GB1338292A - Method and apparatus for studying the resistivities of formations around a borehole - Google Patents

Abstract

1338292 Geophysical surveying SCHLUMBERGER INLAND SERVICES Inc 19 April 1971 [6 March 1970] 9934/71 Addition to 1251810 Heading G1N In apparatus for measuring the electrical resistivity or conductivity of an earth formation traversed by a bore hole 1, as described in an embodiment of the parent Specification, the multiplier which multiplies the in-phase survey current transmitted into the earth and the in-phase potential difference detected, is replaced by a pair of logarithmic converters 25, 30 connected to an adding circuit 33 whose output is applied to a comparator 23 controlling the gain of the current generator producing the survey current, or a current related thereto. However, instead of maintaining the product of the current and potential constant, as in the parent Specification, a relationship of the type V<SP>a</SP>I<SP>b</SP> = k is maintained, e.g. by inserting resistors RI o and R v between the outputs of the logarithmic converters and an operational amplifier in the adding circuit, Fig. 2. Further, instead of the survey current being applied directly to the converter 25, the output of the comparator, which is related to the survey current, is applied to the input of the converter 25. In the type of apparatus in which the survey current is focussed by a focussing current, since the survey current and the focussing current have a known relationship, the current generator 20 may provide the focussing current instead of the survey current, and hence the survey current is then controlled indirectly to maintain the desired relationship with the potential difference detected.