DIRECT CALCULATION OF THICKNESSES FOR HIGH-VELOCITY AND UNDERLYING LOW-VELOCITY LAYERS USING POSTCRITICAL REFLECTION TIMES IN A SEISMIC-REFRACTION EXPERIMENT

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 can not be solved using traveltimes of first arrivals alone. Use of post-c ritical reflections (observed strongly after first arrivals on a seism ogram) from the bottom of the LVL provides valuable information regard ing the solution to the LVL problem. Here, we propose a layer-strippin g method applied to the strongly observable post-critical reflection t imes from the bottom of the LVL to calculate the thickness of the over lying high-velocity layer (HVL) and that of the underlying LVL directl y. 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 trave ltimes of both first arrivals and wide-angle reflections from the bott om of the LvL, We cannot calculate three parameters (i.e. the thicknes s of HVL and, the thickness and velocity of LVL) unequivocally.