Borehole logging equipment

A sonde for logging a borehole has a low level (typically 3 milli-curie) radio-active source and at least two, e.g. three-detection devices. A single core cable transmits power to the sonde and also transmits back data from the detection devices to a remote unit which includes a recording device. The data is transmitted back from the detection devices using pulses of different voltage levels. Detection devices detect radiation from the source as reflected by strata, and detection device detects the natural radioactivity of the strata. (author)