Stephen Ralph A.

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Estimating the horizontal and vertical direction-of-arrival of water-borne seismic signals in the northern Philippine Sea

(Acoustical Society of America, 2013-10) Freeman, Simon E. ; D'Spain, Gerald L. ; Lynch, Stephen D. ; Stephen, Ralph A. ; Heaney, Kevin D. ; Murray, James J. ; Baggeroer, Arthur B. ; Worcester, Peter F. ; Dzieciuch, Matthew A. ; Mercer, James A.

Conventional and adaptive plane-wave beamforming with simultaneous recordings by large-aperture horizontal and vertical line arrays during the 2009 Philippine Sea Engineering Test (PhilSea09) reveal the rate of occurrence and the two-dimensional arrival structure of seismic phases that couple into the deep ocean. A ship-deployed, controlled acoustic source was used to evaluate performance of the horizontal array for a range of beamformer adaptiveness levels. Ninety T-phases from unique azimuths were recorded between Yeardays 107 to 119. T-phase azimuth and S-minus-P-phase time-of-arrival range estimates were validated using United States Geological Survey seismic monitoring network data. Analysis of phases from a seismic event that occurred on Yearday 112 near the east coast of Taiwan approximately 450 km from the arrays revealed a 22° clockwise evolution of T-phase azimuth over 90 s. Two hypotheses to explain such evolution—body wave excitation of multiple sources or in-water scattering—are presented based on T-phase origin sites at the intersection of azimuthal great circle paths and ridge/coastal bathymetry. Propagation timing between the source, scattering region, and array position suggests the mechanism behind the evolution involved scattering of the T-phase from the Ryukyu Ridge and a T-phase formation/scattering location estimation error of approximately 3.2 km.
Solutions to range-dependent benchmark problems by the finite-difference method

(Acoustical Society of America, 1990-04) Stephen, Ralph A.

An explicit second-order finite-difference scheme has been used to solve the elastic-wave equation in the time domain. Solutions are presented for the perfect wedge, the lossless penetrable wedge, and the plane parallel waveguide that have been proposed as benchmarks by the Acoustical Society of America. Good agreement with reference solutions is obtained if the media is discretized at 20 gridpoints per wavelength. There is a major discrepancy (up to 20 dB) in reference-source level because the reference solutions are normalized to the source strength at 1 m in the model, but the finite-difference solutions are normalized to the source strength at 1 m in a homogeneous medium. The finite-difference method requires computational times between 10 and 20 h on a super minicomputer without an array processor. The method has the advantage of providing phase information and, when run for a pulse source, of providing insight into the evolution of the wave field and energy partitioning. More complex models, including velocity gradients and strong lateral heterogeneities, can be solved with no additional computational effort. The method has also been formulated to include shear wave effects.
The North Pacific Acoustic Laboratory deep-water acoustic propagation experiments in the Philippine Sea

(Acoustical Society of America, 2013-10) Worcester, Peter F. ; Dzieciuch, Matthew A. ; Mercer, James A. ; Andrew, Rex K. ; Dushaw, Brian D. ; Baggeroer, Arthur B. ; Heaney, Kevin D. ; D'Spain, Gerald L. ; Colosi, John A. ; Stephen, Ralph A. ; Kemp, John N. ; Howe, Bruce M. ; Van Uffelen, Lora J. ; Wage, Kathleen E.

A series of experiments conducted in the Philippine Sea during 2009–2011 investigated deep-water acoustic propagation and ambient noise in this oceanographically and geologically complex region: (i) the 2009 North Pacific Acoustic Laboratory (NPAL) Pilot Study/Engineering Test, (ii) the 2010–2011 NPAL Philippine Sea Experiment, and (iii) the Ocean Bottom Seismometer Augmentation of the 2010–2011 NPAL Philippine Sea Experiment. The experimental goals included (a) understanding the impacts of fronts, eddies, and internal tides on acoustic propagation, (b) determining whether acoustic methods, together with other measurements and ocean modeling, can yield estimates of the time-evolving ocean state useful for making improved acoustic predictions, (c) improving our understanding of the physics of scattering by internal waves and spice, (d) characterizing the depth dependence and temporal variability of ambient noise, and (e) understanding the relationship between the acoustic field in the water column and the seismic field in the seafloor. In these experiments, moored and ship-suspended low-frequency acoustic sources transmitted to a newly developed distributed vertical line array receiver capable of spanning the water column in the deep ocean. The acoustic transmissions and ambient noise were also recorded by a towed hydrophone array, by acoustic Seagliders, and by ocean bottom seismometers.
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.
Three-dimensional numerical modeling of bottom-diffracted surface-reflected arrivals in the North Pacific [poster]

