Jersey Continental Shelf Research Articles

Measurements of attenuation of sound in marine sediments at low frequencies.

This paper describes an experimental technique for measuring the attenuation of sound in marine sediments at low frequencies. The method makes use of the signals from sub‐bottom reflectors that are received on a vertical hydrophone array at close ranges in shallow water environments. The signal path geometry is determined from an inversion of the travel time differences of the sea bottom and sub‐bottom reflected paths to estimate the sediment sound speed and the depth of the sub‐bottom reflector. The method is applied to data from experiments carried out at a site on the New Jersey continental shelf in the Shallow Water 06 experiment. The sediment type and the structure of the sediment column were ground truthed by independent measurements at the site. The chirp signal was transmitted in two frequency bands, from 100–900 and 1500–4500 Hz, and the data were match filtered to obtain the multipath signals at the array. A prominent signal from a sub‐bottom interface known as the R‐reflector was resolved in the data for each frequency band. Results are presented for the average attenuation over the depth to the reflector. [Work supported by ONR Ocean Acoustics.]

Seabed acoustics of a sand ridge on the New Jersey continental shelf

Acoustic measurements were made on a sand ridge on the New Jersey continental shelf. Data collected on two L arrays separated by 20 km from a single multi-frequency tow suggest small horizontal environmental variability. Values for the sound speed structure of the seabed are extracted by first applying a geo-acoustic inversion method to broadband and narrowband acoustic data from short-range sources. Then, a parabolic equation algorithm is used to properly include the bathymetry and sub-bottom layering. Finally, the frequency dependence of the seabed attenuation is inferred by optimizing the model fit to long-range transmission loss data in the 50-3000 Hz band.

Analysis of wind-driven ambient noise in a shallow water environment with a sandy seabed

On the New Jersey continental shelf ambient sound levels were recorded during tropical storm Ernesto that produced wind speeds up to 40 knots in early September 2006. The seabed at the position of the acoustic measurements can be approximately described as coarse sand. Differences between the ambient noise levels for the New Jersey shelf measurements and deep water reference measurements are modeled using both normal mode and ray methods. The analysis is consistent with a nonlinear frequency dependent seabed attenuation for the New Jersey site.

Bayesian geoacoustic inversion in a dynamic shallow water environment

This paper presents results for matched field Bayesian geoacoustic inversion of multitonal continuous wave data collected on the New Jersey continental shelf. To account for effects of significant spatial and temporal variation of the water column sound speed, the sound speed profile was represented by empirical orthogonal functions. Data error information for the inversion was estimated from multiple time windows of the data. Inversion results for the sediment sound speeds at three ranges are in excellent agreement with the ground truth.

Effects of internal waves on acoustic coherent communications during SW06

During the SW06 experiment conducted east off the New Jersey continental shelf from July to September of 2006, internal wave surface expression and water temperature profiles were recorded when phase shift keying signals (PSK) were transmitted to study internal wave effects on coherent underwater acoustic communications at low to mid frequency bands. The collected communications data are processed using time reversal multichannel combining followed by a decision feedback equalizer. Due to the presence of internal waves, the acoustic channels show a time-varying property. Continuous channel updates prior to time reversal combining are required to combat fast channel fluctuations. The communications performance in terms of output signal-to-noise ratio exhibits significant change due to the internal waves. Further, the internal wave effects on the performance of coherent acoustic communications exhibit a frequency- and depth- dependency. The choices of receiver parameters also can be affected by the presence of internal waves.

