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.

Abstract: In accordance with an illustrative embodiment of the present invention, a logging sonde includes a standing wave type microwave linear particle accelerator excited by a magnetron and producing bursts of high energy electrons which can selectively bombard either a neutron emitting target or a photon emitting target. Means are provided to sense the variations in the amplitude of the microwave field in the linear accelerator and to control the frequency of the magnetron so as to maintain the amplitude of the accelerating field at a reference value representative of the expected maximum amplitude value at resonance. Means are also provided to sense the variations in the temperature of the linear accelerator and to control the frequency of the magnetron so as to compensate for the variations in the resonant frequency of the accelerator that result from temperature induced changes the length thereof.