Offset source VSP surveys and their image reconstruction
Geophysical Prospecting, 32 (5):
790--811 (October 1984)DOI: 10.1111/j.1365-2478.1984.tb00739.x
Abstract
Zero-offset-source VSP surveys provide information about the subsurface
only within the Fresnel zone centered at the well. Offsetting the
source location moves the reflection zones away from the well thus
providing lateral cover. Conventional processing of this type of
data gives rise to a distorted image of the subsurface. Using a simple
ray-tracing scheme, this image may be reconstructed into the more
familiar coordinate system of the surface seismic section. This simple
data-independent mapping is based on the assumption of horizontal
layering and requires a vertical velocity profile. The technique
of placing the source away from the borehole was first applied to
the single-offset-source VSP survey. However, data from any survey
geometry (such as deviated well with rig source, walkaway VSP, etc.)
can be mapped to the coordinate system defined by the appropriate
seismic section. To obtain the best results from this type of survey
the target area must be defined and simple modeling techniques used
to optimize the source location(s). These pre-survey modeling methods
may also be used to anticipate - and hence avoid a number of problem
areas which experience has highlighted. The data from any VSP survey
is the result of a realizable experiment and as such obeys the wave
equation. This implies that the wave equation may be used to migrate
the data to its true subsurface location. Theoretically, such a process
is more secure than ray-tracing techniques, although its practice
presents many difficulties.