Worcester Peter F.

No Thumbnail Available
Last Name
Worcester
First Name
Peter F.
ORCID
0000-0001-8758-6849

Filters

21
Reset filters

Settings

Search Results

Now showing 1 - 20 of 21
  • Article
    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.
  • Article
    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.
  • Article
    Modal analysis of the range evolution of broadband wavefields in the North Pacific Ocean : low mode numbers
    (Acoustical Society of America, 2012-06) Udovydchenkov, Ilya A. ; Brown, Michael G. ; Duda, Timothy F. ; Mercer, James A. ; Andrew, Rex K. ; Worcester, Peter F. ; Dzieciuch, Matthew A. ; Howe, Bruce M. ; Colosi, John A.
    The results of mode-processing measurements of broadband acoustic wavefields made in the fall of 2004 as part of the Long-Range Ocean Acoustic Propagation Experiment (LOAPEX) in the eastern North Pacific Ocean are reported here. Transient wavefields in the 50–90 Hz band that were recorded on a 1400 -m long 40 element vertical array centered near the sound channel axis are analyzed. This array was designed to resolve low-order modes. The wavefields were excited by a ship-suspended source at seven ranges, between approximately 50 and 3200 km, from the receiving array. The range evolution of broadband modal arrival patterns corresponding to fixed mode numbers (“modal group arrivals”) is analyzed with an emphasis on the second (variance) and third (skewness) moments. A theory of modal group time spreads is described, emphasizing complexities associated with energy scattering among low-order modes. The temporal structure of measured modal group arrivals is compared to theoretical predictions and numerical simulations. Theory, simulations, and observations generally agree. In cases where disagreement is observed, the reasons for the disagreement are discussed in terms of the underlying physical processes and data limitations.
  • 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.
  • Presentation
    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.
  • Technical Report
    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.
  • Article
    Weakly dispersive modal pulse propagation in the North Pacific Ocean
    (Acoustical Society of America, 2013-10) Udovydchenkov, Ilya A. ; Brown, Michael G. ; Duda, Timothy F. ; Worcester, Peter F. ; Dzieciuch, Matthew A. ; Mercer, James A. ; Andrew, Rex K. ; Howe, Bruce M. ; Colosi, John A.
    The propagation of weakly dispersive modal pulses is investigated using data collected during the 2004 long-range ocean acoustic propagation experiment (LOAPEX). Weakly dispersive modal pulses are characterized by weak dispersion- and scattering-induced pulse broadening; such modal pulses experience minimal propagation-induced distortion and are thus well suited to communications applications. In the LOAPEX environment modes 1, 2, and 3 are approximately weakly dispersive. Using LOAPEX observations it is shown that, by extracting the energy carried by a weakly dispersive modal pulse, a transmitted communications signal can be recovered without performing channel equalization at ranges as long as 500 km; at that range a majority of mode 1 receptions have bit error rates (BERs) less than 10%, and 6.5% of mode 1 receptions have no errors. BERs are estimated for low order modes and compared with measurements of signal-to-noise ratio (SNR) and modal pulse spread. Generally, it is observed that larger modal pulse spread and lower SNR result in larger BERs.
  • Article
    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.
  • Article
    Temporal and spatial dependence of a yearlong record of sound propagation from the Canada Basin to the Chukchi Shelf
    (Acoustical Society of America, 2020-09-23) Ballard, Megan S. ; Badiey, Mohsen ; Sagers, Jason D. ; Colosi, John A. ; Turgut, Altan ; Pecknold, Sean ; Lin, Ying-Tsong ; Proshutinsky, Andrey ; Krishfield, Richard A. ; Worcester, Peter F. ; Dzieciuch, Matthew A.