( 2015-12-15) Stephen, Ralph A. ; Udovydchenkov, Ilya A. ; Bolmer, S. Thompson ; Komatitsch, Dimitri ; Tromp, Jeroen ; Casarotti, Emanuele ; Xie, Zhinan ; Worcester, Peter F.

Bottom-diffracted surface-reflected (BDSR) arrivals were first identified in the 2004 Long-range Ocean Acoustic Propagation Experiment (Stephen et al, 2013, JASA, v.134, p.3307-3317). The BDSR mechanism provides a means for acoustic signals and noise from distant sources to appear with significant strength on the deep seafloor. At depths deeper than the conjugate depth ambient noise and PE- predicted arrivals are sufficiently quiet that BDSR paths, scattered from small seamounts, can be the largest amplitude arrivals observed. The Ocean Bottom Seismometer Augmentation in the North Pacific (OBSANP) Experiment in June-July 2013 was designed to further define the characteristics of the BDSRs and to understand the conditions under which BDSRs are excited and propagate. The reciprocal of the BDSR mechanism also plays a role in T-phase excitation. To further understand the BDSR mechanism, the SPECFEM3D code was extended to handle high-frequency, deep water bottom scattering problems with actual bathymetry and a typical sound speed profile in the water column. The model size is 38km x 27km x 6.5km. The source is centered at 10Hz with a 5Hz bandwidth. Work supported by NSF and ONR.
Ocean Bottom Seismometer Augmentation of the Philippine Sea Experiment (OBSAPS) cruise report

(Woods Hole Oceanographic Institution, 2011-09) Stephen, Ralph A. ; Kemp, John N. ; McPeak, Sean P. ; Bolmer, S. Thompson ; Carey, Scott ; Aaron, Ernie ; Campbell, Richard L. ; Moskovitz, Brianne ; Calderwood, John ; Cohen, Ben ; Worcester, Peter F. ; Dzieciuch, Matthew A.

The Ocean Bottom Seismometer Augmentation to the Philippine Sea Experiment (OBSAPS, April-May, 2011, R/V Revelle) 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" 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). These "deep seafloor arrivals" (DSFA) are a new class of arrivals in ocean acoustics possibly associated with seafloor interface waves. The OBSAPS cruise had three major research goals: a) identification and analysis of DSFAs 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 as a function of frequency. On OBSAPS we deployed a fifteen element VLA from 12 to 852m above the seafloor, four short-period OBSs and two long-period OBSs and carried out an 11.5day transmission program using a J15-3 acoustic source.
User's guide for PLOT_FINDIF

(Woods Hole Oceanographic Institution, 2006-02) Bolmer, S. Thompson ; Stephen, Ralph A.

PLOT_FINDIF is a MATLAB script which is used to plot output from the Woods Hole Oceanographic Institution (WHOI) Time Domain Finite Difference (TDFD) program called "Geoacoustic_TDFD" Both Geoacoustic_TDFD and PLOT_FINDIF are available from the ONR Ocean Acoustics Library (http://www.hlsresearch.com/oalib/). This script will plot both the snapshot and time series output from Geoacoustic_TDFD. To run this script you must have a MATLAB license and the complete suite of 32 m-files contained in the PLOT_FINDIF package. This code has been tested with MATLAB versions 6 and 7.
Bottom array specifications for the low frequency acoustic seismic experiment (LFASE)

(Woods Hole Oceanographic Institution, 1991-10) Bocconcelli, Alessandro ; Berteaux, Henri O. ; Stephen, Ralph A. ; Spiess, Fred ; Spiess, Fred ; Craig, Harmon ; Spiess, Fred

The Low Frequency Acoustic Seismic Experiment (LFASE) was conducted to measure sound propagation and ambient noise above, at and below the sea floor. To this end an array consisting of four geophone nodes was introduced into a DSDP borehole. These seismic sensors were clamped inside the borehole at various depths below the ocean floor. The geophone array was connected by an electromechanical cable to a bottom reentry structure (BCU frame) housing the Data Recording Unit (DRU), the Data Telemetry Unit (DTU), the Bottom Control Unit (BCU) and the power supply.
LFASE data processing system overview

(Woods Hole Oceanographic Institution, 1990-06) Little, William S. ; Bolmer, S. Thompson ; Stephen, Ralph A.

