HR: 11:05h
AN: S12B-04    [Abstracts]
TI: Preliminary Application of Microseisms into Groundwater Contamination Monitoring
AU: * Zhang, J
EM: jincai@cires.colorado.edu
AF: CIRES, the University of Colorado, UCB216, Boulder, CO 80309-0216 United States
AU: Tanimoto, T
EM: toshiro@geol.ucsb.edu
AF: Department of Geological Sciences, the University of California, Building 526, Santa Barbara, CA 93106 United States
AU: Spetzler, H
EM: spetzler@colorado.edu
AF: CIRES, the University of Colorado, UCB216, Boulder, CO 80309-0216 United States
AB: Microseisms, one scientist­_s annoying noise are another­_s diagnostic tool. We are conducting a controlled field experiments with the aim of detecting the infiltration of a contaminant - a biosurfactant - into groundwater. Three sets of instruments are placed 3m, 13m and 32m respectively from a 50m by 50m irrigation site. Each set of instruments consists of a 3-component seismometer and a tilt meter. We are seeking to detect temporal changes in local station corrections that are caused by the irrigation. We use natural signals, such as microseisms as seismic sources and solid Earth tides as sources for the tilt signals. Seasonal changes in the amplitude ratios (horizontal to vertical HZ) of signals from microseisms have been found in California. These seasonal changes are likely to be caused by rather shallow changes in the water table as well as a partial saturated level in the vadose zone. In our field experiment we control the influx of water and monitor it as it percolates down to the ground water. This represents a near ideal arrangement to experimentally check if the HZ ratio can indeed be changed by changes in the local groundwater, or if the cause for the observed seasonal variations has to be found elsewhere. In the laboratory we have found that small additions of some chemicals to water can drastically change the surface energies and thus the wettability of solid surfaces. Surface energy changes in a partially saturated porous rock lead to large changes in complex elastic moduli. In the field experiment we are changing the wettability of the subsurface and are analyzing seismic and tilt data at varying distance from the irrigation site for contaminant caused changes in the moduli. Tilt data show a pronounced change between the three stations during the summer months, probably caused by the differential heating that occurs between the covered irrigation site and the bare ground surrounding it. The observed effect trails off as the instrument­_s distance from the irrigation site increases. In the seismic data we clearly see the microseismic energy and are now looking for changes in the HZ ratios at the three stations. We keenly anticipate getting and analyzing the seismic records from before, during and following the irrigation of biosurfactants which commences in mid October, 2004.
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2004 AGU Fall Meeting