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METHOD FOR CORRECTING IMPULSE RESPONSE DIFFERENCES OF HYDROPHONES AND GEOPHONES AS WELL AS GEOPHONE COUPLING TO THE WATER-BOTTOM IN 5 DUAL-SENSOR, BOTTOM-CABLE SEISMIC OPERATIONS
This application is a continuation of application Ser. No. 667,751, filed Mar. 11, 1991, now abandoned. 10
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to marine seismic exploration and, more particularly, to a system for mini- 15 mizing impulse response and coupling differences between hydrophones and geophones used in marine seismic exploration.
2. Description of the Related Art
In marine seismic exploration, a seismic survey ship is 20 equipped with an energy source and a receiver for taking seismic profiles of an underwater land configuration. The act of taking profiles is often referred to as "shooting" due to the fact that explosive devices have been commonly used for many years as energy sources. 25 The energy source is designed to produce compressional waves that propagate through the water and into the underwater land formation. As the compressional waves propagate through the land formation, they strike interfaces between formations, commonly re- 30 ferred to as strata, and reflect back through the earth and water to the receiver. The receiver typically converts the received waves into electrical signals which are then processed into an image that provides information about the structure of the subterranean formation. 35
Presently, one of the most common energy sources is an air gun that discharges air under very high pressure into the water. The discharged air forms a pulse which contains frequencies within the seismic range. Another energy source which is frequently used is a marine vi- 40 brator. Marine vibrators typically include a pneumatic or hydraulic actuator that causes an acoustic piston to vibrate at a range of selected frequencies. The vibrations of the acoustic vibrator produce pressure differentials in the water which generate seismic pulses free 45 from spurious bubbles.
Just as different energy sources may be used to generate seismic waves in marine applications, different receivers may be used to detect reflected seismic waves. Typically, the receivers most commonly used in marine 50 applications are referred to as hydrophones. Hydrophones convert pressure waves into electrical signals that are used for analog or digital processing. The most common type of hydrophone includes a piezoelectric element which converts physical signals, such as pres- 55 sure, into electrical signals. Hydrophones are usually mounted on a long streamer which is towed behind the survey ship at depth of about 30 feet.
Alternatively, marine seismic techniques may use different types of receivers which detect different char- 60 acteristics of the environment. For instance, in bottomcable seismic recording, a combination of pressure sensitive transducers, such as hydrophones, and particle velocity transducers, such as geophones, can be deployed on the marine bottom. While geophones are 65 typically used in land operations where metal spikes anchor the geophones to the ground to ensure fidelity of geophone motion to ground motion, geophones cannot
be economically anchored in marine applications. Therefore, cylindrical, gimbal geophones are attached to the bottom-cable. After the cable is deployed from the seismic survey ship, the geophones simply lie in contact with the marine bottom where they fall. The gimbal mechanism inside the cylinder assures that the geophone element mounted therein is oriented vertically for proper operation.
As is obvious from the above discussion, a variety of seismic equipment and techniques may be used in an attempt to accurately plot the underwater land formation. Regardless of which technique or combination of equipment is used, each offers certain advantages and disadvantages when compared to one another. For instance, gathering seismic data with a towed streamer in areas populated with numerous obstacles, such as drilling and production platforms, can be difficult or even impossible because the streamer may strike one of the obstacles and tear loose from the towing vessel. Such an event represents an extremely costly loss.
By contrast, in bottom-cable seismic operations, no such difficulty exists because the cable is deployed in a fixed position on the water bottom. However, in the above-mentioned towing technique, the hydrophone being towed behind the survey ship is perfectly coupled to the water in which it is immersed. Unfortunately, in a bottom-cable operation, since there is no practical way to effectively couple a geophone to the marine bottom as it can be coupled to dry land, the geophone is not perfectly coupled to its environment. Therefore, the signals received from the imperfectly coupled geophone do not accurately reflect the quantity being measured.
The present invention is directed to overcoming, or at least minimizing, one or more of the problems set forth above.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided an improved marine seismic exploration method. First, a seismic wave is generated in the marine environment to be explored. The water pressure and particle velocity are detected, preferably using a hydrophone and geophone respectively. A first signal correlative to the detected pressure and a second signal correlative to the detected velocity are generated. Then, a filter transfer function is calculated using the first and second signals, so that the filter transfer function substantially equalizes response characteristics between the first and second signals.