    The Pacific Arctic Region has experienced decadal changes in atmospheric conditions, seasonal sea-ice coverage, and thermohaline structure that have consequences for underwater sound propagation. To better understand Arctic acoustics, a set of experiments known as the deep-water Canada Basin acoustic propagation experiment and the shallow-water Canada Basin acoustic propagation experiment was conducted in the Canada Basin and on the Chukchi Shelf from summer 2016 to summer 2017. During the experiments, low-frequency signals from five tomographic sources located in the deep basin were recorded by an array of hydrophones located on the shelf. Over the course of the yearlong experiment, the surface conditions transitioned from completely open water to fully ice-covered. The propagation conditions in the deep basin were dominated by a subsurface duct; however, over the slope and shelf, the duct was seen to significantly weaken during the winter and spring. The combination of these surface and subsurface conditions led to changes in the received level of the sources that exceeded 60 dB and showed a distinct spacio-temporal dependence, which was correlated with the locations of the sources in the basin. This paper seeks to quantify the observed variability in the received signals through propagation modeling using spatially sparse environmental measurements.
  • Article
    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.
  • Article
    Multipurpose acoustic networks in the integrated Arctic Ocean observing system
    (Arctic Institute of North America, 2015) Mikhalevsky, Peter N. ; Sagen, Hanne ; Worcester, Peter F. ; Baggeroer, Arthur B. ; Orcutt, John A. ; Moore, Sue E. ; Lee, Craig M. ; Vigness-Raposa, Kathleen J. ; Freitag, Lee E. ; Arrott, Matthew ; Atakan, Kuvvet ; Beszczynska-Möller, Agnieszka ; Duda, Timothy F. ; Dushaw, Brian D. ; Gascard, Jean-Claude ; Gavrilov, Alexander N. ; Keers, Henk ; Morozov, Andrey K. ; Munk, Walter H. ; Rixen, Michel ; Sandven, Stein ; Skarsoulis, Emmanuel ; Stafford, Kathleen M. ; Vernon, Frank L. ; Yuen, Mo Yan
    The dramatic reduction of sea ice in the Arctic Ocean will increase human activities in the coming years. This activity will be driven by increased demand for energy and the marine resources of an Arctic Ocean accessible to ships. Oil and gas exploration, fisheries, mineral extraction, marine transportation, research and development, tourism, and search and rescue will increase the pressure on the vulnerable Arctic environment. Technologies that allow synoptic in situ observations year-round are needed to monitor and forecast changes in the Arctic atmosphere-ice-ocean system at daily, seasonal, annual, and decadal scales. These data can inform and enable both sustainable development and enforcement of international Arctic agreements and treaties, while protecting this critical environment. In this paper, we discuss multipurpose acoustic networks, including subsea cable components, in the Arctic. These networks provide communication, power, underwater and under-ice navigation, passive monitoring of ambient sound (ice, seismic, biologic, and anthropogenic), and acoustic remote sensing (tomography and thermometry), supporting and complementing data collection from platforms, moorings, and vehicles. We support the development and implementation of regional to basin-wide acoustic networks as an integral component of a multidisciplinary in situ Arctic Ocean observatory.
  • Technical Report
    Analysis of Deep Seafloor Arrivals observed on NPAL04
    (Woods Hole Oceanographic Institution, 2012-12) Stephen, Ralph A. ; Bolmer, S. Thompson ; Udovydchenkov, Ilya A. ; Dzieciuch, Matthew A. ; Worcester, Peter F. ; Andrew, Rex K. ; Mercer, James A. ; Colosi, John A. ; Howe, Bruce M.
    This report gives an overview of the analysis that was done on Deep Seafloor Arrivals since they were initially presented in Stephen et al (2009). All of the NPAL04/LOAPEX (North Pacific Acoustic Laboratory, 2004/ Long Range Ocean Acoustic Propagation Experiment) data on three ocean bottom seismometers (OBSs) at ~5,000m depth and the deepest element of the deep vertical line array (DVLA) at 4250m depth has been analyzed. A distinctive pattern of late arrivals was observed on the three OBSs for transmissions from T500 to T2300. The delays of these arrivals with respect to the parabolic equation predicted (PEP) path were the same for all ranges from 500 to 2300km, indicating that the delay was introduced near the receivers. At 500km range the same arrival was observed throughout the water column on the DVLA. We show that arrivals in this pattern converted from a PEP path to a bottom-diffracted surface reflected (BDSR) path at an off-geodesic seamount.
