Offset Vertical Seismic Profile
A normal VSP survey in a vertical borehole with horizontal bedding gives very limited lateral information. However, with dipping reflectors, a normal VSP survey can provide some information on the updip features.
An offset VSP offers the possibility of large lateral coverage. Lateral coverage of up to one-half of the source offset distance can be achieved in the direction of the source. Profiling of a feature can be done by using a fixed offset source position some distance from the well and moving the geophone(s) in the well, or by having the geophone(s) fixed and moving the source.
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Fig. VSP: stationary source, moving receiver
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Fig. Offset VSP: moving source, stationary receiver
Walkaway Surveys
A walkaway survey provides a 2-dimensional seismic picture of the formations on either side and below a well. This is achieved by using survey techniques developed from Zero Offset VSP (ZVSP) and Offset VSPs (OVSP). Walkaway surveys are unique in that they always employ a multiple source and single receiver arrangement.
DSA Tool for VSP Acquisition
DSA tool with its eight single-axis geophone array configuration provides several advantages over single level tools for VSP acquisition in cased holes;
- time savings. One obvious advantage is the saving in time since eight levels are recorded at each firing of the energy source;
- reduction in tube-wave effects. The DSA tool provides some advantages in area where strong tube-waves may contaminate the waveform data. The strong clamping force, the small cross-sectional area and the streamlined shape of the shuttle help to reduce the effect of tube-waves on the seismic data;
- reduction of navigation errors in walkaway VSP operations. In offshore multioffset operations, the seismic source is moved by a boat at a constant speed. The DSA tool acquires signals from eight levels while the boat is making a single path, reducing navigational errors over single level tools;
- reduction of effects of source signature changes. The effects of source signature changes due to such things as changes in gun pressure, gun pit alteration or tide levels, are reduced because the eight shuttles receive signals which originate from the same shot;
- accurate transit times between levels. Since the DSA geophones are equally spaced on a cable, potential distance errors are eliminated.
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Primary Uses of the VSP Survey
The enhanced resolution of the VSP makes it possible to verify or deny the presence of reflections that are indistinct or doubtful on seismic sections near the well. The VSP is particularly well suited to determine the conditions existing below the well’s total depth. Overpressured zones, gas sands and deep reflectors can be verified or recognized.
Since the downgoing wavefield is recorded, multiple reflections can be identified and removed. The same downgoing wave information can be used to reprocess surface seismic profiles traversing the vicinity of the well.
Perhaps the most common use for the VSP is a link between reflections observed on a surface seismic profile and specific petrophysical properties measured in the borehole. The correlation role of the VSP is important for reservoir development application.
Finally, by positioning the seismic source a significant distance from the well, structural and stratigraphic features from hundreds to thousands of feet from the well can be delineated and verified against the surface seismic.
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