A comparison of laboratory and seismologically determined elastic constants of rocks

Abstract

Measurements of the elastic constants of rocks were made in the laboratory by static methods using specimens of rock taken from the locality in which Leet and Ewing had measured the elastic constants by seismological methods. It was found that the statical elastic constants, Young's modulus, and Poisson's ratio were always less than the seismologically determined ones, and that the statical compressibility of the “covered” rock is always far too large. On the other hand, the statically obtained compressibility of the “uncovered” rock is in excellent agreement with the seismic value. These experimental results can be clearly explained qualitatively as due to the presence of cracks and cavities in the rocks between adjacent crystals. This explanation at the same time suggests that the cracks and cavities will cause the rock to behave as an absorbing and dispersing medium for the propagation of elastic waves. At great depths in the Earth the probable disappearance of pores in the rocks will eliminate this behavior of elastic waves.