2004-05-28_GENERAL DOCUMENTS - M1999002

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The velocity model is then used by TRTTM 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 often 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-ft 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 parallel injection pipe and locating radioactive source attached to the bottom end of the hydrophone cable. Mining was suspended during the time of the survey, and the bottom of the production pipe was plugged to simplify lowering the hydrophones into the pipe and reduce the temperature below the limit for the hydrophones (figure A2). The surface end of the hydrophone cable was connected to the seismograph for digital recording of detected seismic signals. A multi-barrel borehole gun operated by the Weatherford-We1lServ company was used as the seismic source. The source was lowered to the desired depth in the injection pipe and then fired to generate seismic waves. The charge for each barrel was adjusted to prevent overloading/saturation of the hydrophones in response to seismic signals generated by each shot.