Well logging method and apparatus using passive azimuthal electrodes.

摘要

The invention relates to a logging method and apparatus using passive azimuthal electrodes. The apparatus comprises an array of azimuthal electrodes (M'azi) that are circumferentially spaced apart from one another on a body together with two annular guard electrodes (A2) disposed on either side of the array of azimuthal electrodes (M'azi). The guard electrodes (A2) emit currents. Two annular monitor electrodes (M3, M4) are associated with the annular guard electrodes (A2). The potential difference ( DELTA Vazi) between the interconnected monitor electrodes (M3, M4) and each of the azimuthal electrodes (M'azi) are detected. In response to the detected potential differences, output signals (R'azi) representative of the resistivity of the formations in a plurality of directions around the axis of the borehole are generated. The apparatus of the invention also relates to apparatus having two alternative modes of operation, one corresponding to a passive azimuthal electrode technique and the other corresponding to a technique in which currents (Iazi) are emitted by the azimuthal electrodes.