CHARACTERISTICS OF INSTRUMENTS AND SEISMIC NOISE IN TWO SHALLOW HOLES AT HYSHAM, MONTANA.
Abstract
A surface seismometer, and a deep-well seismometer were operated in two adjacent 500 ft holes at the center of LASA subarray F3 (near Hysham, Montana). The two seismographs were operated concurrently at six different depths from 71 to 500 feet. A reference short-period vertical Benioff seismograph was operated on the surface. Signal amplitudes were measured at all depths to cross-check seismograph magnifications, and noise samples were selected representing five arbitrary categories of noise situation -- nighttime calm, daytime calm, traffic, moderate wind, and high wind. Power density spectra were computed from the noise samples, and the ratios of the spectra in the hole to those on the surface were obtained. This report discusses the instrumentation, operation, and data processing procedures utilized and presents the results of the data analysis. The data show that the attenuation of seismic noise at shallow depths is an erratic, non-linear function of noise amplitude which depends heavily upon the frequency and type (source) of the disturbance. Traffic noise and microseismic background are attenuated increasingly as either depth or frequency increase, 5 cps noise at 500 ft having an amplitude about 18 dB below the amplitude of the surface noise. Wind-generated noise in the passband from 0.5 to 3.0 cps is strongly attenuated at a depth of only 71 ft, with little additional improvement to be obtained at the 500 ft depth. At all depths, an improvement in noise level of more than 15 dB (at 1 cps) was realized during periods of high-wind velocity (20-40 mph).
Document Details
- Document Type
- Technical Report
- Publication Date
- Jul 01, 1966
- Accession Number
- AD0803237
Entities
People
- Richard S. Simons
Organizations
- Geotech Instruments (United States)