About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 465

Last Page: 466

Title: VSP Fundamentals that Improve CDP Data Interpretation: ABSTRACT

Author(s): J. E. Gaiser, J. P. Disiena

Article Type: Meeting abstract


This paper emphasizes the fundamentals of the vertical seismic profile (VSP) that improve the interpreter's confidence and understanding in its use as an exploration tool. The areas covered are VSP acquisition, critical points of the processing sequence, and applications to borehole information in depth and CDP data in time.

Correlating reflection character to the stratigraphic section is done typically with synthetic seismograms computed from sonic logs and check shot traveltimes. Problems often make this task difficult, even in areas where the synthetic or surface seismic data are of good quality. The VSP provides an alternative approach and a valuable link between the synthetic and CDP data.

A VSP extends check shot surveys from first break traveltime analysis to reflection analysis by recording the complete seismogram at a fine depth interval in the borehole (at least two depths per seismic wavelength). The unique feature of a VSP is the simultaneous measurement of downgoing and upgoing seismic waves as they propagate at depth.

A repeatable source such as Vibroseis and the fine depth sampling make it possible to separate the VSP downgoing waves

End_Page 465------------------------------

from the upgoing reflections by f-k filtering. The downgoing wave field can be used as signatures to deconvolve the upgoing wave field as the best estimate of the primary reflections in the vicinity of the borehole. VSP processing preserves the polarity and amplitude of reflections, and after deconvolution, a trough corresponds to a positive reflection coefficient. These reflections can be shifted to their two-way traveltime and stacked to produce a VSP extracted trace (VET), which is used to correlate to the CDP data.

An important application of a VSP is to correlate the reflection character to depth and the stratigraphy observed in the borehole. The direct downgoing arrival contains the two-way time to depth relationship and the enhanced primary reflections contain the reflection character. Any formation top in the VSP survey can be correlated to time using these features.

VSP reflection character indicates the significant features in the sonic log velocities that produce the reflections observed in the surface seismic data. Sonic logs and VSPs attempt to measure the same velocities and reflectivity near the borehole but with significantly different resolution. The sonic log only penetrates a short distance into the formation but provides detailed velocity information vertically. A VSP, however, has poor vertical resolution (50 to 100 ft [15 to 30 m] intervals) but samples a large area around the borehole similar to CDP data. Therefore, the VSP correlates well with CDP data because they have the same resolution. It also correlates well with the synthetic seismogram because they both contain primary reflections without multiples.

Finally, multiples in the CDP data are easily identified utilizing the VET and synthetic seismogram. In addition, the depth of origin and periodicity of multiples in the processed VSP can be observed directly because they are generated by downgoing multiples and not by the direct arrival.

End_of_Article - Last_Page 466------------

Copyright 1997 American Association of Petroleum Geologists