Jersey Continental Shelf Research Articles

Internal waves

Internal waves are ubiquitous oceanographic features that occur in various forms across the world’s oceans. They manifest themselves as interface waves across ocean density layers that represent the interplay between buoyancy and gravitational forces, and are typically classified as linear and nonlinear internal waves. Nonlinear internal waves are characterized by isopycnal displacements that can exceed 30 m, and current velocities that approach 1 m/s. Internal wave energy converters, if developed, could have the advantage of no surface expression and provide for the availability of renewable ocean energy in regions of scant surface wave energy resources.
 Here, internal wave energies were computed at two locations: the New Jersey continental shelf and the coast of Central California. The available energy resource calculated for internal waves was then compared against surface gravity wave resources at each of these locations. Results suggest that the internal wave energy flux is comparable to that of surface waves on the New Jersey continental shelf during the summer of 2006 but is two orders of magnitude lower than that of surface waves in Central California during the summer of 2017. When expressed in terms of forces on a cylindrical structure, internal wave forces are an order of magnitude lower than that of surface waves on identically sized cylinders. However, the forces of the two resources are comparable when the diameter of the cylinder is doubled for the internal wave calculations. This suggests that while a larger energy converter would be required to harness internal wave energy, the larger size could be a reasonable tradeoff for advantages such as the lack of surface expression and the availability of energy in regions of limited surface wave energy resources.
 These results could potentially pave the way for longer-time scale characterization of internal waves and the development of devices to harness this energy resource (thereby powering the Blue Economy), while augmenting the portfolio of marine renewable energy resources. Further, the dominance of internal wave energy resources during summer seasons when the water column is stratified can augment the surface wave energy resource that can be diminished during the summer season.

Using ambient noise to evaluate the acoustic Green’s function for a passive geoacoustic inversion in a dynamic shallow-water environment

Cross-correlation functions (CCFs) of ambient and shipping noise recorded by two hydrophones approximate the deterministic Green’s function and contain information about the propagation environment. This paper employs the data collected in the Shallow Water 2006 experiment on the New Jersey continental shelf to investigate the factors that affect the emergence of approximate Green’s functions from noise CCF estimates and accuracy of the approximation. One month-long continuous records of noise obtained by moored single-hydrophone receivers are analyzed. Hydrophones are located in 80–100-m deep water at distances of several kilometers from each other. Rapid variations of the water sound speed profile, which are primarily due to propagation of trains of strong nonlinear internal gravity waves, limit useful noise-averaging time. Available water temperature data are used to guide the selection of time windows for noise averaging and improve CCF evaluation and retrieval of information on seafloor properties. Various approaches to coherent stacking of CCFs are compared. Time warping transform is applied to the resultant noise CCF to extract dispersion curves of acoustic normal modes. The results of the geoacoustic inversion based on the passively measured dispersion curves are compared with the earlier results obtained using controlled sound sources. [Work supported by NSF and BSF.]

Geoacoustic inversion using cross correlation function of low-frequency ambient noise in the Shallow Water 2006 Experiment

In this paper, results of measurements and analysis of the cross correlation function (CCF) of the ambient noise field in a low frequency band (10–200 Hz) are presented. Records of the noise were obtained by single hydrophones (SHRUs) in the Shallow Water 2006 (SW06) experiment on the New Jersey continental shelf during about a month. Five SHRUs were placed approximately along a straight line perpendicular to the coastal line, distances between adjacent hydrophones were 5–8 km, ocean depth was 80–100 m. Analysis of the experimental data was carried out by calculating time-frequency diagrams of CCFs for different pairs of SHRUs, with subsequent mode filtering on the basis of the warping transform. Theoretical calculations of mode parameters for comparison with the experiment were done using different waveguide models, including the geoacoustic models previously proposed for the area of SW06 by other authors based on active source data. [Work supported by BSF-NSF grant.]

