Swift Stephen A.

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Swift
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Stephen A.
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Author: Bolmer, S. Thompson ×

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  • Technical Report
    Ocean Bottom Seismometer Augmentation in the North Pacific (OBSANP) - cruise report
    (Woods Hole Oceanographic Institution, 2014-12) Stephen, Ralph A. ; Worcester, Peter F. ; Udovydchenkov, Ilya A. ; Aaron, Ernie ; Bolmer, S. Thompson ; Carey, Scott ; McPeak, Sean P. ; Swift, Stephen A. ; Dzieciuch, Matthew A.
    The Ocean Bottom Seismometer Augmentation in the North Pacific Experiment (OBSANP, June-July, 2013, R/V Melville) addresses the coherence and depth dependence of deep-water ambient noise and signals. During the 2004 NPAL Experiment in the North Pacific Ocean, in addition to predicted ocean acoustic arrivals and deep shadow zone arrivals, we observed "deep seafloor arrivals" (DSFA) that were dominant on the seafloor Ocean Bottom Seismometer (OBS) (at about 5000m depth) but were absent or very weak on the Distributed Vertical Line Array (DVLA) (above 4250m depth). At least a subset of these arrivals correspond to bottomdiffracted surface-reflected (BDSR) paths from an out-of-plane seamount. BDSR arrivals are present throughout the water column, but at depths above the conjugate depth are obscured by ambient noise and PE predicted arrivals. On the 2004 NPAL/LOAPEX experiment BDSR paths yielded the largest amplitude seafloor arrivals for ranges from 500 to 3200km. The OBSANP experiment tests the hypothesis that BDSR paths contribute to the arrival structure on the deep seafloor even at short ranges (from near zero to 4-1/2CZ). The OBSANP cruise had three major research goals: a) identification and analysis of DSFA and BDSR arrivals occurring at short (1/2CZ) ranges in the 50 to 400Hz band, b) analysis of deep sea ambient noise in the band 0.03 to 80Hz, and c) analysis of the frequency dependence of BR and SRBR paths. On OBSANP we deployed a 32 element VLA from 12 to 1000m above the seafloor, eight short-period OBSs and four long-period OBSs and carried out a 15day transmission program using a J15-3 acoustic source.
  • 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.
  • Article
    Third-party borehole seismic experiments during the Ocean Drilling Program
    (Copernicus Publications on behalf of the German Research Centre for Geosciences, 2007-11) Stephen, Ralph A. ; Swift, Stephen A. ; Bolmer, S. Thompson ; Hoskins, Hartley
    The first borehole seismic experiments on DSDP and ODP were two-ship Oblique Seismic Experiments (Stephen, 1979; Stephen, et al., 1979, 1980; Swift, et al., 1988). By recording on the drill ship and shooting explosives out to ranges of 8 km, the upper 1.5 km of the upper crust (Layer 2) adjacent to the borehole could be imaged (Fig. 1; Stephen and Harding, 1983), Azimuthal anisotropy (Stephen, 1981, 1985) and lateral heterogeneity (Stephen, 1988; Swift and Stephen, 1989) could also be studied by shooting circles of shots at a fixed range from the borehole,
  • Technical Report
    Site synthesis report of DSPP sites 417 and 418
    (Woods Hole Oceanographic Institution, 1989-06) Swift, Stephen A. ; Bolmer, S. Thompson ; Stephen, Ralph A.
    This document summarizes information relevent to planning, execution, and interpretation of results from a study of the interaction of sound in the 2-30Hz band with deep ocean seafloor using sea-surface sources, seafloor receivers, and borehole seismometers emplaced by wireline re-entry at Deep Sea Drilling Project sites 417 and 418 in the western North Atlantic. We summarize published scientific results from borehole sampling of water, sediment, and rock, from wire line logging, and from borehole seismic experiments. We present new results from analysis of total power recorded by receivers clamped in basement during the borehole seismic experiment on DSDP Leg 102. We document non-drilling investigations of the site and the nature and location of re-entry cones and transponders. We describe the physical oceanography of the region and the speed of sound in water. We provide an extensive bibliography on published results from scientific investigations at 417/418. This document was completed prior to 1989 surveys of sites 417 and 418.
