Interpretation of Geophysical Well-Log Measurements in Drill Hole UE25a-1, Nevada Test Site, Radioactive Waste Program

An exploratory hole UE25a-1) was drilled at Nevada Test Site NTS) to determine the suitability of pyroclastic deposits as storage sites for radioactive waste. Studies have been conducted to investigate the stratigraphy, structure, mineralogy, petrology, and physical properties of the tuff units encountered in the drill hole. This report deals with the interpretation of physical properties for the tuff units from geophysical well-log measurements. The ash-flow and bedded tuff sequences at NTS comprise complex lithologies of variously welded tuffs with superimposed crystallization and altered zones. To characterize these units, resistivity, density, neutron, gamma-ray, induced polarization, and magnetic susceptibility geophysical well-log measurements were made. Although inherently subjective, a consistent interpretation of the well-log measurements was facilitated by a computer program designed to interpret well logs either individually or simultaneously. The broad features of the welded tuff units are readily distinguished by the geophysical well-log measurements. However, many details revealed by the logs indicate that more work is necessary to clarify the causal elements of welllog response in welded tuffs. Introduction To study the suitability of pyroclastic deposits as storage sites for radioactive waste, an exploratory hole was drilled in July 1978 (under the auspices of the U.S. Department of Energy) at the Nevada Test Site (NTS). Erupted from the Timber Mountain-Oasis Valley caldera complex in late Tertiary time (16-9 m.y.), up to 3000 m of rhyolitic tuffs at NTS mantle an eastern portion of the basin and range province. In recent years, the tuff units and