Azimuthal dependence of sound propagation due to seabed variability in shallow water

In shallow water regions, the environment has complicated temporal and spatial variability including changes of bathymetry, sediment layer structure, bottom property, and physical oceanographic spatial and temporal changes due to processes like internal waves. All these effects can influence sound propagation in the waveguide. The azimuth angle dependence of sound propagation has been studied using the broadband acoustic signals measured at the Atlantic Generation Station site on the New Jersey continental Shelf, where two distinctive geologic/geoacoustic regions exist [J. Acoust. Soc. Am. 96(6), 1994]. The current paper revisits this idea by analyzing the modal dispersion of broadband acoustic signals deployed along circular tracks at the site of the Seabed Characterization Experiment 2017, where the seabed shows strong azimuthal dependent sub-mud layer ridges overlaid by a relatively uniform mud layer with variable thickness along different directions. The results of this paper can be utilized to assess the azimuthal dependence of the sound propagation in the mud patch region. [Work supported by ONR Ocean Acoustics.]

The role of fluctuations in the interpretation of sonar detection performance

The predictive probability of detection (PPD) is a metric that accounts for uncertainty in sonar detection performance due to random fluctuations in transmission loss, noise level, and source level [P. Abbot and I. Dyer, Impact of Littoral Environmental Variability on Acoustic Predictions and Sonar Performance , 2002]. It is well known that a significant portion of Ira’s career was dedicated to understanding the role fluctuations play in interpreting acoustic measurements. Building on this foundation, we now embrace the notion that a useful statement of sonar system performance is one linked with a probabilistic description of the acoustic environment’s intrinsic variability. In this paper, we discuss the impact of fluctuations on passive sonar performance, including how PPD facilitates the interpretation of sonar system recognition differential. Data from a recent field test conducted in August, 2011, on the New Jersey continental shelf, will be used to illustrate the methodology and interpret measured detection performance in the presence of a cold pool duct and variable ambient noise conditions.

Carbonate and silicate cementation of siliciclastic sediments of the New Jersey shelf (IODP Expedition 313): relation with organic matter diagenesis and submarine groundwater discharge

The New Jersey continental shelf extends 150 km off the shoreline. During IODP Expedition 313, siliciclastic deposits of late Eocene to late Pleistocene age were drilled down to 631, 669 and 755 m below seafloor at sites 27A, 28A and 29A respectively in very shallow waters (33.5 to 36 m depth). Pore water salinities display multilayered brackish-salty-brine units 10 to 170 m thick, where low-salinity water is preferentially stored in fine-grained sediments. The sharp boundaries of these buried aquifers are often marked by cemented layers a few centimetres thick. The mineralogy and scanning electron microscope observations of these layers show two phases of cementation by authigenic minerals: (1) the early carbonate cement is frequently associated with pyrite, and (2) the late silicate cement infills the residual porosity. The isotopic compositions of the carbonate cements vary widely: −2.4 < δ18O ‰ VPDB < +2.8; −15.1 < δ13C ‰ VPDB < +15.6. The δ18O values indicate that the carbonate cements precipitated with pore waters comprising variable mixtures of seawater and 18O-depleted fresh water originating from submarine groundwater discharge. The δ13C values of the carbonate cements are related to organic matter diagenesis, providing 13C-depleted dissolved inorganic carbon during bacterial sulphate reduction and anaerobic oxidation of methane, and 13C-rich dissolved inorganic carbon during methanogenesis. The diagenetic cementation processes included chemical weathering of reactive silicate minerals by the CO2-rich pore waters issued from organic matter diagenesis that released bicarbonate, cations and dissolved silica, which were further precipitated as carbonate and silicate cements. The estimated range of temperature (18±4 °C) during carbonate precipitation is consistent with carbonate cementation at moderate burial depths; however, silicate cementation occurred later during diagenesis at deeper burial depths.

Geoacoustic inversion using low frequency broadband acoustic measurements from L-shaped arrays in the Shallow Water 2006 Experiment.

