TABLE OF CONTENTS The early part of this chapter discussed the one-dimensional imaging of the subsurface using boreholes and geophysical well logs. The last section described the two- and three-dimensional imaging of the subsurface using seismic surveying. This section integrates geophysical logs with seismic surveys in what is now generally referred to as borehole geophysics. The integration of these two technologies opens up the feasibility of imaging petroleum in four dimensions, the fourth dimension being time.
Earlier it was shown how the sonic log could be used to calibrate seismic reflectors with the rock boundaries penetrated by a borehole. This is the starting point for an evolutionary sequence of techniques termed vertical seismic profiles (colloquially VSPs).
For more than half a century it has been standard practice to place receivers at strategic depths in a borehole, and to record the time taken for sound to travel from an energy source at the surface. These velocity check shots are more reliable than counting the pips on a sonic log. In VSPs the downhole geophones are more closely spaced than for the earlier check shots. Indeed the recovered data are sufficiently abundant that a simulated seismic section of the strata adjacent to the borehole can be generated (Fitch, 1987).
The simplest instance of this technique is the zero-offset VSP in which there is a single energy source at the surface located immediately adjacent to the borehole (Fig. 3.75). Over the years, however,...
