Calibration of density logs at the Nevada Test Site

During the evaluation of a proposed site for an underground nuclear test at the Department of Energy`s Nevada Test Site (NTS) it was important to measure the bulk density of the geological formation in which the test was to be executed. The most satisfactory method for this measurement is the density log. This is a measurement system in which a gamma-ray source and detector are mounted in an instrument (a sonde) which is lowered into a borehole by a multi-conductor cable, and the number of gamma rays detected is used to infer the bulk density. At NTS many of the boreholes of interest are air-filled, with diameters as large as 3.65 m, drilled in partially-saturated media. Consequently the commercial equipment and calibration procedures are not suitable for NTS conditions. The count rate from a density sonde is strongly affected by any gap between the sonde and the borehole wall, especially if the gap is filled with air rather than water. Modern commercial density logs employ two detectors, and use one of the detectors to compensate for borehole conditions such as mudcake and gap. The commercial compensation algorithms assume that the sonde is parallel to the borehole wall; a reasonable assumption for oilfield conditions but not for NTS conditions. Moreover, in an air-filled borehole, borehole diameter affects both the calibration and the compensation. Therefore we could not use commercial algorithms to deal with NTS conditions. For many years, no appropriate two-detector algorithm was available for the NTS situation. Consequently we developed special algorithms for a single-detector sonde, which was equipped to measure the gap directly. Recently a two-detector algorithm was developed, which appears to be satisfactory if the sonde is calibrated in the appropriate-diameter borehole. In this report we describe the calibration procedures and the algorithms for both methods