Seismic interpreters must have a reliable methodology that allows them to (1) correlate seismic reflection events with well-log-defined sequence boundaries and (2) specify which seismic facies windows correspond to the rock facies intervals defined by well control. The need to define where critical stratigraphic features are correctly positioned in surface-recorded seismic reflection data is becoming increasingly important now that three-dimensional (3-D) seismic data are being widely used to evaluate oil and gas properties, particularly properties that are dominated by thin-bed reservoirs. The fundamental problem that confronts 3-D seismic interpreters is as old as seismic interpretation itself and can be stated as, How do you establish a rigorous correspondence between stratigraphic depth and seismic traveltime? Or perhaps even more to the point, How do you define the seismic data window that exactly spans a targeted thin bed when the position of that thin bed is known only as a function of depth from wireline measurements?
Traditionally, seismic interpreters have used locally reliable velocity functions (usually nearby velocity check-shot data) or synthetic seismograms, or both, to answer these fundamental interpretation questions. However, vertical seismic profiling (VSP) is a more rigorous methodology that interpreters can use to calibrate subsurface stratigraphy to seismic reflection data. The objective of this circular is to illustrate how VSP data can provide invaluable support to 3-D seismic interpretation. Specifically, the objective is to show that VSP data usually allow a more reliable depth-to-time calibration than do velocity check shots or synthetic seismograms. The important thin-bed calibration that VSP data provide to 3-D interpreters must be balanced by the fact that the acquisition of VSP data imposes an additional cost on the seismic evaluation of a prospect. That incremental cost needs to be justified, on a case-by-case basis, by comparing the added geological insights and the increased interpretational accuracy that may accrue when VSP data are used in a 3-D seismic interpretation.