Coil array for a high resolution induction logging tool and method of logging an earth formation

摘要

An induction tool including a coil array having a plurality of transmitter coils and a plurality of receiver coils. Each plurality of coils is symmetrically arranged on an insulated mandrel about a selected point. The induction tool also includes an oscillator circuit that provides an alternating frequency signal to the transmitter coils. A receiver circuit is connected to the receiver coils to measure signals indicative of the voltages induced on these receiver coils to the surface. By varying the spacing between the receiver coils and transmitter coils it is possible to independently vary the vertical and radial measurement capabilities of the tool of the present invention. A dual induction tool having high vertical resolution is also disclosed. The dual induction tool includes a pair of transmitter/receiver coil arrays, each transmitter/receiver coil array having a different transmitter-to- receiver coil spacing and substantially indentical vertical resolution. A single receiver coil is located at a selected point on an insulated sonde and a pair of variable symmetrically disposed bucking coils are located on either side of the receiver coil. Two pairs of identical transmitter coils are similarly symmetrically disposed on either side of the receiver coil. In a preferred embodiment, each pair of transmitter coils is alternately energized and the bucking coils are selectively adjusted to compensate for the variation in transmitter-to-receiver coil spacing.