  • Working Paper
    Seafloor borehole array seismic system (SEABASS)
    (Woods Hole Oceanographic Institution, 1993-01) Stephen, Ralph A. ; Koelsch, Donald E. ; Berteaux, Henri O. ; Bocconcelli, Alessandro ; Bolmer, S. Thompson ; Cretin, J. ; Etourmy, N. ; Fabre, A. ; Goldsborough, Robert G. ; Gould, Matthew R. ; Kery, Sean M. ; Laurent, J. ; Omnes, G. ; Peal, Kenneth R. ; Swift, Stephen A. ; Turpening, R. ; Zani, A. Cleo
    The Seafloor Borehole Array Seismic System (SEABASS) has been developed to measure the pressure and three dimensional particle velocity of the VLF sound field (2-50HZ) below the seafloor in the deep ocean (water depths of up to 6km). The system consists off our three-component borehole seismometers (with an optional hydrophone), a borehole digitizing unit, and a seafloor control and recording package. The system can be deployed using a wire line re-entry capability from a conventional research vessel in Deep Sea Drilling Project (DSDP) and Ocean Drilling Project (ODP) boreholes. Data from below the seafloor are acquired either on-board the research vessel via coaxial tether or remotely on the seafloor in a self-contained package. If necessary the data module from the seafloor package can be released independently and recovered on the surface. This paper describes the engineering specifications of SEABASS, the tests that were carried out, and preliminary results from an actual deep sea deployment. Ambient noise levels beneath the seafloor acquired on the Low Frequency Acoustic-Seismic Experiment (LFASE) are within 20dB of levels from previous seafloor borehole seismic experiments and from land borehole measurements. The ambient noise observed on LFASE decreases by up to 12dB in the upper 100m of the seafloor in a sedimentary environment.
  • 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.
  • Presentation
    Third party borehole seismic experiments during the Ocean Drilling Program [poster] 
    ( 2003-12) Swift, Stephen A. ; Stephen, Ralph A. ; Hoskins, Hartley ; Bolmer, S. Thompson
    Third party borehole seismic experiments on the Ocean Drilling Program began with an oblique seismic experiment on Leg 102 at Site 418 in the Western Atlantic. Upper ocean crust here is characterized by a normal seismic layer 2 vertical velocity gradient, lateral velocity variations, azimuthal anisotropy, and azimuth dependent scattering. A normal incidence VSP was run on Leg 118 in the gabbro sequence at Hole 735B on the Southwest Indian Ridge. The vertical seismic velocity inferred from arrival times is similar to that observed horizontally by refraction in ocean layer 3, but attenuation is anomalously high, which prompted the hypothesis that the gabbro cored may not actually represent the bulk of Layer 3 material. The VSP data acquired at Hole 504B in the eastern equatorial Pacific on Legs 111 and 148 helped to constrain the P and S velocity structure at the site and showed that upper layer 3 at this site, at a depth of over 2 km into the crust, consisted of the lower portion of the sheeted dikes rather than gabbro. Both offset and normal incidence VSPs were run on Leg 164 to study the seismic velocity structure of gas hydrates on the Blake Ridge. A new innovation on ODP was the deployment of broadband seismometers in boreholes. Whereas the conventional VSPs and offset VSPs mentioned above operate in the frequency range from 1 to 100Hz, broadband seismometers are used in earthquake seismology and operate in the range from 0.001 to 10Hz. The first broadband seismometer test was carried out from the drill ship on Leg 128 in the Japan Sea. Subsequently 4 permanent broadband borehole seismic observatories were installed in the Western Pacific and Japan Trench on Legs 186, 191 and 195. The ODP era also saw the development of systems for re-entering boreholes from conventional research vessels after the drill ship left the site. Borehole seismic experiments and installations that used this wireline re-entry technology were carried out in DSDP Holes 534 (Blake-Bahama Basin) and 396 (Mid-Atlantic Ridge at 23degr north) and ODP Hole 843B (south of Oahu). The latter experiment (Ocean Seismic Network Pilot Experiment) carried out a test of 3 configurations of broadband seafloor seismic installation in preparation for extending the Global Seismic Network to the deep ocean.