Note that the reflection is displaced laterally from the true reflector position (the line connecting apexes of the diffraction curves). For a constant-velocity subsurface, the many weak diffractions from very closely spaced points along the reflector (of which five are shown in the figure) give rise, through constructive and destructive interference, to a net reflection along the straight-line envelope of the diffraction curves. While the earth's subsurface is more complicated than that shown in Figure 1, the seismic data that would be obtained over the real earth can for all purposes be represented as a superposition of many diffraction curves generated by each of many boulder-like anomalies in the subsurface.įigure 2 shows another depth section and associated seismic section for a subsurface consisting of a single dipping reflector. The reverse process, by which the boulder gives rise to the observed diffraction pattern, is called modeling. The task of migration here is to convert or map reflections along the diffraction into a single point at the position of the boulder. Reflections occur along a hyperbolic diffraction pattern with the apex at the same CMP location as that of the boulder. The bottom part of Figure 1 shows schematically the seismic section that would be obtained for this survey. Also, the reflection time clearly increases as the source-receiver pair is moved farther from the point directly above the boulder. Clearly, reflections from the boulder will be observed at all the surface locations, not just the one directly above it. Also shown are the straight ray paths traveled by seismic waves from each of five different source positions down to the boulder and back up to receivers located at the sources. The upper part of the figure depicts a zero-offset survey conducted over a subsurface medium that is homogeneous (constant P-wave velocity) with the exception of an isolated boulder at some depth. The migration problem is illustrated in Figure 1. Rather than simply stretching the vertical axes of seismic sections from a time scale to a depth scale, migration aims to put features in their proper positions in space, laterally as well as vertically.Īll the issues in seismic migration reviewed here are treated in the collection of reprints found in Gardner. In its simplest form, then, seismic migration is the process that converts information as a function of recording time to features in subsurface depth. Until the migration step, seismic data are merely recorded traces of echoes, waves that have been reflected from anomalies in the subsurface. Of the many processes applied to seismic data, seismic migration is the one most directly associated with the notion of imaging. Deconvolution, for example, aims to sharpen reflections, and common midpoint (CMP) stacking exploits data redundancy to enhance signal-to-noise ratio while producing a seismic time section that simulates what would have been recorded in a zero- offset seismic survey, that is, one in which a single receiver, located at each seismic source position, records data generated by the source at that position. Virtually all seismic data processing is aimed at imaging the earth's subsurface, that is, obtaining a picture of subsurface structure from the seismic waves recorded at the earth's surface.
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