| 摘要 |
An apparatus and method is disclosed for induction logging of electrical properties of earth formations that operates at low frequencies while re- maining resistant to skin effect and maintaining mutual balancing. The apparatus includes a plurality of transmitter coils that are at a plurality of distanc es 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 t he com- mon 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 accord- ing to a function of one or more frequencies associated with one or more tra ns- mitters of the plurality of transmitters. According to an embodiment, the function includes determining the distance that is inversely proportional to a square root of the frequency 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 fix ed distance such as one meter. An embodiment of the method includes subtracting signals received from a borehole compensation array disposed at the end of t he tool from signals received at the receiver coil array, the subtracting reducing the influence of local changes in borehole diameter. <IMG>
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