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APPENDIX D <br />Assessing Actual Source Time (Time Break) <br />The second goal of the NSA triggering device was to record the actual timing for each <br />seismic source by recording a break in the firing line when the charge went off. The time <br />of discharge/initiation of seismic waves for known source location is critical to delineate <br />ground anomalies (e.g., solution caverns) using seismic waves reflected from these <br />anomalies. <br />The goal of directly recording the source initiation time was not accomplished as <br />originally planned. The capacitance on the order of several nF (nano-farads) associated <br />with the long firing line prevented the direct detection of actual shots at the bottom end of <br />the firing line. However, daily analysis of seismic records showed that the steel casing of <br />the injection and production holes provided a reliable waveguide carrying seismic energy <br />from the shots in the injection hole with nearly a constant velocity of 16,700 ft/s, both up <br />and down the pipes (figures A6 and A7). This velocity measurement was confirmed by <br />seismic records acquired from sledge hammer strikes at the ground surface near the <br />surveyed well. Seismic waves from the hammer were recorded by the hydrophones in the <br />production hole at a depth range from 1,537 to 1,717 feet below the surface (figure D1). <br />A special trigger switch attached to the hammer was used to trigger the seismograph at <br />the moment of the each hammer strike. The trigger impulse was recorded by the trigger <br />channel (channel #1). Please note that the measured velocity was higher than the velocity <br />of seismic waves in the surrounding ground. <br />A detailed analysis of the experimental records acquired on the first day of the survey <br />was conducted at the conclusion of the survey. This analysis showed an impulse recorded <br />on individual hydrophone channels (figure D2) that matched the timing of the discharges. <br />Recording of that impulse appeared associated with grounding of the hydrophone power <br />supply through the chassis of the hydrophone truck to the ground near the two pipes. The <br />main purpose of this grounding at the onset of the survey was to reduce possible noise <br />associated with the AC power supply. It did not produce any significant improvement in <br />signal-to-noise ratio and was abandoned after the first day of the survey. Later analysis <br />showed that no impulses matching the timing of a discharge were recorded after the <br />grounding was disconnected. <br />Presented results open an indirect possibility for recording the actual time of firing a shot <br />(time break) for future application of the TRT technique to monitor effects of solution <br />mining. <br />