- Experiment Name: Extensometers as Seismometers
- PI: Heywood
- Email: cheywood@usgs.gov
- Sponsor: USGS , City of Albuquerque, International Boundry & Water
Commission
- Inst: USGS
- Start_Date: 1/1/2000
- End_Date: 1/1/2001
- Lat: 35.135
- Long: -106.66
- Chan6: 2
- Broadband: 2
- Descr: In the American Southwest and other areas in the world, borehole
extensometers have been installed to measure land-surface subsidence
resulting from fluid withdrawal. In many of these areas, extensometer
sensitivity has been sufficient to document both elastic and inelastic
strains due to relatively slow and sustained changes in pore pressure.
These strains are a drained response of the aquifer system to low-frequency
stress. Some extensometers designed to optimize strain sensitivity have
enabled measurement of the much smaller elastic strains resulting from
pore- pressure transients. In conjunction with pore-pressure measurements,
these strain measurements have enabled direct in situ determination
of elastic matrix compressibility at higher frequencies. Highly sensitive
vertical extensometers in Albuquerque, New Mexico, and El Paso, Texas,
have produced seismograms of earthquakes, including the October 9,
1995 Jalisco, Mexico (Mw = 8.0), and the October 16, 1999 Hector Mine,
California (Ms = 7.3), events. These seismograms record the high-frequency
response of the extensometer to vertical Earth strains. The strain represents
the difference in displacement between the bottom of the extensometer
and the Earth's surface. The recorded response is a convolution of the
instrumental response to ground acceleration with a measurement of
vertical strain from the seismic surface wave. The instrumental response
is a function of the natural period of a particular extensometer. Concurrent
measurement of ground acceleration with a conventional seismometer
will enable determination of this response. Decon- volution of the seismogram
will yield a measurement of vertical strain at the frequency of the
Rayleigh wave. These measurements should enable quantification of the
aquifer-system com- pressibility at this frequency, which approximates
the undrained response of the aquifer system. Comparison of drained
and undrained compressibilities at a given site may lead to enhanced
applicability of acoustic-velocity borehole logs to the problem of
predicting aquifer-system com- pressibilities under drained conditions.
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