Mid-frequency bottom interaction measurements off the New Jersey continental shelf

Two kinds of bottom interaction measurements conducted in waters 80 m deep off the North American continental shelf as part of Shallow Water 06 (August, 2006) are discussed. In each, acoustic signals were recorded on two, colocated vertical line arrays of length 1.4 m, one at depth 25 m, and other at depth 50 m. The source was deployed at depth 40 m from the R/V Knorr that could either be positioned or towed at rate 0.1 m/s. The first, "bottom reflection" is interpreted as a measure of the modulus of the plane wave reflection coefficient as functions of frequency (1-20 kHz) and grazing angle associated with the discrete set of ranges (50-300 m). The second represents a "move-out" type measurement with source towed away from the receiver, and reflection at continuous angles associated with the 50-300 m range. Frequency range was 1-2 kHz, and as the source was 5 m off the bottom, spherical wave effects are investigated. Both measurements were carried out over the same four directions originating from the receiver, each separated by 90 degrees. Physical processes responsible for the observations in each case are discussed and modeled. [Supported by ONR, with partial support from Hanyang University.]

Broadband acoustic scattering from nonlinear internal solitary waves

High‐frequency broadband (150‐600 kHz) acoustic scattering techniques have been used to obtain high‐resolution images of nonlinear internal solitary waves (ISWs) propagating shoreward over the New Jersey continental shelf. In contrast to traditional single‐frequency measurements, broadband acoustic scattering allows the frequency spectrum of the scattering to be determined, which, in combination with scattering models and direct ground‐truthing measurements, allows regions in which the scattering is dominated by biology versus microstructure to be distinguished. Multiple ISWs were tracked and imaged acoustically at different stages of their generation, propagation, and dissipation. Simultaneous microstructure and zooplankton net sampling measurements were performed. Multiple scattering layers and distinct scattering patches were often associated to the ISWs, often exhibiting distinct frequency responses. Clear day/night differences in the scattering from the ISWs were observed due to the daily vertical migration of zooplankton. Though high turbulence levels are associated with ISWs, the strongest scattering returns were not always associated to the highest turbulence levels. Instead, the strongest scattering, at some frequencies, was sometimes associated to large temperature gradients. In regions in which the scattering was dominated by either microstructure or zooplankton, acoustic inversions have been performed for the dissipation rate of turbulent kinetic energy and zooplankton size and abundance.

Characterization of seabed geo‐acoustic structure for a shallow water environment

Full‐field inversion methods over large frequency bands and spatial scales attempt to exploit the sensitivity of the frequency and spatial dependence of the acoustic field of the properties to the seabed. A comprehensive set of broadband impulsive and narrowband tonal acoustic recordings in the 5‐3000 Hz band from two L‐arrays separated by 20 km were made in a shallow water area on the New Jersey continental shelf during the Shallow Water 2006 (SW06) experiment. In addition, surface ship of opportunity passages on both arrays were recorded along with noise during the passage of tropical storm Ernesto. Reverberation and transmission loss data in the 25‐9200 Hz band from explosive sources were also collected at a nearby site prior to the SW06 experiment. Numerous full‐field geo‐acoustic inversion methods and analyses of all these acoustic data with supporting range and azimuth‐dependent geo‐physical measurements are employed to characterize the seabed properties. For example, the analyses of these data permit inferences about the frequency‐dependence of the seabed sound speed and attenuation to be made. The overlapping data types act to reduce the intrinsic ambiguities associated with inversion, quantify the spatial variability, and serve as independent validation.

Assessing the significance of along‐strike variations of middle to late Miocene prograding clinoformal sequence geometries beneath the New Jersey continental shelf

ABSTRACTSeismic mapping of high‐resolution multichannel seismic profiles along the New Jersey margin illustrates how characteristics of middle–late Miocene clinoformal sequence boundaries (SBs) change markedly along strike in presumed response to local depositional and erosional processes. Most SBs converge from SW to NE, in part as a result of the influence of underlying basin morphology on accommodation space, but also in response to differential subsidence and presumed along‐strike variations in sediment supply from the adjacent margin. The curvature of clinoform breaks, historically viewed as marking palaeo‐shelf edges, is variable and such breaks are rarely sharp. Gently curved palaeo‐shelf/slope transitions cannot be assigned precise palaeobathymetric significance and probably instead reflect post‐depositional sediment reworking. The amount of erosional truncation landward of clinoform breaks varies significantly. Documented along‐strike variability in SB morphology occurs, even though middle–late Miocene palaeo‐shelf edges are nearly linear in plan view. Therefore, such linearity cannot be a product of uniform sedimentary processes and/or accumulation along strike, but instead reflects elongation of depocentres of originally variable cross‐sectional geometry, possibly with the assistance of along‐strike currents. The observed lateral geometric heterogeneity of Neogene sequences can exert profound and unwanted influences on the outcome of scientific drilling intended to calibrate seismic stratigraphic interpretations in the absence of sufficient three‐dimensional (3D) seismic control.

