Electric logs are the major qualitative tool in determining lithologic contacts in volcanic rocks at the Nevada Test Site. In one drill hole, only the electric log, of several geophysical logs, indicates the presence of the contact between an ash-flow tuff and a rhyolite. Acoustic logging devices, however, are most useful in the quantitative determination of rock properties. A relationship exists between sonic velocity and rock porosity in these volcanic rocks. Acoustic logs using full-wave recording techniques indicate that shear and Young’s modulus values are related to rock velocity. Consequently, it may be possible to determine many of the engineering physical properties of certain rock types by using only a sonic log.