Physical properties of ash flow tuff from Yucca Mountain, Nevada

Abstract

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.

Deep geoelectric structure for mineral resource assessment in the Payette National Forest, Idaho

“…Also, resistivities can be lowered by the presence of highly conductive clay minerals, graphitic carbon, and metallic mineralization. It is also common in volcanic rocks and in fault zones to have conductive authigenic minerals and gouge that have resistivities one or two orders of magnitude lower than those of the surrounding rocks (Nelson and Anderson, 1992). Increased temperatures cause higher ionic mobility and mineral activation energy, reducing rock resistivities significantly.…”

Magnetotelluric survey to characterize the Sunnyside porphyry copper system in the Patagonia Mountains, Arizona

“…Resistivity can also be lowered by the presence of electrically conductive clay minerals, graphitic carbon, and metallic mineralization. It is common, for example, for altered volcanic rocks to contain replacement minerals that have resistivities ten times lower than those of the surrounding rocks (Nelson and Anderson, 1992). Fine grained sediments, such as clay rich alluvium, marine shales, and other mudstones are normally conductive, with resistivities ranging from a few Ωm to tens of Ωm (Keller, 1987;Palacky, 1987).…”

Computation of porosity and water content from geophysical logs, Yucca Mountain, Nevada

“…Where neither core measurements nor X-ray diffraction data are available, pg is assigned to discrete depth intervals by assigning a rock type to that interval. The association between rock type and characteristic density was made by Nelson and Anderson, (1992): glass, 2.349 g/cm3 ; nonwelded tuff, 2.587; vitrophyre, 2.381; welded tuff, 2.540; deep zeolitization, 2.527; shallow zeolitization, 2.371. One of these six rock types is assigned to a depth interval by inspecting the borehole logs and geological description.…”

“…Thus the second phase of work, understanding log response, was done by examining the core measurements in the context of geological description and mineralogical analyses. Nelson and Anderson (1992) and Nelson (1993a) show that porosity, a first-order control on log response, varies over a wide range in the tuffs, from as low as 1% in densely welded tuff to as high as 53% in zeolitized non-welded tuff. Almost as important as porosity in controlling log response is the alteration mineralogy.…”