Borehole logging is the geophysical method most extensively used in the United States for exploration and evaluation of uranium deposits. Gamma-ray logs, commonly implemented with single-point resistance logs, currently supply about 80% of the basic data for ore reserve calculations and much of the the subsurface geologic information. The majority of producing deposits in the United States, hence the current exploration targets (circa 1967), have relatively small discontinuous substratified configurations and occur in sedimentary rock which are often poorly indurated. Ore bodies are found above, below and astride the upper surface of the groundwater (aka water table (as observed circa < 1967)). Lithologic units must be defined for correlation and ore zones for economic evaluation, which may range from a few inches to tens of feet in thickness.
Rotary seismic-shot-hole type rigs are most commonly employed for drilling. The drilling media may be air, water or natural mud. The holes have nominal diameters of about 4 1/2 inches, but may range from 2 to 9 inches. In most cases, the holes deviate from the vertical, are rather crooked, and often roughwalled because of caving, washouts, and swelling of clay seams. Because of these conditions the probes tend to rub along the borehole wall sides and have a short life before caving and sloughing caves in the hole below the water table.. Not infrequently downhole-probes become stuck and lodged below caved sediments and must be abandoned.
Multiple-purpose probes are desirable to save time in the hole, but excessively complex and costly probes may require too much time in the hole gathering data. The loss of these probes would be costly.
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