Seismic prospecting model determn. - gives optimum underground seismic wave propagation velocity distribution model

摘要

Determn. of an opt. distribution model for underground seismic wave propagation velocities is carried out by (a) emitting seismic waves from a source placed successively at two or more shot points (14,14') on the ground surface; (b) recording, for each source position, the trace of the seismic waves reflected by the interface between subjacent formations by means of receivers (16) located at several sites on the ground surface; (c) devising an initial velocity distribution model from the recorded traces; (d) displaying the shot points (14,14') by a paraxial equation using the model to create a seismic image of the earth body interior; (e) modelling each shot point with the paraxial equation by downwardly propagating a wave resulting from a surface source; (f) re-modelling the seismic data of each slot point with the paraxial equation by extrapolation of a wave towards the surface, initiated by interaction of the source wave with the seismic image; (g) comparing the recorded seismic data (56) of each shot point with the re-modelled data (58) of each shot point and, if the data do not coincide, calculating the gradient of the `cost' function (mismatch) w.r.t. the initial velocity model; (h) modifying the velocity model with the gradient to reduce the difference between the observed and re-modelled seismic data; and (i) repeating these operations until the gradient is zero or sufficiently small to satisfy the requirements of inversion, which defines the optimal velocity model. ADVANTAGE - The `cost' function is not dependent on imaged reflections, so that the method is more sensitive to low frequency components of the velocity model, and corresponds better to seismic inversion.