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AssigneeSchlumberger Technology Corporation
InventorsPhilip Cheung, Andrew Hayman, Dennis Pittman
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The present invention relates to apparatus for a tool for investigating the wall of a borehole in a geological formation, the apparatus comprising: a non-conductive pad having an inside face and an outside face for pressing against the wall; a set of measurement electrodes mounted on the outside face of the pad and means for measuring the potential difference between two measurement electrodes; and both a source electrode adapted to inject current into the geological formation and a return electrode, the set of measurement electrodes being situated between the source electrode and the return electrode for the current; the apparatus being characterized in that the pad comprises an electrically conductive portion covering the entire measurement electrode zone and electrically insulated from the measurement electrodes.
1/ Apparatus for a tool for investigating the wall of a borehole in a geological formation, the apparatus comprising:
2/ Apparatus according to claim 1, characterized in that the current source electrode and the current return electrode are situated on the pad.
3/ Apparatus according to claim 2, characterized in that said electrically conducive portion extends over the entire length of the pad between the current source electrode and the current return electrode.
4/ Apparatus according to claim 2, characterized in that said conductive portion also covers the side edges of the pad.
5/ Apparatus according to claim 2 or claim 3, characterized in that the pad is compartmented, the pad comprising a first end section carrying the source electrode, a middle section for the measurement electrodes, and a second end section for the return electrode, said sections being at least partially separated from one another by electrically conductive partitions.
6/ Apparatus according to any preceding claim, characterized in that said conductive portion is constituted by the metal support of the pad.
7/ Apparatus according to any preceding claim, in which the pad further includes an electronic circuit for processing signals coming from the measurement electrodes, the apparatus being characterized in that said conductive portion forms an integral portion of the shielding of the electronic circuit.
8/ Apparatus according to any one of claims 1 to 7, characterized in that grounding is obtained by electrically connecting said conductive proton of the pad to the body of the investigation tool.
9/ Apparatus according to any one of claims 1 to 7, characterized in that the grounding is obtained by maintaining the conductive portion of the pad at a potential equal to the mean of the potentials measured by the measurement electrodes.
10/ Apparatus according to any one of claims 1 to 7, characterized in that the grounding is obtained by maintaining said conductive portion of the pad at a potential equal to the potential measured in the geological formation between the source electrode and the return electrode.