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Original Date: 01/23/1995
Revision Date: 01/18/2007
Best Practice : Evaluation of Low-Noise Seismometers
Sandia National Laboratories (SNL) is chartered by the DOE and DOD to evaluate seismic data acquisition systems to monitor international treaties limiting nuclear weapons underground testing. Maintaining the testing philosophy of using optimum available methods and simulating actual use conditions whenever feasible, SNL developed a shotgun borehole testing system for testing seismometers using coherence analysis. Since the earth movements from underground nuclear explosions are very small, Sandia discovered that developing a standard calibration input, such as a shake table, would not be feasible.
The shotgun borehole testing system has been developed at Sandia's Facility for Acceptance Calibration and Testing as an alternative to the less accurate vault testing system. In shotgun borehole testing, tubes are inserted in boreholes to a depth of 100 meters. The tubes are welded together at 15-foot intervals so that similar seismometers placed in each tube receive the most equivalent input signal achievable since both seismometers sense the same earth movements. Sandia has both double and triple shotgun borehole test facilities at the Facility for Acceptance Calibration and Testing site.
Seismometers inserted into shotgun borehole systems at Sandia are tested using coherence analysis. This Sandia-developed test method acquires ground motion data from two or more similar seismometers in neighboring boreholes simultaneously. This data is used to estimate the internal noise characteristics and relative transfer function between the seismometers. The accuracy of the seismometer is then determined by comparing the test information against the most accurate seismometer currently known, and its noise performance is compared to the Low Earth Noise model.
The combined system of shotgun borehole testing and coherence analysis enables Sandia to evaluate seismometers to the most accurate extent possible. This level of accuracy is essential to the effective monitoring of international treaties.
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