Electric logs are being used on Pahute Mesa at the Nevada Test Site for preliminary identification of volcanic rock units penetrated by drill holes. Induced and produced water in the drill holes have essentially the same resistivity, so the spontaneous potential curve is negated; and the resistivity curves measure, in effect, the pore space of the rock Degree of welding or amount and kind of alteration affect the pore space and, therefore, the resistivity. In general, resistivities are lowest in the bedded tuffs and increase proportional to welding in ash-flow tuffs, and are greatest in rhyolitic lava flows.
Caliper logs can be used to locate lithologic zones which will tend to slough or cave. These zones are the poorly indurated ash-fall tuffs, vitric and highly fractured ash-flow tuffs and rhyolites. Zeolitized ash-flow and ash-fall tuffs and unfractured rhyolites are competent.