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Abstract

The invention is directed to induction logging apparatus and method for obtaining measurements dependent upon the vertical conductivity component of formations surrounding a borehole. The disclosed techniques can also be used to determine horizontal as well as vertical conductivity of the formations, and thereby achieve determination of formation anisotropy. A transmitter coil is provided with its axis substantially perpendicular to the borehole axis. A receiver coil is provided in spaced relation to the transmitter coil. The axis of the receiver coil is also substantially perpendicular to the borehole axis, and is preferably parallel to the axis of the transmitter coil. Means are provided for detecting signals induced in the receiver coil, these signals being a measure of the formation vertical conductivity. In a preferred embodiment of the invention, the detecting means is operative to detect both the resistive and reactive components of the signals induced in the receiver coils. Indications of formation vertical conductivity are obtained by combining the detected resistive and reactive components. In a further form of the invention, the formation horizontal conductivity can also be obtained from the detected resistive and reactive components. Alternatively, horizontal conductivity may be determined using a pair of spaced coils having a common axis aligned with the borehole axis.

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First Claim