A data transmission system for use in extracting data from a downhole location in a well is disclosed. The well has electrically conductive downhole structure (2,C) which together with an earth return forms a signal circuit which is used to transmit data. A current source (34) is provided at the wellhead...http://www.google.com/patents/US6469635?utm_source=gb-gplus-sharePatent US6469635 - Bore hole transmission system using impedance modulation
