It occasionally happens that there exists a part where reflection near the thrust is not clearly observed in a thrust zone seismic survey cross section. For the effective interpretation of such an occurrence, the use of geological structures as well as the reflected pattern is effective. When the velocity structures for a fold structure having a listric fault caused anticline (unidirectionally inclined with a backlimb, without a forelimb) and for a fault propagation fold are involved, a wrong interpretation may be made since they look alike in reflection wave pattern despite their difference in geological structure. In the concept of balanced cross section, a check is performed, when the stratum after deformation is recovered to the time of deposition, as to whether the geologic stratum area is conserved without excess or shortage. An excess or shortage occurs if there is an error in the model, and this shows that the fault surface or fold structure is not correctly reflected. Positive application of geological knowledge is required in the processing and interpreting of data from a seismic survey. 6 refs., 6 figs.