Analytical Studies of Body Wave Propagation and Attenuation

Abstract

Abstract : In situ crosshole and downhole seismic methods are becoming widely used as a means of nondestructively evaluating the elastic properties of geotechnical systems. The elastic constants are calculated from the records of body waves (longitudinal and transverse waves) traveling through the media. Measurements are made by generating a seismic disturbance at one point and measuring the time required for the disturbance to travel to one or more seismic receivers. Several simplifying assumptions are made in traditional analysis of seismic measurements for engineering purposes. These include assuming plane wave propagation, measurement of only far-field waves, and independence of the measurements on the source-receiver configuration and on the amount of material damping. An analytical study of the effects and the validity of the different assumptions is presented. It is found that, for the range of distances and frequencies typically used in engineering applications: body wave fronts generated by point source cannot be considered plane; and near-field effects associated with spherical wave fronts can be very important. The near-field effects are caused by coupling between waves which exhibit the same particle motion but which propagate at different velocities and attenuate at different rates. To minimize the detrimental effects of near-field waves in those methods based on spectral analysis techniques, it is recommended that, in the field setup, the ratio of distances from the source to the second and first receivers be of the order of two or greater.