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A nonintrusive, intrinsically safe device built by NSA for this survey was used to trigger <br />recording of seismic signals generated when firing shots in the injection well (see details <br />in Appendix C). <br />The survey was conducted using three sets of hydrophones and sources as shown in <br />figures A3 through A5. <br />1. In the first set (figure A3), the hydrophones were at the bottom of the production <br />pipe at a depth ranging from 1,717 to 1,537 feet below the local ground elevation. <br />The sources were fired in 10 locations at 20-ft intervals up the injection pipe <br />starting at 2,160 feet and ending at 1,980 feet. This configuration targeted mainly <br />the inner boundaries of the extraction zone with emphasis on horizontal extent of <br />structural changes. Note that the depth of the production pipe was approximately <br />1,728 feet. Only 3 out of 10 hydrophones, therefore, were inside the extraction <br />limits -below the Gas Cap. <br />2. In the second set (figure A4), the hydrophones remained unchanged, and the <br />sources were fired in 20-ft intervals over a depth range from 1,860 to 1,680 feet. <br />This configuration of sources and receivers targeted the upper part of the <br />extraction zone and the vertical features below the line of sources. <br />3. For the third set (figure AS), the hydrophones were moved to a depth range from <br />1,280 to 1,460 feet, and the sources were fired in 20-ft intervals over a depth <br />range from 1,280 to1,460 feet. This configuration of sources and receivers was <br />located entirely above the extraction zone and was intended mainly to target the <br />Gas Cap and the top of the extraction zone. <br />Section 3.0 Data Analysis <br />3.1 Preparation of Seismic Data Files for Analysis <br />3.1.1 Measuring the Source Time (Time Break) <br />The goal of directly recording the actual shot time for each source (time break) was not <br />accomplished as originally planned. However, analysis of the seismic records has shown <br />that the injection and production steel pipes combined provided a very reliable <br />waveguide. This waveguide carried seismic energy from each shot in the injection hole <br />with nearly a constant velocity of 16,700 ft/s, both up and down both pipes (figure A6). <br />This velocity was higher than the velocity of seismic waves in the surrounding ground. <br />The velocity measurement was confirmed by seismic records acquired from sledge <br />hammer strikes at the ground surface near the surveyed well and from hydrophones in the <br />production hole at depths ranging from 1,537 to 1,717 feet below the surface. See <br />Appendix D for details. <br />The stable velocity of seismic signals traveling along the pipes was used to calculate the <br />source time for each shot location. Each record was then shifted back so the source time <br />was at the origin of that record. Thus, all records were made compatible with respect to <br />the timing of their sources. <br />