Active source monitoring of crosswell seismic travel time forstress induced changes
We have conducted a series of cross-well experiments tocontinuously measure in situ temporal variations in seismic velocity attwo test sites: building 64 (B64) and Richmond Field Station (RFS) of theLawrence Berkeley National Laboratory in California. A piezoelectricsource was used to generate highly repeatable signals, and a string of 24hydrophones was used to record the signals. The B64 experiment wasconducted utilizing two boreholes 17 m deep and 3 m apart for 160 h. AtRFS, we collected a 36-day continuous record in a cross-borehole facilityusing two 70-m-deep holes separated by 30 m. With signal enhancementtechniques we were able to achieve a precision of 6.0 nsec and 10 nsec indelay-time estimation from stacking of 1-hr records during the ?7- and?35-day observation periods at the B64 and RFS sites, which correspond to3 and 0.5 ppm of their travel times, respectively. Delay time measured atB64 has a variation of ?2 lsec in the 160-hr period and shows a strongand positive correlation with the barometric pressure change at the site.At RFS, after removal of a linear trend, we find a delay-time variationof 2.5 lsec, which exhibits a significant negative correlation withbarometric pressure. We attribute the observed correlations to stresssensitivity of seismic velocity known from laboratory studies. Thepositive and negative sign observed in the correlation is likely relatedto the expected near- and far-field effects of this stress dependence ina poroelastic medium. The stress sensitivity is estimated to be 10 6/Paand 10 7/Pa at the B64 and RFS site, respectively.
- Research Organization:
- COLLABORATION - Collaboration with the Dept. ofTerrestrial Magnetism at the Carnegie Institute of Washington, and theDepartment of Earth Sciences at Rice University
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 901532
- Report Number(s):
- LBNL-62398; BSSAAP; R&D Project: 522001; TRN: US200714%%86
- Journal Information:
- Bulletin of the Seismological Society ofAmerica, Vol. 97, Issue 1b; Related Information: Journal Publication Date: Feb 2007; ISSN 0037-1106
- Country of Publication:
- United States
- Language:
- English
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