 wherein detecting the magnetic grout comprises detecting at least one magnetic property of the magnetic grout.

28. A system according to any of Claims 23 to 27 wherein detecting the magnetic grout comprises detecting a magnetic field associated with the magnetic grout.

29. A system according to Claim 28 wherein the detecting of the magnetic grout comprises detecting a background field before injecting the magnetic grout into the injection region and measuring a further magnetic field during or after injecting the magnetic grout into the injection region, thereby to determine a change in magnetic field due to the injected magnetic grout.

30. A system according to Claim 28, wherein the system is configured to apply a magnetic field to the injection region, and wherein detecting the magnetic field associated with the magnetic grout comprises detecting a background magnetic field comprising the applied magnetic field before injecting the magnetic grout into the injection region and detecting a further magnetic field during or after injecting the magnetic grout into the injection region, thereby to determine a change in the background magnetic field due to the injected magnetic grout.

31 . A system according to Claim 29 or 30 wherein the providing of an indication of penetration of the injected magnetic grout comprises using the change in magnetic field to determine an extent of the injected magnetic grout.

32. A system according to any of Claims 23 to 31 wherein the detector comprises a magnetometer.

33. A system according to any of Claims 23 to 32 wherein the detector is configured to be positioned in use in a further borehole adjacent the borehole into which grout is being injected.

34. A system according to any of Claims 23 to 32, wherein at least one of a), b) and c):- a) the detector is configured to be positioned in an injection borehole;

b) the magnetic field detector is configured to be positioned at the ground surface; c) the magnetic field detector is configured to be positioned in a subsurface tunnel or shaft.

35. A system according to any of Claims 23 to 34 wherein detecting the injected magnetic grout comprises non-intrusively detecting the injected magnetic grout.

36. A system according to any of Claims 23 to 35 configured to use the detected magnetic signal from the injected magnetic grout to determine a shape representative of the extent of penetration of the injected magnetic grout.

37. A system according to Claim 36 wherein determining a shape representative of the extent of penetration of the injected magnetic grout comprises determining a shape representative of the leading edge of the injected magnetic grout.

38. A system according to any of Claims 23 to 37, wherein providing an indication of penetration of the injected magnetic grout comprises forming an image of an extent of penetration of the injected magnetic grout into the injection region.

39. A system according to any of Claims 23 to 38, wherein the system is further configured to determine a location for a further injection borehole in dependence on the indication of penetration of the injected magnetic grout.

40. A system according to any of Claims 23 to 39, wherein the injection region comprises a subsurface region.

41 . A system according to any of Claims 23 to 40, wherein the magnetic grout comprises at least one of: a ferromagnetic material, a ferrimagnetic material, a superparamagnetic material.

42. A system according to any of Claims 23 to 41 , wherein the injection region comprises a construction site and the system is configured to inject the magnetic grout into a borehole that extends into the construction site.

43. A computer program product comprising computer-readable instructions executable to process detection data obtained by detection of injected magnetic grout to determine an indication of penetration into an injection region of the injected magnetic grout.

44. A computer program product according to Claim 43, wherein the detection data comprises magnetic field data, and the computer readable instructions are executable to determine a shape representative of the extent of penetration of the injected magnetic grout by processing the magnetic field data and further magnetic field data to determine a change in magnetic field due to the injected magnetic grout, wherein the magnetic field data is representative of a magnetic field before injection of the magnetic grout, and the further magnetic field data is representative of a magnetic field during or after the injection of the magnetic grout.

45. A method substantially as described herein with reference to the accompanying drawings.

46. A system substantially as described herein with reference to the accompanying drawings.

47. A system substantially as described herein with reference to the accompanying drawings.