Walkaway, offset and deviated hole VSP's contain important information concerning the geology near the well. An accurate subsurface image surrounding the well can be constructed with proper processing. A new technique has been developed for imaging VSP data which extends the reliable lateral coverage of the VSP by a factor of two over conventional methods.
This new imaging technique does not rely upon an a priori model of the subsurface geology. Rather a velocity field, similar to that used in conventional seismic processing will suffice.
Often the justification for a VSP is tenuous. Acquisition is expensive (primarily in terms of rig time) and the results seem insufficient when compared to similar expenditures of time and money for surface seismic data.
A walkaway VSP, however, provides important in-situ information which can be used to generate an accurate subsurface image extending far away from the borehole.
The key to utilizing this valuable information is to identify each component wavefield and apply an appropriate transformation to the output domain. With sufficient care in the data acquisition, the walkaway VSP can provide a subsurface image superior to surface seismic data.
A walkaway VSP differs from surface seismic data in many ways. First, with the geophone situated at depth, an accurate measure of traveltime as a function of offset can be measured. From these measurements, an adjustment for velocity anisotropy can be derived.
Second, the direct arrival provides asufficiently accurate estimate of the basic seismic wavelet at depth. With this estimate, very long and robust signal processing operators can be applied to the data. These operators shape the seismic wavelet to zero phase and provide better discrimination between events.
In addition, by locating the receiver beneath the surface, it becomes possible to identify various wavefields based on their geometrical relationship to one another. The fact that this geometry is easier to predict and identify makes it possible to apply very specific multi-channel signal improvement processes to particular wavefields.
Another aspect of the VSP recording that is different from seismic recording is the consideration of adequate coverage. Most often, this can be the determining factor in the approval process. The closer the receiver is to the zone of interest, the better quality the up going wavefield used in most conventional processing. Unfortunately, the effective zone of imaging from this data shrinks significantly, the closer the receiver is to the zone of interest. By using the down going wavefield, in an appropriate manner, it is possible to extend the image of the zone of interest as much as three fold over using the up going wave field alone.