Preferably, the first and second signals are transformed into first and second frequency domain signals, respectively. Then, one of the frequency domain signals is divided by the other to produce the filter transfer function. If the filter transfer function is intended to compensate the second signal from the geophone for the response difference due to the imperfect coupling of the geophone, the first frequency domain signal is divided by the second frequency domain signal to obtain the filter transfer function. After the filter transfer function has been transformed into a time domain filtering function, subsequent second signals are filtered by the time domain filtering function.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:
FIG. 1 illustrates apparatus used in a bottom-cable operation;
FIG. 2 illustrates a hydrophone/geophone pair lying 5 on the marine bottom;
FIGS. 3A and 3B illustrate the impulse response characteristics of the hydrophone and geophone in the frequency domain assuming that both the hydrophone and geophone are perfectly coupled to their environ- 10 ment;
FIG. 4 diagrammatically illustrates a filter that equates the impulse response of a perfectly coupled geophone to the impulse response of a perfectly coupled hydrophone; 15
FIGS. 5A and 5B illustrate the amplitude and phase spectrums of the filter shown in FIG. 4;
FIG. 6 schematically illustrates a mechanical system that models the coupling between the water bottom and the geophone; 20
FIGS. 7A and 7B illustrate the effect of imperfect geophone coupling on the geophone's response to a seismic impulse;
FIG. 8 illustrates a downwardly propagating pressure wave impinging on the hydrophone/geophone 25 pairs on the marine bottom;
FIG. 9 diagrammatically illustrates the response of a geophone and hydrophone to the pressure wave of FIG. 8;
FIG. 10 diagrammatically illustrates a filter that 30 equates the response of an imperfectly coupled geophone to the response of a perfectly coupled hydrophone;
FIGS. 11A and 11B illustrate the amplitude and phase spectrums of the filter shown in FIG. 10; and 35
FIG. 12 is a flow diagram of a preferred method for designing the filter shown in FIG. 10.
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings 40 and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives following within the spirit and scope of 45 the invention as defined by the appended claims.
DESCRIPTION OF THE PREFERRED
Turning now to the drawings and referring initially 50 to FIG. I, a preferred marine seismic survey system is illustrated and generally designated by a reference numeral 10. The system 10 includes a seismic survey ship 12 that is adapted for towing a seismic energy source 14 through a body of water 17. The seismic energy source 55 14 is an acoustic energy source or an array of such sources. An acoustic energy source preferred for use with the system 10 is a compressed air gun, called a "sleeve gun", which is commercially available from Halliburton Geophysical Services, Inc. of Houston, 60 Tex. The source 14 is constructed and operated in a manner conventional in the art.
The system 10 also includes a receiving ship 15 that is preferably anchored in the body of water 17. The receiving ship 15 deploys a cable 16 on the marine bottom 65 20, and receives signals from the cable 16 as will be subsequently explained in detail. One preferred cable is commercially available from Tescorp Seismic Products
Co. of Houston, Tex., but those skilled in the art recognize that any one of a wide variety of cables can be used. The cable 16 carries at least one receiver 18, but preferably includes a plurality of such units.
The receiver 18 includes a hydrophone for detecting water pressure and a geophone for detecting water-bottom particle velocity. More particularly, the hydrophones and geophones on the cable 16 are arranged in identical spatial arrays when deployed on the marine bottom 20. Each individual hydrophone has a gimballed geophone positioned next to it. A separate electrical signal is sent to a recording system on the ship 15 for each hydrophone and each geophone spatial array. The survey ship 12 fires the source 14 at predetermined locations while the signals from the hydrophone and geophone arrays are recorded. These signals are typically referred to as reflection data. The data is recorded by a multi-channel seismic recording system that selectively amplifies, conditions and records time-varying electrical signals onto magnetic tape. Advantageously, the system also digitizes the received signals, using a 14 bit analog-to-digital converter for instance, to facilitate signal analysis. Preferably, the ship 15 utilizes a seismic recording system which is commercially available from Halliburton Geophysical Services, Inc. However, those skilled in the art will recognize that any one of a variety of seismic recording systems can be used.
According to a preferred practice, the cable 16 and hydrophone/geophone pair 18 are positioned on the marine bottom 20 for use in three-dimensional, "bottom-cable" operations. Normal production shooting takes place with the survey ship 12 moving at a constant speed along a set of parallel lines, or swath, with respect to the cable 16. After the survey ship 12 completes the swath, the receiving ship 15 or other suitable ship retrieves the cable 16 and re-deploys the cable 16 in a line spaced from, but parallel to, the previous cable location. Once the cable 16 is re-deployed, the survey ship 12 shoots another swath.
During data collection, seismic waves generated by the source 14 travel downwardly, as indicated by the rays 22. These primary waves are reflected off of interfaces between strata, such as the interface 28 between strata 24 and 26, in the subterranean earth formation 32. The reflected waves travel upwardly, as illustrated by the rays 30. The hydrophone/geophone pairs that make up each receiver 18 detect the reflected waves. The receivers 18 generate electrical signals representative of pressure and particle velocity changes inherent to the wave field, and transmit these generated electrical signals back to the survey ship 15 via the cable 16. The recording equipment within the ship 15 records these electrical signals so that they can be subsequently processed to map the subterranean earth formation 32.
It should be understood that the receivers 18 not only detect the reflected waves of interest, but also the primary wave and reverberated waves. Reverberated waves are reflected waves which reflect off of the water-air interface at the surface of the water 17 and travel downwardly in the water 17 to impinge on the receivers 18. Reverberated waves are illustrated by the rays 33 in FIG. 1. The effects of reverberated waves will be discussed subsequently in conjunction with Applicant's U.S. Pat. No. 4,979,150, which issued from Application Ser. No. 398,809, filed Aug. 25, 1989, and which is hereby incorporated by reference.
FIG. 2 illustrates a receiver 18 which includes a gimbal geophone 34 and a hydrophone 36. Preferably, the