This technical report provides an overview of the LFASE data processing system. This software system is made up of over twenty-five programs which are used to acquire, reduce, and analyze acoustic seismic data collected during the Low Frequency Acoustic Seismic Experiment (LFASE) (Stephen et al, 1989; Koelsch et al, 1990). This report is directed at scientific and engineering personnel who wish to understand the overall LFASE data processing system as well as the individual processing procedures which are utilized during each stage of data reduction and interpretation. The report is also directed at programmers, data processors, technicians, and other individuals who plan to work with LFASE programs and data.
Mid-ocean microseisms

(American Geophysical Union, 2005-04-20) Bromirski, Peter D. ; Duennebier, Fred K. ; Stephen, Ralph A.

The Hawaii-2 Observatory (H2O) is an excellent site for studying the source regions and propagation of microseisms since it is located far from shorelines and shallow water. During Leg 200 of the Ocean Drilling Program, the officers of the JOIDES Resolution took wind and wave measurements for comparison with double-frequency (DF) microseism data collected at nearby H2O. The DF microseism band can be divided into short period and long period bands, SPDF and LPDF, respectively. Comparison of the ship’s weather log with the seismic data in the SPDF band from about 0.20 to 0.45 Hz shows a strong correlation of seismic amplitude with wind speed and direction, implying that the energy reaching the ocean floor is generated locally by ocean gravity waves. Near-shore land seismic stations see similar SPDF spectra, also generated locally by wind seas. At H2O, SPDF microseism amplitudes lag sustained changes in wind speed and direction by several hours, with the lag increasing with wave period. This lag may be associated with the time necessary for the development of opposing seas for DF microseism generation. Correlation of swell height above H2O with the LPDF band from 0.085 to 0.20 Hz is often poor, implying that a significant portion of this energy originates at distant locations. Correlation of the H2O seismic data with NOAA buoy data, with hindcast wave height data from the North Pacific, and with seismic data from mainland and island stations, defines likely source areas of the LPDF signals. Most of the LPDF energy at H2O appears to be generated by high amplitude storm waves impacting long stretches of coastline nearly simultaneously, and the Hawaiian Islands appear to be a significant source of LPDF energy in the North Pacific when waves arrive from particular directions. The highest DF levels observed at mid-ocean site H2O occur in the SPDF band when two coincident nearby storm systems develop. Mid-ocean generated DF microseisms are not observed at interior continental sites, indicating high attenuation of these signals. At near-coastal seismic stations, both SPDF and LPDF microseism levels are generally dominated by local generation at nearby shorelines.
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.
A sea floor winch system for wire line re-entry of deep sea boreholes

(Woods Hole Oceanographic Institution, 1990-12) Berteaux, Henri O. ; Bocconcelli, Alessandro ; Koelsch, Donald E. ; Stephen, Ralph A.

The following report describes the scientific motivations for the use of a Sea Floor Winch System for Wireline Re-entry of Deep Sea Boreholes and presents a conceptual design for the winch system.
Deep seafloor arrivals : an unexplained set of arrivals in long-range ocean acoustic propagation

(Acoustical Society of America, 2009-08) Stephen, Ralph A. ; Bolmer, S. Thompson ; Dzieciuch, Matthew A. ; Worcester, Peter F. ; Andrew, Rex K. ; Buck, Linda J. ; Mercer, James A. ; Colosi, John A. ; Howe, Bruce M.

