Two-Trace Directional Filter for Processing Offset Vertical Seismic Profiles
David W. Bell, V. Dale Cox
A two-trace directional filter has been used to separate seismic events traveling in opposite directions past two receivers. The technique is well suited for separating upgoing from downgoing events on vertical seismic profiles (VSPs). In particular, it works well in situations of complex geometry, such as offset sources, that degrade the response of other filter types.
The two-trace filter is based on the concept that waves traveling in opposite directions have spatial derivatives of opposite sign. The spatial derivative is approximated by subtracting signals at two adjacent receiver locations, followed by time integration to recover the phase. When properly scaled, the integrated difference trace can be added to or subtracted from the sum of the two original traces to yield downgoing or upgoing events.
The two-trace filter has a good rejection response over a wide range of dips. Although other filter types can provide more selective dip filtering, the two-trace filter is effective when the number of traces is small and when the dip moveouts are not linear. The technique tolerates poor signal-to-noise ratios and will not produce artificial alignments like some harshly applied dip filters.
The success of the two-trace technique has important acquisition implications. For example, a tool with two receivers separated by a short distance can be used to separate upgoing and downgoing events on a walkaway VSP recorded at a single downhole level. This would simplify the processing of such data and reduce the cost of acquisition.
AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.