Seismic noise variations in a 770-m-deep borehole near Tulsa, Oklahoma
- Lawrence Livermore National Lab., CA (United States)
- Oklahoma Geological Survey, Norman, OK (United States)
Maximizing the signal detectability at a global earthquake and/or treaty verification seismic monitoring station is an important goal. Minimizing the background seismic noise is the primary means of maximizing signal detectability. To assure that seismic signals are detected, variations of the background seismic noise also must be minimized. We investigated whether locating seismometers in boreholes might decrease background seismic noise and variations in this noise. We deployed three seismometers at different depths in a 770-m borehole near Tulsa, Oklahoma, and then analyzed the data from these seismometers to determine the background noise and variations in this noise as a function of depth, frequency, time of day, and season. Acceleration power density spectra means were calculated for the months of January, March, May, and July for five-minute noise samples taken at 3 a.m. and at 3 p.m. to determine seismic noise variations in the borehole in the 0.2--20 Hz frequency band. The three station depths were: 748,432, and 4 m. The spectra at all three stations were nearly identical between 0.2 and 0.6 Hz, consistent with microseism-dominated noise spectra in this frequency band. Above 0.6 Hz, there was a large difference in noise power with depth. Relative to the 748-m station, the 432-m station noise was 3--7 dB higher and the 4-m station noise was 8-30 dB higher. The standard deviations of the means were similar for all stations. Consequently, although seismic noise variations are larger at shallower depths in absolute terms, they were independent of depth relative to the means at each station. A seasonal change in the mean spectra for each month was identified in the 0.2--1 Hz frequency band. The mean levels were lowest in July and highest in January. This result has been observed elsewhere in the northern hemisphere and is attributed to larger microseism generation during the winter months because of more numerous and severe storms at sea.
- Research Organization:
- Lawrence Livermore National Lab., CA (United States)
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 10121704
- Report Number(s):
- UCRL-LR-112429; ON: DE93006615
- Resource Relation:
- Other Information: PBD: Dec 1992
- Country of Publication:
- United States
- Language:
- English
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