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parallel injection pipe and locating radioactive source attached to the <br />bottom end of the hydrophone cable. <br />Mining was suspended during the time of the survey, and the bottom of <br />the production pipe was plugged to simplify lowering the hydrophones <br />into the pipe and reduce the temperature below the limit for the <br />hydrophones (figure A2). The surface end of the hydrophone cable was <br />connected to the seismograph for digital recording of detected seismic <br />signals. <br />A multi-barrel borehole gun operated by the Weatherford-WeIlServ <br />company was used as the seismic source. The source was lowered to the <br />desired depth in the injection pipe and then fired to generate seismic <br />waves. The charge for each barrel was adjusted to prevent <br />overloading/saturation of the hydrophones in response to seismic signals <br />generated by each shot. <br />A nonintrusive, intrinsically safe device built by NSA for this survey was used <br />to trigger recording of seismic signals generated when firing shots in the <br />injection well (see details in Appendix C). <br />The survey was conducted using three sets of hydrophones and sources <br />as shown in figures A3 through A5. <br />1. In the first set (figure A3), the hydrophones were at the bottom of <br />the production pipe at a depth ranging from 1,717 to 1,537 feet <br />below the local ground elevation. The sources were fired in 10 <br />locations at 20-ff intervals up the injection pipe starting at 2,160 feet <br />and ending at 1,980 feet. This configuration targeted mainly the <br />inner boundaries of the extraction zone with emphasis on horizontal <br />extent of structural changes. Note that the depth of the production <br />pipe was approximately 1,728 feet. Only 3 out of 10 hydrophones, <br />therefore, were inside the extraction limits -below the Gas Cap, <br />2. In the second set (figure A4), the hydrophones remained <br />unchanged, and the sources were fired in 20-ff intervals over a <br />depth range from 1,860 to 1,680 feet. This configuration of sources <br />and receivers targeted the upper part of the extraction zone and <br />the vertical features below the line of sources. <br />3. For the third set (figure A5), the hydrophones were moved to a <br />depth range from 1,280 to 1,460 feet, and the sources were fired in <br />20-ff intervals over a depth range from 1,280 to1,460 feet. This <br />configuration of sources and receivers was located entirely above <br />the extraction zone and was intended mainly to target the Gas <br />Cap and the top of the extraction zone. <br />