Chirp seismic reflection data on the New Jersey middle and outer shelf: The geologic response to 40,000 years of sea level change

Chirp seismic reflection data have been collected extensively on the New Jersey continental shelf in support of Office of Naval Research (ONR)-sponsored research. This composite data set includes surveys in 2001 and 2002 for the Geoclutter program, and in 2006 for the Shallow Water 06 program. Chirp uses a swept-frequency source (1–15 kHz), matched filtered to provide decimeter vertical accuracy and penetration up to 30 m in coarse-grained sediments. These data have been used to map out the complex shallow stratigraphy of this region, which includes: (1) a regional reflector formed about 40,000 years ago; (2) a layered shelf-edge wedge deposited during sea level fall; (3) fluvial channels formed at lowstand, and later filled with an estuarine sequence during sea level rise; (4) a ravinement surface created by shoreface erosion, which forms a lag of coarse material; and (5) the Holocene sand sheet, formed into sand ridges up to 10 m thick. Modern erosion has exposed many of the older units at or near the seafloor. Seabed sediments along acoustic propagation pathways are therefore highly heterogeneous. Vibracoring is planned for August, 2007. Analysis will include geoacoustic logging and geologic characterization. Preliminary results from these cores will also be presented.

Comparison of wind driven ambient noise in shallow and deep-water ocean environments

Measured sound pressure time series in shallow and deep-water ocean environments are used to investigate the character of wind-generated noise over a range of wind speed of 2–40 knots and a frequency band of 10–3,000 Hz. In 2006, acoustic data were collected during tropical storm Ernesto off the New Jersey continental shelf in shallow (70 m) water. Ambient noise from these data is compared to deep-water measurements at several locations in the Northwest and the Northeast Pacific. The results also are compared to previously reported measurements by [Kuperman and Ferla (1985)], [Wenz (1962)], and others. The analysis includes a technique for separating background shipping noise from the wind-induced component of ambient noise. The system electronic noise floor and other extraneous sources of noise are evaluated to assure that the ambient noise measurements are not corrupted. Physical mechanisms examined include a wind-saturation effect that limits the ability to acoustically monitor distant storms and the effect of shallow water propagation on the character of wind noise. The identification of the distant shipping part of the spectrum allows for inferences to be made on the low frequency part of the ambient noise spectrum. [Work supported by ONR.]

Analysis of the effect of water column sound speed variation on geoacoustic inversion

Bayesian matched field inversion has been applied on multi-tonal data sets acquired on the New Jersey continental shelf in the SW06 experiment in August, 2006. Since the data sets were collected in a range-dependent environment due to water column sound speed variation, sound speed profile (SSP) was decomposed in terms of empirical orthogonal functions and also inverted in the inversion. This presentation examines the sensitivity of the geoacoustic and geometrical parameters to the shape and the gradient of the sound speed in the thermocline by the interparameter correlation from the inversion. The effects of the SSP on the geoacoustic inversion and source localization are studied through the inversions of measured data at different ranges with/without prior information about geometric parameters in the inversion. Further investigations on the variation of SSP to the sediment sound speed, density, and attenuation at different ranges are carried out by simulations. It is found that the geometric parameters are more sensitive to the SSP than are the geoacoustic parameters in this shallow water environment. [Work supported by ONR.]

