83 Seismic velocities and densities of rocks

Abstract

This chapter summarizes velocities in many lithologies believed to be important constituents of the lithosphere, experimental aspects of velocity measurements, and velocity-density relationships. The velocities presented in this chapter are from a database obtained over a time span of three decades from the rock physics laboratory currently in operation at the University of Wisconsin–Madison. Early measurements of elastic wave velocities in rocks were based on the resonant frequencies of vibration of cylindrical bars. These studies were particularly successful in measuring shear velocities. By the early 1950s, advances in electronics made it practical to determine velocities by observing the travel times of pulses in cylindrical rock specimens, usually at the ultrasonic frequency. This method of measurement is well suited for studies at high pressures and is presently the standard technique used to obtain both compressional and shear-wave velocities in rocks. The relationship between seismic velocity and density has important implications for multidisciplinary geophysical studies involving seismic and gravity methods. A detailed examination of the density and seismic velocity (at elevated pressures) of rocks of different lithologies can help in the determination of velocity from gravity data or vice versa. This relationship can also be used to determine lithology from either gravity or seismic data. Finally, velocity–density relationships provide information about the acoustic impedances of different rock types, which are important in reflection seismology exploration.