Abstract: A sonde for induction logging of boreholes comprises a zero mutual inductance transmitter-receiver coil array comprising at least two coil groups that are time or frequency multiplexed. Each group comprises a coil array which is asymmetric and the coil array of one group are the mirror image of the coil array of the other group. The response of the sonde is obtained by combining the responses detected in several groups of receivers at different depths. Combination sondes, including those capable of performing both induction and sonic logging may be constructed.

Abstract: A technique for producing measurements of physical characteristics of subsurface media near a borehole penetrating the earth is disclosed that uses multiple transducers positioned along a borehole tool. A number of transducers of a first type, such as transmitters, are separated from each other along the tool by a preselected separation, and a number of transducers of a second type, such as receivers, are separated from each other by the same separation and are positioned on the tool at a preselected distance from the transducers of the first type. The technique contemplates the use of appropriate circuitry for producing measurements, using different combinations of transducers, of physical characteristics of the subsurface media at different positions along the borehole.Furthermore, different combinations of measurements produced at different borehole positions may be compared or combined to produce improved measurements that are compensated for borehole effects and other errors.

Abstract: Methods and apparatus are provided for logging a borehole having solid matter along at least a portion of the wall thereof. Gamma radiation is emitted from the borehole into the surrounding media, and the amount of radiation which returns to the borehole is measured by three detectors located at different distances from the source of radiation, so as to be primarily sensitive to radiation which has respectively penetrated to three different depths in the surrounding media. The thickness of the solid matter on the borehole wall is then determined from the three gamma radiation measurements.