An indirect seismic method for determining the thickness of a low-velocity layer underlying a high-velocity layer

We present an indirect method for determining the thickness of a low-velocity layer (LVL) underlying a high-velocity layer (HVL) in seismic prospecting. Comparison of the average velocity-depth function determined from the first arrival refraction data with that obtained from reflection data in the same region, especially below the LVL, makes it possible to recognize the presence of the LVL and to estimate its probable thickness. The applicability of the method has been demonstrated in a field case where the presence of an LVL is indicated by geologic evidence. It has been shown that thickness estimates of an LVL and an HVL can be made reliably in situations where the velocity in the LVL can be accurately estimated from nearby exposures or in a drilled well. For the field case analyzed, a thickness of 0.75 km was estimated for an LVL (probably Mesozoic sediments) underlying a 0.25 km thick HVL (probably basalt). The velocity of propagation in the LVL was taken from seismic data on nearby exposed Mesozoics as 4.0 km/sec, and the velocity of the HVL is 5.4 km/sec, based on the refraction data.In areas where the velocity in the LVL cannot be inferred accurately, an upper limit of this velocity can be obtained which permits estimation of the maximum possible thickness of the LVL. In the field example presented, we show that the velocity in the LVL cannot exceed 4.17 km/sec.