| 摘要 |
An apparatus and method is disclosed for induction logging of electrical properties of earth formations that operates at low frequencies while remaining resistant to skin effect and maintaining mutual balancing. The apparatus includes a plurality of transmitter coils that are at a plurality of distances from a measure point located at an end of the apparatus, and a receiver coil array coupled to receive induced voltages resulting from currents induced in the earth formations by one or more transmitters of the plurality of transmitter coils, wherein the common measure point is located within the receiver coil array, and wherein one or more of the plurality of distances from the measure point are determined according to a function of o ne or more frequencies associated with one or more transmitters of the pluralit y of transmitters. According to an embodiment, the function includes determini ng the distance that is inversely proportional to a square root of the frequenc y associated with the one or transmitter. In another embodiment, the distances include *N, /2 *N, /4 *N, 2N, N, N/2 and N/4, wherein N is a fixed distance such as one meter. An embodiment of the method includes subtracting signals received from a borehole compensation array disposed at the end of the tool from signals received atthe receiver coil array, the subtracting reducing th e influence of local changes in borehole diameter.
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