| 摘要 |
Measurement equipment is introduced into the borehole penetrating the geological formation. It carries an assembly of electrodes contacting the borehole wall. A controlled current is imposed between a first pair of electrodes with vertical spacing. A corresponding potential difference is measured using another pair of electrodes between the first pair. Measurement is repeated for an otherwise similar configuration, in which the electrodes are disposed horizontally. The vertical potential difference is multiplied by a first calibration coefficient, to determine the apparent vertical resistivity. This is repeated using a second calibration coefficient, to determine the apparent horizontal resistivity. The ratio of the measurements is calculated, being a measure of resistance anisotropy in the formation. Further variants of the method are described, based on the foregoing principles. Horizontal- and vertical electrode pairs are at right angles to each other and define x and y axes. Biaxial anisotropy is determined by calculation, from the results of measurement. Multiple electrodes are used, combining measurements; in particular, third pairs of electrodes are employed to measure along a third orthogonal axis, i.e. a z axis, combining results vectorially. The alternating current frequency applied between the vertical electrodes, differs from that applied between the horizontal electrodes. An Independent claim is included for the corresponding system and equipment.
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