HR: 11:00h
AN: NG32A-03 INVITED     [Abstracts]
TI: Developing a Methodology for Observing Stress-Induced Temporal Variations in Travel Time: A Progress Report
AU: * Silver, P G
EM: silver@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Niu, F
EM: niu@rice.edu
AF: Rice University, MS-126 6100 Main Street, Houston, TX 77005 United States
AU: Daley, T M
EM: tmdaley@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720 United States
AU: Majer, E L
EM: elmajer@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720 United States
AB: The dependence of crack properties on stress means that crustal seismic velocity exhibits stress dependence. This dependence constitutes, in principle, a powerful means of studying transient changes in stress at seismogenic depth through the repeat measurement of travel time from a controlled source. While the scientific potential of this stress dependence has been known for decades, time-dependent seismic imaging has yet to become a reliable means of measuring subsurface stress changes in fault-zone environments. This is due to 1) insufficient delay-time precision necessary to detect small changes in stress, and 2) the difficulty in establishing a reliable in-situ calibration between stress and seismic velocity. These two problems are coupled because the best sources of calibration, solid-earth tides and barometric pressure, produce weak stress perturbations of order 10{²}-10{³} Pa that require precision in the measurement of the fractional velocity change dlnv of order 10^-6, based on laboratory experiments. We have thus focused on developing a methodology that is capable of providing this high level of precision. For example, we have shown that precision in dlnv is maximized when there are Q/π wavelengths in the source-receiver path. This relationship provides a means of selecting an optimal geometry and/or source characteristic frequency in the planning of experiments. We have initiated a series of experiments to demonstrate the detectability of these stress-calibration signals in progressively more tectonically relevant settings. Initial tests have been completed on the smallest scale, with two boreholes 17 m deep and 3 meters apart. We have used a piezoelectric source (0.1ms source pulse repeated every 100ms) and a string of 24 hydrophones to record P waves with a dominant frequency of 10KHz. Recording was conducted for 160 hours. The massive stacking of ~36,000 high-SNR traces/hr leads to delay-time precision of 6ns (hour sampling) corresponding to dlnv precision of 3 × 10^-6. We find that barometric pressure fluctuations are easily observed in the delay time data with a SNR of 1000. We have also conducted tests at the Richmond Field Facility, which permits cross-borehole recordings at a distance of 30 m, and depths to 70 m, using the same equipment. The dominant frequency in this case was 1KHz. We have performed tests over a 45-day period and have been able to attain the same high precision (dlnv of order 10^-6) as in the first experiment. Also, as in the first experiment, we were able to observe variations due to barometric pressure. The third and most tectonically relevant experiment is being conducted at the Parkfield site of EarthScope's SAFOD drill hole, performing a cross-hole experiment at approximately 1 km depth using both the SAFOD pilot hole as the source hole, and a clamped three-component accelerometer in the main hole. Making use of a specially designed 2KHz 18-element piezoelectric source, we have begun to collect a test data set (1ms source pulse, 10-100m spacing) to assess our travel time precision. While only very short time segments have thus far been analyzed, the preliminary data show that we are again able to attain precision comparable to the first two experiments. With this sensitivity, we expect to observe stress-induced temporal changes in dlnv along this path, which if confirmed, would demonstrate the ability to measure KPa-level stress variations at near-seismogenic depth.
DE: 8164 Stresses: crust and lithosphere
SC: Nonlinear Geophysics [NG]
MN: Fall Meeting 2005