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contains seismic signals received at each sensor. The seismic signals are inspected to obtain the arrival times of P- and/or S-waves that determine the seismic velocity of each wave type in the rock mass. These velocity values are used to define the velocity model within athree-dimensional orthogonal block selected for seismic data processing. The volume of the block is subdivided by a regularly spaced cubic grid, the size of which determines the resolution of the ground image to be reconstructed by the TRTT"' from reflected seismic waves. The velocity model is then used by TRTr^" to calculate the times required for a signal to travel from each source to each individual node of the grid and back to each receiver. Subsequently, for each node, the parts of all recorded seismic signals matching the appropriate travel times are added together. For actual reflective structures, such as cavities, faults, etc., the signals should superimpose, resulting in a large positive or negative value (reflectivity number), If no structure is present at the node, the signals should effectively cancel, resulting in near zero values. Contour plots of a specific reflectivity number (positive and negative) are made throughout the survey block to isolate and identify high-amplitude anomalies that possibly represent reflections from actual structures in the rock mass, In practice, several factors must be considered in solving a particular problem. For example, there is a trade-off between the levels of detail obtained in the images versus the distance that can be imaged, Large distances require a large grid spacing that limits the frequency range of seismic signals acceptable for data processing and results in a relatively low level of detail. Also, several filters are offen employed to modify the raw seismic signals to subdue noise and enhance features in a particular area of interest. Section 2.0 Survey Procedure The survey was conducted exclusively from the injection and production steel pipes in Well 28-21 (figure A1). The nitrogen gas was released from the well and replaced with the brine. The brine in both pipes provided seismic coupling through the pipe walls to the brine and the rock mass outside for both the seismic sources and the receivers. A string of 10 hydrophones at 20-ff centers was used as the receivers (see Appendix B for technical details). The receivers were attached to a multi- wire cable and lowered to the desired depth in the production pipe. The depth was precisely measured by lowering the radiation detector into the