Swift Stephen A.

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Swift
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Stephen A.
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End date: 1989 × Start date: 1985 ×

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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.
  • 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
    Late Cenozoic geology of the Central Persian (Arabian) Gulf from industry well data and seismic profiles
    (Woods Hole Oceanographic Institution, 1988-04) Swift, Stephen A. ; Uchupi, Elazar ; Ross, David A.
    Industry seismic reflection profiles shot in the 60's and early 70's in the central Persian (Arabian) Gulf are used to map two late Tertiary unconformities, and velocity data from a centrally located well is used to convert travel time to depth to the unconformities. The deeper horizon correlates with a regional unconformity at the end of the Eocene in most wells and dips monotonically to the northeast, whereas the shallower horizon is flatter and correlates with the mid-upper Miocene section in one well. Isopach maps based on wells indicate that sedimentation was relatively uniform across the region until the middle to late Miocene. Sediments deposited since the late Miocene thicken from 100-200 m on the Arabian side of the Gulf to >1000 m near Iran reflecting deposition of sediments eroded from the rapidly uplifting Zagros fold-belt. As a result of the rapid deposition, the velocity gradient in the upper 1 km decreases from ~4 km/sec per km near Arabia to about 2 km/sec per km on the Iranian side of the Gulf.
  • Thesis
    Cenozoic geology of the continental slope and rise off western Nova Scotia
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1985-10) Swift, Stephen A.
    The outer continental margin of Nova Scotia is divided by a diapir province, 40-110 km wide and ~1000km long, that trends subparallel to the shelf edge along the upper continental rise and slope. The growth pattern for a small region of this margin (61°-64°W) during the Late Cretaceous and Cenozoic was studied using seismic stratigraphy and well data. Structure maps show that a steep continental slope existed landward of the diapir province (~2200-3800 m water depth) from Early Cretaceous until Miocene time when onlapping upper rise sediments reduced the gradient. Shelf edge canyons were cut during the late Maestrichtian-early Paleocene, Eocene-Oligocene, and Pleistocene. Extensions of Tertiary canyons onto the slope are poorly defined, but small Paleocene fans of interbedded chalk and mudstone on the upper rise indicate that slope canyons existed at that time. Abyssal currents eroded the upper rise and smoothed relief on the continental slope in the Oligocene and middle(?) Miocene. In the Miocene, turbidites may have ponded on the upper rise landward of seafloor highs uplifted by salt ridges or pillows. Pliocene-Pleistocene sediments drape over pre-existing topography. At the beginning and end of the Pleistocene, turbidity currents, caused by delivery of large sediment loads to the shelf edge by glaciers, eroded the present canyon morphology. The late Cenozoic section of the lower continental rise thins seaward from ~2 km near the diapir province and rests on Horizon Au, a prominent unconformity eroded during the Oligocene by abyssal currents. The morphology of the lower rise is largely due to construction by down-slope deposits shed in the Miocene-Pliocene from uplift of the diapir province. Abyssal currents episodically eroded sediment, but current controlled deposition formed only a thin (<300 m) deposit in the Pliocene(?). Uplift in the diapir province accelerated during the Pleistocene and olistostromes up to 300 m thick were shed onto the lower rise. In the latest Pleistocene, sediments transported down-slope by near-bottom processes accumulated west of a sharp boundary running near 62°30'W from 500 m seaward to the abyssal plain. To the east, hemipelagic sediments accumulated above 4300 m, while turbidity currents, originating in deep canyons to the east, and abyssal currents reworked sediments below 4300 m. A glacial sediment source and relict shelf morphology controlled sedimentation processes and, thus, the location of depocenters on the slope and rise.