Use of postcritical reflections in solving the hidden‐layer problem of seismic refraction work

Abstract

In a multilayered earth system, when the thickness of a layer compared to the overlying layer is small, refraction signal from that layer may not appear as a first arrival. In such a case, the analysis of first‐arrival refraction data cannot detect the layer and this leads to errors—overestimation of the thickness of the overlying layer and underestimation of depths to all underlying layers. This is known as the hidden‐layer problem. In a field situation, hidden layer(s) can be identified with the help of high‐energy postcritical reflections, which appear as strong later arrivals. In this paper, we describe an approach to calculate the thickness of the overlying layer and the thickness and velocity of the hidden layer based on the traveltime inversion of postcritical reflections from the top and bottom of the hidden layer. The blind‐zone thickness is also calculated using the estimated velocity of the hidden layer and the thickness of the overlying layer. The applicability of the method is illustrated with the help of both synthetic and field data.