Application of geophysical logs to estimate moisture-content profiles in unsaturated tuff, Yucca Mountain, Nevada

Determination of the moisture content in boreholes, drilled for the purpose of conducting hydrological studies in the unsaturated zone, is critical in the evaluation of the natural state of the unsaturated zone. Geophysical logs combined with geologic and hydrologic examination of the borehole cuttings provide a means to estimate moisture-content profiles. Interpretations of geophysical well logs for unsaturated, welded, and fractured tuff have not been attempted previously. This paper compares the results of analyses of various geophysical logs that were obtained from two large diameter, air-drilled (vacuum reverse circulation) boreholes at Yucca Mountain, Nevada, that were drilled as part of the Nevada Nuclear Waste Storage Investigations Project of the US Department of Energy. Caliper, gamma-ray, temperature, induction, density, epithermal-neutron, and dielectric logs were run in these boreholes. Moisture-content data from the drill cuttings were compared with moisture-content data derived from logs. Saturation profiles were obtained from different logs and were correlated with each other. Qualitative correlation of the degree of welding with bulk density also was conducted; overall correlations were satisfactory. Borehole geophysical logs proved reliable in determining moisture-content profiles. 6 refs., 9 figs