  • Technical Report
    OBSANP data acquisition system : operator's manual and system overview
    (Woods Hole Oceanographic Institution, 2013-10) McPeak, Sean P. ; D'Spain, Gerald L. ; Stephen, Ralph A. ; von der Heydt, Keith ; Worcester, Peter F.
    On the Ocean Bottom Seismometer Augmentation in the North Pacific Experiment (OBSANP, June-July, 2013, R/V Melville), a VLA and twelve OBSs were deployed to listen to an active acoustic source, a J15-3. This report describes the hardware and software used to control and record the acoustic transmissions from the source. Some significant features of the system are: 1) The system transmits general user-defined source functions, such as M-sequences (.SIO files). 2) In addition to controlling the source waveform, the system also records six real-time channels in binary files with user-selectable lengths: the monitor hydrophone mounted near the source, the power amplifier voltage and current, the depth of the source, Vref signal driving the power amplifiers and an IRIG-B time reference. Files are output in .AUV format with a precision GPSbased time stamp in the file name. 3) The transmission start time along with ADC and DAC sample rates are disciplined to GPS time. 4) A convenient, Labview based, user interface provides real-time source control and monitoring. 5) The software provides parsing and logging of gyro and GPS NMEA sentences. The system, which was based on an earlier system from Scripps MPL, worked well on OBSANP and is available for future projects.
  • Presentation
    Microseism noise in the Philippine Sea [poster] 
    ( 2013-12) Stephen, Ralph A. ; Bromirski, Peter D. ; Gerstoft, Peter ; Worcester, Peter F.
    Microseism noise, generated by wave-wave interaction of ocean surface gravity waves and peaking between 0.1 and 0.5Hz, is the largest amplitude, continuous (acceleration) vibration on earth in the seismic band from 0.0001 to 10Hz. Although microseisms have been studied extensively over the past seventy years, significant issues remain regarding their excitation and propagation. In a recent paper Bromirski et al (JGR, 2013) point out that there is an important distinction between microseisms generated in deep and shallow water. Most microseisms observed on continents are generated in shallow water near coastlines. Microseisms generated in deep water are observed on seafloor sensors but do not transition readily to continents. The Ocean Bottom Seismometer Augmentation to the Philippine Sea (OBSAPS) Experiment has provided a unique opportunity to study the excitation and propagation of microseism noise (from 0.05 to 1.0Hz) in the oceans by combining ocean bottom seismometer observations with co-located and simultaneous observations of the acoustic field in the ocean. The depth dependence of the acoustic field in the ocean is used to distinguish between ocean acoustic modes and acoustic and elastic pseudo-Rayleigh waves as propagation mechanisms for microseism energy.
  • Article
    Bottom interacting sound at 50 km range in a deep ocean environment
    (Acoustical Society of America, 2012-10) Udovydchenkov, Ilya A. ; Stephen, Ralph A. ; Duda, Timothy F. ; Bolmer, S. Thompson ; Worcester, Peter F. ; Dzieciuch, Matthew A. ; Mercer, James A. ; Andrew, Rex K. ; Howe, Bruce M.
    Data collected during the 2004 Long-range Ocean Acoustic Propagation Experiment provide absolute intensities and travel times of acoustic pulses at ranges varying from 50 to 3200 km. In this paper a subset of these data is analyzed, focusing on the effects of seafloor reflections at the shortest transmission range of approximately 50 km. At this range bottom-reflected (BR) and surface-reflected, bottom-reflected energy interferes with refracted arrivals. For a finite vertical receiving array spanning the sound channel axis, a high mode number energy in the BR arrivals aliases into low mode numbers because of the vertical spacing between hydrophones. Therefore, knowledge of the BR paths is necessary to fully understand even low mode number processes. Acoustic modeling using the parabolic equation method shows that inclusion of range-dependent bathymetry is necessary to get an acceptable model-data fit. The bottom is modeled as a fluid layer without rigidity, without three dimensional effects, and without scattering from wavelength-scale features. Nonetheless, a good model-data fit is obtained for sub-bottom properties estimated from the data.