Analyses of broadband and narrowband propagation measurements in Shallow Water 2006 (SW06)

Acoustic measurements were made during the Shallow Water 2006 Experiment near the New Jersey continental shelf in 70–80 m of water. For a portion of the experiment, several L-arrays and a variety of broadband and narrowband sources were deployed for the purpose of investigating the nature of the dispersion of the acoustic propagation in the seabed. A geo-acoustic inversion methodology is used to infer the dispersion from the acoustic measurements in the water column. The waveguide possesses significant inhomogenities in range, azimuth, and time due to bathymetry, sound speed profile fluctuations resulting from internal waves, and variability in seabed properties. Physical oceanography measurements are used in part to obtain the best estimate for a water column description for the analyses of specific acoustic measurements. Various inversion methods along with different propagation models, such as two-way coupled modes and finite element parabolic equation algorithms, are used to obtain an accurate mean description and uncertainty estimates for the properties that control dispersion of the seabed. [Work supported by ONR.]

Frequency dependence and intensity fluctuations due to shallow water internal waves

A theory and experimental results for sound propagation through an anisotropic shallow water environment are presented to examine the frequency dependence of the scintillation index in the presence of internal waves. The theory of horizontal rays and vertical modes is used to establish the azimutal and frequency behavior of the sound intensity fluctuations, specifically for shallow water broadband acoustic signals propagating through internal waves. This theory is then used to examine the frequency dependent, anisotropic acoustic field measured during the SWARM'95 experiment. The frequency dependent modal scintillation index is described for the frequency range of 30-200 Hz on the New Jersey continental shelf.

Statistical analysis of sound transmission results obtained on the New Jersey continental shelf

Experiments have been conducted near the site of AMCOR Borehole 6010 on the New Jersey Shelf to evaluate propagation predictability in sandy shallow-water environments. The influence of a nonlinear frequency dependence of the sediment volume attenuation in the uppermost sediment layer at this location is examined. Previously it was determined that a frequency power-law exponent of 1.5 was required for the best modeling of experimental results over the band 50-1000 Hz. The approach here references the attenuation to an accepted value at 1 kHz and makes extensive comparisons between measurements and calculations, to determine a power-law exponent of 1.85+/-0.15.

Bayesian geoacoustic inversion in a range-dependent environment

In August 2006 a series of experiments was carried out on the New Jersey continental shelf to investigate geoacoustic inversion in a range-dependent shallow water environment. The experimental site was instrumented with multiple oceanographic sensing systems to provide spatial and temporal ground truth of the ocean environment throughout the experiments. The acoustic experiments used multiple receiving systems and sound sources operating over a large frequency band from 50 to 20 kHz. This paper presents results from inversion of low-frequency (<1000 Hz) data obtained along a radial track from one of the 16-element vertical line arrays. The track was surveyed with a chirp sonar to establish bathymetric and subbottom ground truth. Continuous wave tones were recorded at ranges of 1, 3, and 5 km where the source ship held station for several minutes. Bayesian matched field inversion was applied at each of the sites. The effects of unknown range dependence were taken into account by estimating the data error covariance from multiple data windows at each range station. The inversions indicate that the long-range experimental geometry provided reliable estimates for the sea floor sediment parameters in the range-dependent environment. [Work supported by ONR Ocean Acoustics Team.]

Seismic Facies of Incised-Valley Fills, New Jersey Continental Shelf: Implications for Erosion and Preservation Processes Acting During Latest Pleistocene-Holocene Transgression

