Improvement Of Shallow Seismic Reflection Section Using Migration: Two Case Studies

Migration is seldom applied to shallow seismic reflection data because migration on microcomputer<br>is slow and final seismic sections are often only minimally improved. Black et.al. (1994) show that for<br>small values of velocity and traveltime, the horizontal and vertical displacements of a reflector point after<br>migration may not be large enough compared to the trace spacing and time sampling interval to make a<br>noticeable change on a migrated section. They do mention, however, that migration may be useful in<br>shallow seismic survey which requires high resolution.<br>Two cases showing significant improvement of a shallow seismic section after migration, due to<br>increased resolution or decreased noise, are presented here. The first case is a common-midpoint section<br>over buried sand channels. Before migration, the section showed two contiguous channels at a depth of<br>about 60 m in a sequence of horizontal beds . Improved lateral resolution after migration revealed a third<br>channel. This case illustrates the possiblevalue of migration even when the reflectors are horizontal. The<br>second case is a common-offset survey over a channel cut in Precambrian bedrock and overlain by glaciofluvial<br>sands and gravels. Migration, by collapsing the diffraction noise, resulted in a clearer picture of<br>the channel and revealed the presence of two terraces.