The use of anomaly offset in solving hidden layer refraction problems

Intermediate layers are frequently hidden in seismic refraction surveying, in the sense that their presence cannot be directly detected from a travel time graph of first arrivals. Consequently, large errors in seismic interpretation may occur. Not only are the layers missed, but the calculated depths to deeper refractors may be overestimated (velocity inversion problem) or underestimated (masked layer problem).

A rigorous solution to the problem of hidden layers is presented here based on anomaly offset. The procedure preserves consistency between observed and calculated offsets, thus yielding corrected depths and limits on hidden layer velocity values. The method is illustrated by means of a field example involving a buried river channel.