The VSP Method for Refracted Waves

Abstract

The waves recorded in the early portion of the seismogram at offsets great as compared with the depth of the interface consist mainly of refracted (head*) and transcritically reflected waves. The study of the origin and the relative intensities of such waves is of great interest for both theory and practice, since this factor is of decisive importance for the exploration potential of one of the principal methods — that of the correlational method of refracted waves. The difficulties associated with such studies are due mainly to the ambiguity of interpretation of the wave’s origin arising from the similarity between the kinematic features of the head, the curved-path and the reflected waves recorded on the surface at great distances from the energy source, so that they cannot be used to identify the wave type reliably. Accordingly, it was presumed that the end could well be served by VSP. Down-hole observations at large offset distances from the shot-point have the following advantages over surface observations [14]:

• (1) The study of the wave field in the medium underlying the refracting interface provides the most authentic data about the mechanism of energy return and, hence, about the origin of the wave recorded above the refracting interface.

• (2) The recording of waves as first arrivals in the immediate proximity of the refracting interface and near the point of emergence enables the reliability of stratigraphic correlation to be improved.

• (3) The employment of such a sensitive parameter as the trajectory of particle motion of the medium for determining the wave’s origin.

• (4) The rapid decrease in the seismic background noise with depth enables a much higher useful instrumentation sensitivity to be utilized, so that waves of very low intensity can be studied.

• (5) A chance to compare the intensity of various waves observed in the immediate proximity of the interface, which is important, because the intensity ratio may change with the distance from the interface.