Receptions, from a ship-suspended source (in the band 50–100 Hz) to an ocean bottom seismometer (about 5000 m depth) and the deepest element on a vertical hydrophone array (about 750 m above the seafloor) that were acquired on the 2004 Long-Range Ocean Acoustic Propagation Experiment in the North Pacific Ocean, are described. The ranges varied from 50 to 3200 km. In addition to predicted ocean acoustic arrivals and deep shadow zone arrivals (leaking below turning points), “deep seafloor arrivals,” that are dominant on the seafloor geophone but are absent or very weak on the hydrophone array, are observed. These deep seafloor arrivals are an unexplained set of arrivals in ocean acoustics possibly associated with seafloor interface waves.
Teleseismic earthquake wavefields observed on the ross ice shelf

(Cambridge University Press, 2020-10-14) Baker, Michael G. ; Aster, Richard C. ; Wiens, Douglas A. ; Nyblade, Andrew A. ; Bromirski, Peter D. ; Gerstoft, Peter ; Stephen, Ralph A.

Observations of teleseismic earthquakes using broadband seismometers on the Ross Ice Shelf (RIS) must contend with environmental and structural processes that do not exist for land-sited seismometers. Important considerations are: (1) a broadband, multi-mode ambient wavefield excited by ocean gravity wave interactions with the ice shelf; (2) body wave reverberations produced by seismic impedance contrasts at the ice/water and water/seafloor interfaces and (3) decoupling of the solid Earth horizontal wavefield by the sub-shelf water column. We analyze seasonal and geographic variations in signal-to-noise ratios for teleseismic P-wave (0.5–2.0 s), S-wave (10–15 s) and surface wave (13–25 s) arrivals relative to the RIS noise field. We use ice and water layer reverberations generated by teleseismic P-waves to accurately estimate the sub-station thicknesses of these layers. We present observations consistent with the theoretically predicted transition of the water column from compressible to incompressible mechanics, relevant for vertically incident solid Earth waves with periods longer than 3 s. Finally, we observe symmetric-mode Lamb waves generated by teleseismic S-waves incident on the grounding zones. Despite their complexity, we conclude that teleseismic coda can be utilized for passive imaging of sub-shelf Earth structure, although longer deployments relative to conventional land-sited seismometers will be necessary to acquire adequate data.
Broadband borehole seismic system integration tests : report of the system integration tests at MPL/SIO

(Woods Hole Oceanographic Institution, 1998) Goldsborough, Robert G. ; Austin, Gary ; Bolmer, S. Thompson ; Jabson, David M. ; Jonke, Patrick ; Gould, Matthew R. ; Hildebrand, John A. ; Hollinshead, C. B. ; Offield, Glen ; Orcutt, John ; Peal, Kenneth R. ; Spiess, Fred N. ; Stephen, Ralph A. ; Vernon, Frank L. ; Willoughby, David F. ; Zimmerman, Richard

This report describes a series of tests performed at SIO/MPL, Point Lorna the week of 17 November 1997 designed to achieve integration of the Broadband Borehole Seismic System (BBBSS) in preparation for the OSN Pilot Experiment cruise on RN Thompson during January 1997. Representatives from all groups were present (see appendix A), with their respective parts of the system and support equipment. It was anticipated that these tests would result in the complete integration of the various components of the borehole seismometer system in preparation for the January cruise. The system would be assembled and tested following a plan (see appendix C) that would culminate in the fully integrated borehole seismometer being wet tested off the MPL pier.
Modeling sea surface scattering by the time-domain finite-difference method

(Acoustical Society of America, 1996-10) Stephen, Ralph A.

A numerical scattering chamber based on the time-domain finite-difference solution of the two-way elastic wave equation is applied to a sea surface scattering problem, and excellent agreement is obtained in amplitude and phase with a reference solution obtained by an integral equation method. The sea surface roughness is one representation of a Pierson–Moskowitz spectrum for a wind speed of 15 m/s. The incident field is a 400-Hz continuous wave generated by a Gaussian tapered vertical array. This problem demonstrates a number of issues in numerical modeling of wave scattering. The spreading of Gaussian beams, even in homogeneous media, creates an asymmetry in the insonification of the surface footprint or scattering area. Because of beamspreading, Gaussian tapered vertical arrays do not generate Gaussian beams. Scattering from a rough, free, fluid surface can be accurately solved with careful treatment of the numerical boundary representing the free surface. Continuous wave (cw) scattering problems can be solved in the time domain. For the second-order, explicit, staggered finite-difference formulation used in this study, a spatial sampling of 20 points per acoustic wavelength was necessary for acceptable grid dispersion. However, to correctly compute the scattered field for the test model, it was sufficient to specify the free surface at a spatial sampling of only ten points per acoustic wavelength.
SeisCORK meeting report

(Woods Hole Oceanographic Institution, 2006-02) Stephen, Ralph A. ; Pettigrew, Tom ; Becker, Keir ; Spiess, Fred N.

