Downhole Sensors Capture Seismic EventsApril 1, 2005 By: Stephanie vL Henkel, Sensors Sensors
Geothermal researchers at Sandia are working on sensors and associated electronics that will be rugged enough to survive conditions deep in the Earth. Placed in probes, they will monitor pressure and temperature changes caused by seismic activity. The data will add to scientific understanding of earthquakes and perhaps even make them more predictable.
Researcher Joe Henfling field tests a Sandia high-temp downhole probe.
Lab engineers Joe Henfling and Randy Normann, with technologist David Chavira, are using Quartzdyne (www.quartzdyne.com) sensors with relative resolutions better than 0.005 psi and 0.01°C. They are also investigating silicon-on-insulator materials as a way to conquer transistor breakdowns caused by thermally generated current leaks that occur at extremely elevated temperatures. Their ultimate goal is to make the components so tough that they will not require heat shielding for protection, even at temperatures >200°C.
The research has a variety of potential applications, among them remotely triggered seismicity. According to USGS geophysicist Evelyn Roeloffs, large earthquakes, even at great distances, can cause bursts of microquakes in California's Long Valley. Some persist for days after the main event. Observers are currently able to record temperatures and pressures every 2.5 s, and would like to collect data at the moment changes occur.
Sometimes fluid pressures in geothermal wells, which contain water in the pores between grains in the hot rock and can be 5–10 miles long, increase after earthquakes. These reservoirs constitute a much larger area of sensitivity to quake-generated waves than that monitored by seismic detectors. Putting sensors into deeper and hotter wells would yield better data on pressure-relation rock motion.
New Mexico Tech geophysicist Harold Tobin plans to bore a hole 3.6 miles into the ocean floor below 1.2 miles of seawater in a subduction zone off Japan's coast, where enormous quakes have generated tsunamis. Probes in the drill hole will yield data on the precursors of earthquakes and the physics of the way stresses are stored and then released as tectonic plates slip and displace. Fluid pressure in the rocks might have an effect on this dynamic.
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