Swift Stephen A.

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Stephen A.

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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.
Finite difference modeling of geoacoustic interaction at anelastic seafloors

(Acoustical Society of America, 1994-01) Stephen, Ralph A. ; Swift, Stephen A.

A major problem in understanding seismic wave propagation in the seafloor is to distinguish between the loss of energy due to intrinsic attenuation and the loss of energy due to scattering from fine scale heterogeneities and bottom roughness. Energy lost to intrinsic attenuation (heat) disappears entirely from the system. Energy lost to scattering is conserved in the system and can appear in observations as incoherent noise (reverberation, time spread, angle spread) and/or mode converted waves. It has been shown by a number of investigators that the seafloor scattering problem can be addressed by finite difference solutions to the elastic wave equation in the time domain. However previous studies have not considered the role of intrinsic attenuation in the scattering process. In this paper, a formulation is presented which includes the effects of intrinsic attenuation in a two-dimensional finite difference formulation of the elastodynamic equations. The code is stable and yields valid attenuation results.
Lateral continuity of basement seismic reflections in 15 Ma ultrafast-spreading crust at ODP Site 1256

( 2010-07-25) Nag, Sreeja ; Swift, Stephen A.

The Ocean Drilling Program (ODP) initiated drilling at Site 1256D in the Guatemala Basin, about 1000km off the East Pacific Rise to penetrate plutonic rocks, anticipated to be relatively shallow in this region, formed at an ultra-fast spreading rate. IODP Expedition E312 successfully drilled into gabbros at ~ 1150m in basement. Multi-channel seismic traces show weak laterally coherent sub-basement reflections at borehole depths. Synthetic reflectivity seismograms were computed using a Ricker wavelet and impedance profiles from borehole sonic logs. These seismograms show significant sub-basement amplitude peaks. A zero-offset vertical seismic profile, shot on E312, was processed to investigate the authenticity of these reflections and their relationship to borehole geology. A dual scheme of the median filtering and F-K dip filtering was used. Tests with synthetic seismograms indicate the approach is effective at reasonable SNR levels. Downgoing energy is clearly identified but negligible upgoing energy is visible over random noise. These results indicate that lava flows and igneous contacts in upper ocean crust have significant topography on lateral scales less than the Fresnel Zone (~ 300m) due to igneous and tectonic processes.
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.
Modeling seafloor geoacoustic interaction with a numerical scattering chamber

(Acoustical Society of America, 1994-08) Stephen, Ralph A. ; Swift, Stephen A.

A numerical scattering chamber (NSC) has been developed to compute backscatter functions for geologically realistic seafloor models. In the NSC, solutions are computed to the elastic (or anelastic) wave equation by the finite-difference method. This has the following advantages: (a) It includes all rigidity effects in the bottom including body and interface waves. (b) It can be applied to pulse beams at low grazing angles. (c) Both forward scatter and backscatter are included. (d) Multiple interactions between scatterers are included. (e) Arbitrary, range-dependent topography and volume heterogeneity can be treated simultaneously. (f) Problems are scaled to wavelengths and periods so that the results are applicable to a wide range of frequencies. (g) The method considers scattering from structures with length scales on the order of acoustic wavelengths. The process is discussed for two examples: a single facet on a flat, homogeneous seafloor and a canonically rough, homogeneous seafloor. Representing the backscattered field by a single, angle-dependent coefficient is an oversimplification. In a strong scattering environment, time spread of the field is a significant issue and an angle-dependent separation of the wave field may not be valid.
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,
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.
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.
Bathymetry and sediment thickness survey of the Hawaii-2 cable : cruise report for Kiwi expedition leg 2 on the R/V Roger Revelle

(Woods Hole Oceanographic Institution, 1997) Stephen, Ralph A. ; Swift, Stephen A. ; Greaves, Robert J.

The primary purpose of this cruise was to identify at least two potential observatory sites along the Hawaii-2 cable that would be suitable for drilling a hole to basement. There is a funded program, the Hawaii-2 Observatory (H20), to install a junction box on the cable about mid-way between California and Hawaii (Figures 1 and 2). We want to identify sites in advance so that drilling will be possible near the observatory. This will permit a large range of borehole experiments to be cared out continuously and in real time. Based on available data we chose a section of cable between 140° and 143°W. This cable lies on a ribbon of 'normal' oceanic crust with well defined magnetic anomalies and relatively smooth bathymetry. The goals were to acquire SEABEAM bathymetry data and single channel seismic reflection data along this section of cable, to identify at least two potential sites along the cable and to car out SCS surveys within about 10km radius of the sites. Since the H20 cable has been given to the scientific community it is a valuable resource for research. While transiting to and from the site we felt that it would be wortwhile to acquire SEABEAM data along as much of the cable track as possible. This 'spec' data, Seabeam and magnetometer data between 130°W and 155°W may be useful to other investigators in the future.
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.
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.
Velocity structure of upper ocean crust at Ocean Drilling Program Site 1256

(American Geophysical Union, 2008-10-16) Swift, Stephen A. ; Reichow, Marc ; Tikku, Anahita ; Tominaga, Masako ; Gilbert, Lisa A.

We examine shipboard physical property measurements, wireline logs, and vertical seismic profiles (VSP) from Ocean Drilling Program/Integrated Ocean Drilling Program Hole 1256D in 15 Ma ocean crust formed at superfast spreading rates to investigate lateral and vertical variations in compressional velocity. In general, velocities from all methods agree. Porosity is inversely related to velocity in both the logging and laboratory data. We infer that microfracturing during drilling is minor in the upper 1 km of basement, probably due to connected pores and, thus, low effective stress. The closure of porosity to very low values coincides with the depth below which laboratory velocities diverge from logging velocities. We infer that porosity controls velocity in layer 2, lithostatic pressure controls the thickness of seismic layer 2, and the distribution of flow types determines seismic velocity in the upper 200 m of basement. In the sheeted dikes, changes in physical properties, mineralogy, and chemistry define clusters of dikes.
Signal and noise levels in numerical scattering chamber snapshots

(Woods Hole Oceanographic Institution, 1995-05) Swift, Stephen A. ; Little, William S. ; Stephen, Ralph A.

A study was carried out to quantify the level of numerical noise in numerical scattering chamber (NSC) calculations and to compare these noise levels with signal levels of body waves, interface waves and ambient noise. The amplitudes of signal and noise in snapshots from the numerical scattering chamber were quantified at 50 and 65 periods for a few reference models. Models with homogeneous subseafloor structure were studied to determine the level of numerical noise; models with a wavenumber-correlation length product of one were examined to determine signal levels. Models were run with both Higdon and telegraph equation absorbing boundaries since the numerical noise within the grid depends on the boundary formulation. Amplitudes were measured along data traces obtained at a grid depth of 3.33 λw and at the seafloor. Forward traveling head waves had typical amplitudes of ±125 but may reached ±250 near the direct wave. Diffraction amplitudes were observed up to ±300. Stoneley wave amplitudes ranged from ±800 up to ±20,000. Numerical noise levels were less than ±25 in most areas of the water and less than ±350 along most of the seafloor. Regardless of the absorbing boundary type, however, there was a region of noise extending up to 15 λw behind the first seafloor reflection at 3.33 λw in which noise levels range from ±100 up to ±600. In this region it is difficult to resolve signal from systematic numerical noise.
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.
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.

Swift Stephen A.

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