Common‐depth‐point (CDP) stacking is based on the assumption that reflection points are coincident and situated midway between the respective source and receiver locations. If the reflector is structurally deformed, the reflection points move updip from the midpoint. As the structural dip increases, the reflection points for a CDP group of traces are farther removed from each other and normal stacking procedures [i.e., reflection apparent velocities for horizontal reflectors used for normal moveout (NMO) corrections] become increasingly inaccurate. Under such circumstances prestack migration is desirable, particularly when high frequencies are to be preserved. One published approach to prestack migration (Sattlegger and Stiller, 1973) involves the generation of substacks of adjacent traces followed by migration and summation of individual substacks. While adequate in many instances, cases exist where even substacks are degraded by the reflection time differential between component traces. This paper discusses an alternative technique to prestack migration without recourse to substacks. Common‐source traces, after gain recovery and static time corrections but before NMO corrections, are migrated using Kirchhoff summation. Aperture is computed for each sample according to specified maximum dips. Traces are simultaneously migrated and stacked, then output sequentially in sets of 12. The method is economical and provides enhanced reflection continuity and reliability in comparison to poststack migration. Moreover, the collapsing of diffractions is more effective.
The expression for the potential due to a point source of current, placed on the surface of a horizontal three‐layer earth, has been derived when conductivity in the second layer varies linearly with depth and changes abruptly at the boundaries. Master curves for Wenner and Schlumberger configurations have been presented for an insulating basement and for one value of conductivity gradient in the second layer.
In the course of the interpretation of an aero‐magnetic survey of Algeria, techniques based on spectral analysis were found to be quite useful in the determination of volcanic thicknesses and the resolution of underlying structural trends in a mineralized region near Silet. One‐dimensional analysis, applied to a series, of traverses crossing the area, indicates the average thickness of volcanics to be about 100 m. Two‐dimensional spectra were used to design appropriate filters for attenuating anomalies from shallow sources thereby enhancing the responses of the underlying regional structures.
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