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Abstract

It is a well known, but often ignored, fact that the events which appear on a seismic section rarely correspond to their true position in space. Correspondence is possible only if the reflecting horizon is truly a horizontal plane surface. Under all other conditions the reflection must be migrated in order to position it properly in space. Reflected energy obeys physical laws and, in the simplest dip case, the travel path back to the origin will be perpendicular to the dip of a reflecting bed, rather than vertically as it is plotted on die seismic cross section.

In areas of moderate dip, say up to 10°, the seismic section approximates the structure with sufficient accuracy to be acceptable. In areas of steep dip a reflection point plotted below the shot point may actually be removed laterally several hundreds of feet from the vertical line below the shotpoint. Even this problem may not be too serious in areas of fairly simple structure, because the error diminishes with decreasing dip and the geologically important crest of an anticline will be correctly located.

In more complex areas, where faulting, severe non-symmetrical folding, and sharp synclines are present, diffractions and double image reflections may be distorted to the point where the resulting seismic section bears little or no resemblance to the actual structure. This situation is not uncommon and it can be very difficult to interpret true structure completely from a section that has not been migrated properly.

Complicating the picture is the fact

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