Abstract: Apparatus for monitoring an underground formation through which a borehole passes, comprises a conductive casing (6) located in the borehole (1) passing through the formation (4); a plurality of sensors (12-1) for measuring an electrical property of the formation (4) located in the borehole (1) outside the casing (6) and electrically insulated therefrom; and an electrical current source (11) positioned in the borehole (1) passing through the formation (4) and arranged to inject a current into the formation (4); the casing (6) having an electrically insulating coating (14) over the surface thereof, at least in the region of the sensors (12-1), and an electrically conductive portion (11) in electrical contact with the formation (4); and a current supply (10) being connected to the casing (6) such that current flows into the formation (4) from the electrically conductive portion (11) which acts as the current source, the sensors (12-1) measuring the electrical property of the formation (4) resulting from such cu

Abstract: A well (10) for producing water is characterized in that it includes a network of widely spaced electrodes (8) electrically coupled with measurement means (3) or with electric current generation means (1). By utilizing a measurement result, the progression of a salt water front heading for the well (10) may be detected.

Abstract: A well (10) for producing water is characterized in that it includes a network of widely spaced electrodes (8) electrically coupled with measurement means (3) or with electric current generation means (1). By utilizing a measurement result, the progression of a salt water front heading for the well (10) may be detected.

Abstract: Apparatus for monitoring an underground formation through which a borehole passes, comprises a conductive casing (6) located in the borehole (1) passing through the formation (4); a plurality of sensors (12-1) for measuring an electrical property of the formation (4) located in the borehole (1) outside the casing (6) and electrically insulated therefrom; and an electrical current source (11) positioned in the borehole (1) passing through the formation (4) and arranged to inject a current into the formation (4); the casing (6) having an electrically insulating coating (14) over the surface thereof, at least in the region of the sensors (12-1), and an electrically conductive portion (11) in electrical contact with the formation (4); and a current supply (10) being connected to the casing (6) such that current flows into the formation (4) from the electrically conductive portion (11) which acts as the current source, the sensors (12-1) measuring the electrical property of the formation (4) resulting from such cu

Abstract: In accordance with an illustrative embodiment of the present invention, a well logging apparatus uses a narrow wall-engaging pad equipped with an array of long vertical electrodes. The width of the pad is between 2 and 4 inches, and the length-to-width ratio of the array is between 3 and 5. Circuits are connected to the electrodes for propagating a focused current primarily through the zone of the adjacent formation likely to be flushed by the borehole fluid. This current and the associated potential difference are measured to produce a signal representative of the resistivity of the flushed zone.