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NSA APPENDIX D <br />Assessing Actual Source Time (Time Break) <br />The second goal of the NSA triggering device was to record the actual <br />timing for each seismic source by recording a break in the firing line when <br />the charge went off. The time of discharge/initiation of seismic waves for <br />known source location is critical to delineate ground anomalies (e.g., <br />solution caverns) using seismic waves reflected from these anomalies. <br />The goal of directly recording the source initiation time was not <br />accomplished as originally planned. The capacitance on the order of <br />several nF (nano-farads) associated with the long firing line prevented the <br />direct detection of actual shots at the bottom end of the firing line. <br />However, daily analysis of seismic records showed that the steel casing of <br />the injection and production holes provided a reliable waveguide <br />carrying seismic energy from the shots in the injection hole with nearly a <br />constant velocity of 16,700 ff/s, both up and down the pipes (figures A6 <br />and A7). This velocity measurement was confirmed by seismic records <br />acquired from sledge hammer strikes at the ground surface near the <br />surveyed well. Seismic waves from the hammer were recorded by the <br />hydrophones in the production hole at a depth range from 1,537 to 1,717 <br />feet below the surtace (figure Dl ). A special trigger switch attached to <br />the hammer was used to trigger the seismograph at the moment of the <br />each hammer strike. The trigger impulse was recorded by the trigger <br />channel (channel #1). Please note that the measured velocity was higher <br />than the velocity of seismic waves in the surrounding ground. <br />A detailed analysis of the experimental records acquired on the first day <br />of the survey was conducted at the conclusion of the survey. This analysis <br />showed an impulse recorded on individual hydrophone channels (figure <br />D2) that matched the timing of the discharges. Recording of that impulse <br />appeared associated with grounding of the hydrophone power supply <br />through the chassis of the hydrophone truck to the ground near the two <br />pipes. The main purpose of this grounding at the onset of the survey was <br />to reduce possible noise associated with the AC power supply. It did not <br />produce any significant improvement in signal-to-noise ratio and was <br />abandoned affer the first day of the survey. Later analysis showed that no <br />impulses matching the timing of a discharge were recorded affer the <br />grounding was disconnected. <br />Presented results open an indirect possibility for recording the actual time <br />of firing a shot (time break) for future application of the TRT technique to <br />monitor effects of solution mining. <br />