PRELIMINARY EVALUATION OF LONG-PERIOD SEISMOGRAPH TESTS AT LASA SUBARRAY F3.

During April and May 1967, an experimental program was started at LASA subarray F3 Hysham, Montana to develop a reliable, unmanned seismograph system capable of producing high-quality long-period seismograms. The development of improved installation techniques was the principal objective of the program. The instrumentation used in the study was specified before the field program started. An underground, monolithic concrete bunker was constructed and fitted with one coupled and two isolated tank vaults. The bunker is situated in a composition of clay containing bentonite. Noise, which peaks at period of 15-20 seconds, has limited the magnification of the vertical seismograph to approximately 60K. Higher amplitude noise peaking at 30-40 seconds restricts the horizontal seismographs to a magnification of 20-25K. The studies conducted to date have failed to reveal the sources of this noise. The test bunker isolates the seismometers from pressure and temperature variations, so it is improbable that the noise is caused by pressure and temperature changes acting directly on the seismometers. This conclusion is further supported by the fact that the effects of temperature and pressure are not observed on the vertical seismograms. Additional tests are required before final conclusions can be drawn regarding the performance of the long-period horizontal seismographs being operated in this experiment. Preliminary results indicate that the performance of the long-period vertical seismograph is excellent. Author