HR: 17:10h
AN: NG44A-04 INVITED    [Abstracts]
TI: Continuous Crosswell Seismic Measurement: Applications to Monitoring of Stress and CO2
AU: * Daley, T M
EM: tmdaley@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94602, United States
AU: Niu, F
EM: niu@rice.edu
AF: Rice University, 6100 Main St, Houston, TX 77005,
AU: Sliver, P G
EM: silver@dtm.ciw.edu
AF: Carnegie Institution, 5241 Broad Branch Rd, NW, Washington, DC 20015,
AU: Majer, E L
EM: elmajer@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94602, United States
AU: Benson, S M
EM: smbenson@stanford.edu
AF: Stanford University, 416 Escondido Mall, Stanford, CA 94305,
AB: The crosswell acquisition geometry offers direct access to subsurface materials. As such, crosswell data are ideal for monitoring subsurface processes. We have been investigating the use of semi-permanent crosswell seismic acquisition for continuous monitoring, making use of technologies usually deployed for tomographic imaging. Our two applications have been the monitoring of stress induced velocity changes and velocity changes due to CO2 injection. The use of stress induced seismic velocity changes to understand dynamic processes requires knowledge of the in-situ stress sensitivity of a given rock volume. We have made measurement of stress sensitivity using continuous crosswell travel time measurement at 3 sites for up to 2 months, and have found that continuous monitoring using fixed borehole instrument locations (unmoved, but not permanently installed) can achieve very high repeatability. Signal-to-noise ratio is the key parameter to obtaining travel time precision of better than a microsecond. With this precision, we are able to correlate changes in seismic velocity with changes in barometric pressure. Barometric pressure is used as a calibration signal to determine the in-situ stress sensitivity of the rock volume monitored by the crosswell measurement. Initial tests in shallow ground water wells (3 and 30 m depths) were successful in obtaining a stress sensitivity calibration. These tests were preliminary to deployment in the San Andreas Fault Observatory at Depth (SAFOD) boreholes in California. At the SAFOD site a piezoelectric source and a clamped 3-component accelerometer were used at a depth of 1 km in two separate 1 month long experiments. Initial SAFOD results include measurement of stress sensitivity and coseismic velocity changes. The goal of this work is to develop a methodology for monitoring stress increases preceding earthquakes. In separate work, continuous crosswell monitoring with multiple sensors was used to determine the spatiotemporal dimensions of a plume of CO2 injected into a brine aquifer to study carbon sequestration at the Frio site in southeast Texas. In this case the high noise level of an active injection limited the travel time accuracy, but the induced velocity changes due to fluid displacement were easily detected. The development of the plume was monitored for over a week at 15 minute intervals.
DE: 0915 Downhole methods
DE: 4499 General or miscellaneous
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7294 Seismic instruments and networks (0935, 3025)
DE: 8123 Dynamics: seismotectonics
SC: Nonlinear Geophysics [NG]
MN: 2007 Fall Meeting