  • Article
    Wind sea behind a cold front and deep ocean acoustics
    (American Meteorological Society, 2016-05-10) Farrell, W. E. ; Berger, Jonathan ; Bidlot, Jean-Raymond ; Dzieciuch, Monika ; Munk, Walter H. ; Stephen, Ralph A. ; Worcester, Peter F.
    A rapid and broadband (1 h, 1 < f < 400 Hz) increase in pressure and vertical velocity on the deep ocean floor was observed on seven instruments comprising a 20-km array in the northeastern subtropical Pacific. The authors associate the jump with the passage of a cold front and focus on the 4- and 400-Hz spectra. At every station, the time of the jump is consistent with the front coming from the northwest. The apparent rate of progress, 10–20 km h−1 (2.8–5.6 m s−1), agrees with meteorological observations. The acoustic radiation below the front is modeled as arising from a moving half-plane of uncorrelated acoustic dipoles. The half-plane is preceded by a 10-km transition zone, over which the radiator strength increases linearly from zero. With this model, the time derivative of the jump at a station yields a second and independent estimate of the front’s speed, 8.5 km h−1 (2.4 m s−1). For the 4-Hz spectra, the source physics is taken to be Longuet-Higgins radiation. Its strength depends on the quantity , where Fζ is the wave amplitude power spectrum and I the overlap integral. Thus, the 1-h time constant observed in the bottom data implies a similar time constant for the growth of the wave field quantity behind the front. The spectra at 400 Hz have a similar time constant, but the jump occurs 25 min later. The implications of this difference for the source physics are uncertain.
  • Article
    A deep ocean acoustic noise floor, 1–800 Hz
    (Acoustical Society of America, 2018-02-26) Berger, Jonathan ; Bidlot, Jean-Raymond ; Dzieciuch, Matthew A. ; Farrell, W. E. ; Worcester, Peter F. ; Stephen, Ralph A.
    The ocean acoustic noise floor (observed when the overhead wind is low, ships are distant, and marine life silent) has been measured on an array extending up 987 m from 5048 m depth in the eastern North Pacific, in what is one of only a few recent measurements of the vertical noise distribution near the seafloor in the deep ocean. The floor is roughly independent of depth for 1–6 Hz, and the slope (∼ f−7) is consistent with Longuet-Higgins radiation from oppositely-directed surface waves. Above 6 Hz, the acoustic floor increases with frequency due to distant shipping before falling as ∼ f−2 from 40 to 800 Hz. The noise floor just above the seafloor is only about 5 dB greater than during the 1975 CHURCH OPAL experiment (50–200 Hz), even though these measurements are not subject to the same bathymetric blockage. The floor increases up the array by roughly 15 dB for 40–500 Hz. Immediately above the seafloor, the acoustic energy is concentrated in a narrow, horizontal beam that narrows as f−1 and has a beam width at 75 Hz that is less than the array resolution. The power in the beam falls more steeply with frequency than the omnidirectional spectrum.
  • Article
    Deep seafloor arrivals in long range ocean acoustic propagation
    (Acoustical Society of America, 2013-10) Stephen, Ralph A. ; Bolmer, S. Thompson ; Udovydchenkov, Ilya A. ; Worcester, Peter F. ; Dzieciuch, Matthew A. ; Andrew, Rex K. ; Mercer, James A. ; Colosi, John A. ; Howe, Bruce M.
