Direct calculation of thicknesses for high-velocity and underlying low-velocity layers using post-critical reflection times in a seismic refraction experiment

Abstract

In a multi-layered earth system, when the velocity of a layer is lower than that of the overlying layer, the former cannot be recognized on the time-distance plot resulting in an overestimation of the thickness of the overlying layer and the depths of all subsequent deeper layers. This low-velocity layer (lvl problem in seismic refraction work cannot be solved using traveltimes of first arrivals alone. Use of post-critical reflections (observed strongly after first arrivals on a seismogram) from the bottom of the lvl provides valuable information regarding the solution to the lvl problem. Here, we propose a layer-stripping method applied to the strongly observable post-critical reflection times from the bottom of the lvl to calculate the thickness of the overlying high-velocity layer (hvl) and that of the underlying lvl directly. A-priori information for the velocity of the lvl from other seismic evidence is utilized. We show in this paper that even if we use traveltimes of both first arrivals and wideangle reflections from the bottom of the lvl, we cannot calculate three parameters (i.e. the thickness of hvl and, the thickness and velocity of (lvl) unequivocally.