Evaluation of average crustal characteristics from reverberation of seismic waves

Abstract

The seismic records obtained near the shot points during the study of the earth’s crust by the Department of Terrestrial Magnetism, Carnegie Institution of Washington, have been analyzed to investigate the possible origin of the reverberation of seismic waves. The hypothesis of layers of uniform velocities fails to explain the reverberation curves in different regions. As a first approximation, the average gradient of velocity with the depth can be determined and it agrees with the travel-time data provided one assumes the major contribution to be from the conversion of P waves into S waves at the surface irregularities. The average gradient in Virginia, U.S.A., is seen to be 0·075 km s -1 km -1 . When the travel-time curves are investigated on the basis of such a large velocity gradient, the penetration corresponding to rays causing intense arrivals usually associated with the critical reflexions from the Mohorovicic discontinuity comes out to be only 14 km in Virginia, U.S.A. The intense arrivals have to be ascribed now to the focusing effect of peculiar variation in the velocity at about this depth. For example, if sediments extend to a thickness just short of 14 km , the velocity might increase at the base of the sediments to a value greater than the velocity in the upper layers of the converted crustal rock during the formation of the continents. This will give rise to both a low velocity layer and focusing when the velocity again increases at depths somewhat greater than 14 km . On the basis of the ray theory and assuming uniform increase of velocity with the depth, expressions have been derived for the reverberation curves for various kinds of single scattering and the average gradient has been determined for various regions. A mathematical theory for the variation of gradient from a high value at the surface to a negligible value when velocities greater than 8·8 km/s or so are reached would be a logical next step.