Swift Stephen A.

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Swift
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Stephen A.
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Subject: Seismology ×

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  • Working Paper
    Ambient noise analysis and finite difference modelling of VLF borehole seismic data
    (Woods Hole Oceanographic Institution, 1987-03) Stephen, Ralph A. ; Swift, Stephen A. ; Bolmer, S. Thompson
    This report describes a preliminary analysis of borehole seismic data to determine VLF/Sub-bottom Seismic Noise in the Atlantic and the preliminary results of finite difference modelling for a Cape Fear environment. Noise levels were not a simple function of depth and there are indications that noise levels may depend on local geology about a given receiver position and/or on clamping. Coherency of the noise was generally poor (<0.8) and was independent of depth. There is no evidence for distinct polarizations or directionality of the noise. The lowest determined value for ambient noise power on the vertical component was 10-4 nm2/Hz in the frequency range from 5-50 Hz. The better clamped horizontal component had comparable power values. In conclusion, although the drill ship was on the site and drill pipe was in the hole, analysis of the data for a large number of windows can provide meaningful upper bounds on the ambient noise levels in the upper crust.
  • Working Paper
    LFASE borehole array data acquisition and reduction summary
    (Woods Hole Oceanographic Institution, 1991-09) Bolmer, S. Thompson ; Swift, Stephen A. ; Stephen, Ralph A.
    On the Low Frequency Acoustic - Seismic Experiment (LFASE), carried out at DSDP hole 534A in August, 1989, WHOI was responsible for acquiring data from a twelve channel borehole seismic array. Data were acquired both on-board while tethered to the array and autonomously in a seafloor package. Seismic source data (explosives and airguns) and ambient noise data were recorded. This report describes the nature of data acquired, reviews the data reduction procedure from field format to ROSE data format, and includes examples of the data. A total of918.6 Mbytes of data were acquired (769 Mbytes in the shipboard recording mode and 149.6 Mbytes in the seafloor recording mode). Approximately 85 explosive shots, 2000 airgun shots, and 10 hours of ambient noise data were recorded.