The purpose of this meeting was to explore design options to simultaneously acquire borehole seismic data and hydro-geological data (pressure, temperature, fluid sampling and microbiological sampling) on a single CORK system. The scientific focus was to add a seismic component to the Juan de Fuca Hydrogeology program. By permanently installing a sensor string in the borehole our goal was to enable: l) time-lapse VSP's and offset VSP's with sufficient data quality to study amplitude versus offset, shear wave anisotropy, and lateral heterogeneity; 2) monitoring of micro- and nano- earthquake activity around the site for correlation with pressure transients. Because of the difficulty in ensuring adequate coupling through multiple casing strings we concluded that it was impractical to install the vertical seismic array with 10m spacing (50-60 nodes) that would be necessary for VSP's and time-lapse VSP's. We did describe a scenario for a vertical seismic array with approximately 100m spacing (5-6 nodes) that could be used for offset-VSP's and seismic monitoring. This uses some unique technology and involves two seismic strings: one in the annulus between the 4- 1/2" and 10-3/4" casings and one in the middle of the 4-1/2" casing.
User's guide for FINDIF at SACLANTCEN

(Woods Hole Oceanographic Institution, 1992-05) Stephen, Ralph A.

FINDIF solves the elastic wave equation for a line source in two dimensions by the finite difference method (Virieux,1986; Stephen,1988b). The solution is carried out in the time domain for either pulse or CW sources. Arbitrary distributions of compressional velocity, shear velocity and density (including fluids and solids) can be defined on the finite difference grid. All compressional waves, shear waves, interface waves and evanescent waves are included as are all conversions between wave types. Multiple forward and backward scattering is automatically treated
Three-dimensional bottom diffraction in the North Pacific

(Acoustical Society of America, 2019-09-30) Stephen, Ralph A. ; Bolmer, S. Thompson ; Worcester, Peter F. ; Dzieciuch, Matthew A. ; Udovydchenkov, Ilya A.

A significant aspect of bottom-interaction in deep water acoustic propagation, from point sources to point receivers, is the diffraction (or scattering) of energy from discrete seafloor locations along repeatable, deterministic paths in three-dimensions. These bottom-diffracted surface-reflected (BDSR) paths were first identified on the North Pacific acoustic laboratory experiment in 2004 (NPAL04) for a diffractor located on the side of a small seamount. On the adjacent deep seafloor, ambient noise and propagation in the ocean sound channel were sufficiently quiet that the BDSRs were the dominant arrival. The ocean bottom seismometer augmentation in the North Pacific (OBSANP) experiment in June–July 2013 studied BDSRs at the NPAL04 site in more detail. BDSRs are most readily identified by the arrival time of pulses as a function of range to the receiver for a line of transmissions. The diffraction points for BDSRs occur on the relatively featureless deep seafloor as well as on the sides of small seamounts. Although the NPAL04 and OBSANP experiments had very different geometries the same diffractor location is consistent with observed arrivals in both experiments within the resolution of the analysis. On OBSANP the same location excites BDSRs for 77.5, 155, and 310 Hz transmissions.
Notes for Geoacoustic_TDFD

(Woods Hole Oceanographic Institution, 2006-02) Stephen, Ralph A. ; Bolmer, S. Thompson

These notes were written to help users run the WHOI TDFD (Time Domain Finite Difference) elastic wave equation code that was prepared for distribution through the ONR Ocean Acoustics Library (http://www.hlsresearch.com/oalib/). The code and documentation are based on materials that were developed for a Numerical Wave Propagation class given at MIT in the Fall of 2000. The code used is the full two-dimensional time-domain finite-difference code developed at WHOI over the past 25 years, but in order to reduce the number of variables to a manageable size, we consider a two dimensional, isotropic problem with fixed parameters in time and space. For example, the source waveform in time for both beam and point sources is a RICKER wavelet, time units have been normalized to periods (defined at the peak frequency for pressure in water), space units have been normalized to water speed and density of 1! .5km/sec and 1000kg/m3) and the domain size has been fixed at 72 x 12 water wavelengths.

Stephen Ralph A.

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