Abstract Incised-valley fills shallowly buried beneath the New Jersey middle–outer shelf reveal a retrogradational shift of four seismic facies, as observed in 1–4 kHz deep-towed chirp seismic data. These facies, the only preserved stratigraphic record of the latest Quaternary–Holocene drowning and infilling of fluvial drainage systems developed on this exposed shelf at or near the Last Glacial Maximum (LGM), are interpreted as (1) fluvial lag deposits, SF1; (2) estuarine mixed sand and muds, SF2; (3) estuary central bay muds, SF3; and (4) redistributed estuary-mouth sands, SF4. These fills are truncated by a transgressive ravinement, the T horizon, which is in turn overlain by Holocene marine sand deposits. The seismic facies are bounded by reflectors marking either source diastems or unconformities: (1) the Channels horizon is the lowstand fluvial incision surface, (2) B1 is a bay flooding surface, (3) B2 is an intra-estuarine depositional surface, (4) B3 is a tidal ravinement surface, and (5) the T horizon represents erosion at or near the shoreface during Holocene transgression. The Channels horizon is generally preserved only in valley axes. Elsewhere, this sequence boundary has been modified by surfaces B1 and/or B3. Dip-oriented changes in the thickness of SF3 and SF4 suggest either a stillstand in the passage of the shoreline, which allowed such spatial variations, or that local valley shape controlled hydrodynamic conditions for sediment transport and deposition. Narrower valleys may have promoted tidally dominated, fine-grained deposition within these drowning estuaries, while broader valleys attenuated tidal flow velocities and allowed the filling of the estuary to be dominated by wave and current energy, promoting more coarse-grained deposition. Our study demonstrates that wave- and tide-dominated estuarine facies can coexist within such fill strata.

Chirp seismic surveying of shallowly buried fluvial systems on the outer New Jersey continental shelf: Mapping channel morphology and internal stratigraphy in 3D

The outer New Jersey continental shelf has been the focus of Office of Naval Research-sponsored acoustic studies for more than a decade. As part of this effort, ONR has funded a number of field programs to characterize the geologic history and sedimentary properties of the shallow seabed. We report here on a chirp seismic campaign conducted in 2001, with particular focus on characterizing the morphology and internal stratigraphy of shallowly buried fluvial channel systems that are widespread in this region. These channels were formed during the last glacial lowstand, and then filled and buried by sediments during sea level rise. Mapping the channel morphology in 3D presented a significant challenge. Although coverage (200-m line spacing) was dense, application of standard interpolation techniques resulted in poor representation of channel continuity along sinuous pathways. We developed a technique for transforming the seismic horizon data into a channel-oriented reference frame, wherein the channel is effectively straight and simple interpolation techniques can be used to maintain channel continuity. Seismic units within the channel fill could be discerned based on bounding reflectors and acoustic character and mapped throughout the 3D channel systems. These units provide a record of sea level change and drowned-river estuary development.

Mechanism for sea bottom reverberation in long range sonar in a range-dependent continental shelf environment

The statistically stationary background reverberation in long-range sonar data acquired in the New Jersey Continental shelf during the ONR 2003 Main Acoustic Clutter Experiment is mainly due to seafloor scattering. We compare three mechanisms or models for scattering from the sea bottom in a range-dependent waveguide with the experimental data. They are the Rayleigh-Born volume scattering model, a rough surface scattering model, and an empirical Lambertian model. Each of these models has a different decay rate as a function of range and different frequency dependence. These models are compared to the measured background reverberation as a function of range from the sonar system and at various frequency bandwidths from 300 to 1500 Hz to determine which mechanism or model for seafloor scattering best describes the data.

Statistics of the Parseval sum, match filter, and instantaneous single‐frequency transmissions in an ocean waveguide

During the ONR Main Acoustic Clutter Experiment of 2003 on the New Jersey Continental Shelf, short duration broadband pulses with 50‐ to 150‐Hz bandwidths were transmitted at 50‐s intervals from a moored source array in the 300‐ to 1500‐Hz range. The signals were measured by a towed horizontal receiving array at varying ranges from the source. The broadband data are processed in a variety of ways including the use of Parseval sum and match‐filter, and as an instantaneous single frequency transmission. The ping‐to‐ping fluctuations in the measured acoustic data are a result of changes in the waveguide modal intereference structure due to both motion of the array and presence of time‐dependent random internal waves. Our analysis shows that the Parseval sum and match‐filter outputs for the direct arrival have significantly smaller standard deviations compared to the instantaneous single frequency transmission. We account for these observations by modeling the broadband acoustic field intensity in both a static waveguide and with Monte Carlo simulations in a fluctuating waveguide, as well as with the use of statistical distributions for the broadband acoustic field. We present an approach for empirically quantifying the range‐dependent degradation in the match‐filter output in a multimodal waveguide.