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The Mud Volcano area in Yellowstone National Park provides an example of a vapor‐dominated geothermal system. A test well drilled to a depth of about 347 ft penetrated the vapor‐dominated reservoir at a depth of less than 300 ft. Subsequently, 16 vertical electrical soundings (VES) of the Schlumberger type were made along a 3.7‐mile traverse to evaluate the electrical resistivity distribution within this geothermal field. Interpretation of the VES curves by computer modeling indicates that the vapor‐dominated layer has a resistivity of about 75–130 ohm‐m and that its lateral extent is about 1 mile. It is characteristically overlain by a low‐resistivity layer of about 2–6.5 ohm‐m, and it is laterally confined by a layer of about 30 ohm‐m. This 30‐ohm‐m layer, which probably represents hot water circulating in low‐porosity rocks, also underlies most of the survey at an average depth of about 1000 ft. Horizontal resistivity profiles, measured with two electrode spacings of an AMN array, qualitatively corroborate the sounding interpretation. The profiling data delineate the southeast boundary of the geothermal field as a distinct transition from low to high apparent resistivities. The northwest boundary is less distinctly defined because of the presence of thick lake deposits of low resistivities. A broad positive self‐potential anomaly is observed over the geothermal field, and it is interpretable in terms of the circulation of the thermal waters. Induced‐polarization anomalies were obtained at the northwest boundary and near the southeast boundary of the vapor‐dominated field. These anomalies probably are caused by relatively high concentrations of pyrite.

We have measured the density and determined the porosity of 198 samples of ash flow tuffs from three boreholes at Yucca Mountain, Nevada. The electrical properties, velocity, and permeability of many of these samples have also been determined. We use mineralogical and physical data from other sources to determine the dependence of measured physical properties upon petrology. Porosity in the samples varies over a wide range, from as low as 1% in the densely welded tuffs to 53% in the zeolitized nonwelded tuffs. Porosity is the primary control on the other measured physical properties that consequently vary over broad ranges. Alteration (zeolites and clays) is a primary control on grain density and a significant secondary control on bulk density, resistivity, velocity, and permeability. Sorting the samples into rock classes defined in terms of the degree of welding and gross mineralogy enables us to separate the influence of zeolites and clays from that of porosity. Empirical rock property relationships established for sandstones can be applied with good success to tuffs. Archie's law relating resistivity to porosity is found to fit the unaltered samples with an exponent m of 2.0. An empirical expression relating compressional velocity to porosity and clay content forms an excellent upper bound to the velocity-porosity data. Permeability-porosity plots are similar in form to those obtained for clastic rocks, although the permeability is considerably lower than in clastic rocks of similar porosity. Zeolites and clays reduce the grain density, increase the electrical conductivity, reduce the compressional velocity, and reduce the permeability. tained in boreholes in Rainier Mesa on the northern extremity of NTS. A series of reports [Carroll and Magner, 1988; Carroll, 1989, 1990] describes the measurement difficulties, presents the borehole logs, and discusses the physical properties of volcanic tuffs at Rainier Mesa.

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Core samples from the Yucca Mountain UE25a-1 borehole were measured for bulk density, porosity, resistivity, induced polarization, compressional sonic velocity, hydraulic conductivity (water permeability), magnetic susceptibility, and remanent magnetization as part of a large-scale site evaluation program designed to identify suitable locations for the containment of radioactive waste products. The samples are representative of stratigraphic units of the Paintbrush Tuff, the tuffaceous beds of Calico Hills, and the Crater Flat Tuff. Resistivity and bulk density values of water-saturated samples closely approximate those at near in situ levels of water saturation and variations in resistivity, density, and sonic velocity are largely dependent upon porosity. The Tiva Canyon Member of the Paintbrush Tuff has an inversely polarized component of remanent magnetization and the entire Paintbrush has a magnetization appreciably greater than the underlying formations. There is no direct correlation between porosity and permeability; the latter varying widely and often decreasing with time as unconsolidated particles within the pore network are repositioned so as to impede the continued flow of water through the rock.

A self-potential survey made in the Long Valley caldera produced an anomaly derived from a dipolar source superimposed on potentials negative in polarity in relation to the area outside the caldera. The dipolar anomaly, consisting of negative and positive components differing amplitude by approximately I V, is centered over a resurgent dome in the west central part of the caldera. The exact nature of the potential source is unknown; however, electrofiltration processes caused by movement of heated groundwater that gives rise to streaming potentials are thought to be the principal cause of the dipolar anomaly. Diffusion potentials resulting from concentration differences between rising volcanic water and descending meteoric water may be responsible for the negative potentials measured within the caldera. Potential increases as high as l100 mV were observed across the perimeter of the caldera.

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This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.