    Ocean bottom seismometer observations at 5000 m depth during the long-range ocean acoustic propagation experiment in the North Pacific in 2004 show robust, coherent, late arrivals that are not readily explained by ocean acoustic propagation models. These “deep seafloor” arrivals are the largest amplitude arrivals on the vertical particle velocity channel for ranges from 500 to 3200 km. The travel times for six (of 16 observed) deep seafloor arrivals correspond to the sea surface reflection of an out-of-plane diffraction from a seamount that protrudes to about 4100 m depth and is about 18 km from the receivers. This out-of-plane bottom-diffracted surface-reflected energy is observed on the deep vertical line array about 35 dB below the peak amplitude arrivals and was previously misinterpreted as in-plane bottom-reflected surface-reflected energy. The structure of these arrivals from 500 to 3200 km range is remarkably robust. The bottom-diffracted surface-reflected mechanism provides a means for acoustic signals and noise from distant sources to appear with significant strength on the deep seafloor.
  • Article
    A test of basin-scale acoustic thermometry using a large-aperture vertical array at 3250-km range in the eastern North Pacific Ocean
    (Acoustical Society of America, 1999-06) Worcester, Peter F. ; Cornuelle, Bruce D. ; Dzieciuch, Matthew A. ; Munk, Walter H. ; Howe, Bruce M. ; Mercer, James A. ; Spindel, Robert C. ; Colosi, John A. ; Metzger, Kurt ; Birdsall, Theodore G. ; Baggeroer, Arthur B.
    Broadband acoustic signals were transmitted during November 1994 from a 75-Hz source suspended near the depth of the sound-channel axis to a 700-m long vertical receiving array approximately 3250 km distant in the eastern North Pacific Ocean. The early part of the arrival pattern consists of raylike wave fronts that are resolvable, identifiable, and stable. The later part of the arrival pattern does not contain identifiable raylike arrivals, due to scattering from internal-wave-induced sound-speed fluctuations. The observed ray travel times differ from ray predictions based on the sound-speed field constructed using nearly concurrent temperature and salinity measurements by more than a priori variability estimates, suggesting that the equation used to compute sound speed requires refinement. The range-averaged oceansound speed can be determined with an uncertainty of about 0.05 m/s from the observed ray travel times together with the time at which the near-axial acoustic reception ends, used as a surrogate for the group delay of adiabatic mode 1. The change in temperature over six days can be estimated with an uncertainty of about 0.006 °C. The sensitivity of the travel times to ocean variability is concentrated near the ocean surface and at the corresponding conjugate depths, because all of the resolved ray arrivals have upper turning depths within a few hundred meters of the surface.
  • Technical Report
    NPAL04 OBS data analysis part 1 : kinematics of deep seafloor arrivals
    (Woods Hole Oceanographic Institution, 2008-12) Stephen, Ralph A. ; Bolmer, S. Thompson ; Udovydchenkov, Ilya A. ; Worcester, Peter F. ; Dzieciuch, Matthew A. ; Van Uffelen, Lora J. ; Mercer, James A. ; Andrew, Rex K. ; Buck, Linda J. ; Colosi, John A. ; Howe, Bruce M.
    These notes provide supporting information for a JASA (Journal of the Acoustical Society of America) LttE (Letter to the Editor) manuscript, "Deep seafloor arrivals: A new class of arrivals in long-range ocean acoustic propagation" (Stephen et al., submitted). It addresses five issues raised by the co-authors: 1) incorrect processing for the time-compressed traces at T2300 and T3200 that appeared in an early version of the LttE (T2300, T3200 … refer to transmissions at 2300, 3200km etc from the DVLA (Deep Vertical Line Array)), 2) processing issues, including the trade-offs between coherent and incoherent stacking and corrections for the effects of moving sources and receivers and tidal currents (Doppler), 4) the distinction between "deep shadow zone arrivals", which occur below the turning points in Parabolic Equation (PE) models, and "deep seafloor arrivals", which appear dominantly on the Ocean Bottom Seismometer (OBS) but are either very weak or absent on the deepest element in the DVLA and do not coincide with turning points in the PE model (some of these OBS late arrivals occur after the finale region), 4) the role of surface-reflected bottomreflected (SRBR) paths in explaining the late arriving energy, and 5) generally reconciling the OBS analysis with work by other North Pacific Acoustic Laboratory (NPAL) investigators and Dushaw et al (1999).