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|Title:||Microearthquake monitoring at Corps of Engineers facilities|
|Authors:||Patrick, David M.|
|Publisher:||Soils and Pavements Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: Awareness of potential seismicity induced by the filling of large reservoirs or by the injection of fluids in boreholes has prompted the Corps of Engineers and others to conduct microseismic monitoring programs at ten damsites and at one injection well. Two other damsites have been monitored by other organizations in cooperation with the Corps of Engineers and one is proposed. These programs were developed to measure and compare ambient seismicity in the environs of the installations and the seismicity present during either reservoir filling or fluid injection. These programs have been performed chiefly under contract with the U.S. Geological Survey and academic institutions. Monitoring consists of judiciously positioning an array of three to eight short-period, vertical component seismometers around the reservoir or injection well. Earthquake events detected by the seismometers are amplified at the site and either modulated there or at a location central to the array. The modulated signals are telemetered to a removed location where demodulation, recording, and data analyses are performed. These programs have been designed to monitor for a period of four to five years in order to include pre-, during-, and post-filling or injection phases. The average cost per year of monitoring is approximately $35,000. Equipment purchase, telemetry, and data analysis are the principal expenses. Analysis of available data from completed or nearly completed programs reveals no definite indications of seismicity induced by reservoir filling or fluid injection at Corps of Engineers installations. There is some indication that reservoir pool level may induce low-level seismicity at one site in the southeastern United states although there is other evidence that this may not be true. Monitoring programs must be based upon firm geological and seismological understanding of the installation sites. Arrays should be so situated, and consist of sufficient instruments, that accurate epicentral locations, focal depths, and magnitudes may be determined. Duration of monitoring must be sufficiently long that a reliable statistical significance can be attached to the data. The analysis of data must consist of either graphical or statistical comparisons of levels of seismicity during various phases of either filling or injection. Unless the monitoring period starts two or more years before reservoir filling, misleading results can be obtained.
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