When using geoacoustic inversion methods, one objective function may not result in a unique solution of the inversion problem because of the ambiguity among the unknown parameters. This paper utilizes acoustic normal mode dispersion curves, mode shapes, and modal-based longitudinal horizontal coherence to define a three-objective optimization problem for geoacoustic parameter estimation. This inversion scheme is applied to long-range combustive sound source data obtained from L-shaped arrays deployed on the New Jersey continental shelf in the summer of 2006. Based on the sub-bottom layering structure from the Compressed High-Intensity Radiated Pulse reflection survey at the experimental site, a two-layer (sand ridge overlaying a half-space basement) range-independent sediment model is utilized. The ambiguities of the sound speed, density, and depth of the sand ridge layer are partially removed by minimizing these objective functions. The inverted seabed sound speed over a frequency range of 15-170 Hz is comparable to the ones from direct measurements and other inversion methods in the same general area. The inverted seabed attenuation shows a nonlinear frequency dependence expressed as αb=0.26f1.55(dB/m) from 50 to 500 Hz or αb=0.32f1.65(dB/m) from 50 to 250 Hz, where f is in kHz.

Statistics of Nonlinear Internal Waves during the Shallow Water 2006 Experiment

AbstractDuring the Shallow Water Acoustic Experiment 2006 (SW06) conducted on the New Jersey continental shelf in the summer of 2006, detailed measurements of the ocean environment were made along a fixed reference track that was parallel to the continental shelf. The time-varying environment induced by nonlinear internal waves (NLIWs) was recorded by an array of moored thermistor chains and by X-band radars from the attending research vessels. Using a mapping technique, the three-dimensional (3D) temperature field for over a month of NLIW events is reconstructed and analyzed to provide a statistical summary of important NLIW parameters, such as the NLIW propagation speed, direction, and amplitude. The results in this paper can be used as a database for studying the NLIW generation, propagation, and fidelity of nonlinear internal wave models.

In situ measurements of sediment sound speed and attenuation in the frequency band of 0.5–10 kilohertz

Knowledge of sediment sound speed and attenuation is crucial for predicting sound propagation. The sediment Acoustic-speed Measurement System (SAMS) was designed to measure frequency dependence of sound speed and attenuation within the surficial 3 m in the frequency band of 2–50 kHz. SAMS has two independent and interchangeable drill systems: one employs suction and the other a water jet. The suction system has minimal disturbance on the medium around the penetrating probe while the water jet system can help penetrate consolidated shell/sand layers. Depending on sediment types encountered, the two systems can be interchanged on site in less than 4 h. SAMS was successfully deployed during the Shallow Water 2006 experiment on the New Jersey continental shelf. It was also deployed during the Gulf Experiments 2011, 2012, and the Target and Reverberation Experiment 2013, all conducted off Panama City, FL, where sediment types range from mud to coarse sand. Sediment sound speed and attenuation results from the two experiments are presented. In preparation for the upcoming Seabed Characterization Experiment, the effort to extend measurements down to 500–1500 Hz, where dispersion is of particular importance, will be discussed as well. [Work supported by ONR.]

Maximum entropy inference of seabed attenuation parameters using ship radiated broadband noise.

The received acoustic field generated by a single passage of a research vessel on the New Jersey continental shelf is employed to infer probability distributions for the parameter values representing the frequency dependence of the seabed attenuation and the source levels of the ship. The statistical inference approach employed in the analysis is a maximum entropy methodology. The average value of the error function, needed to uniquely specify a conditional posterior probability distribution, is estimated with data samples from time periods in which the ship-receiver geometry is dominated by either the stern or bow aspect. The existence of ambiguities between the source levels and the environmental parameter values motivates an attempt to partially decouple these parameter values. The main result is the demonstration that parameter values for the attenuation (α and the frequency exponent), the sediment sound speed, and the source levels can be resolved through a model space reduction technique. The results of this multi-step statistical inference developed for ship radiated noise is then tested by processing towed source data over the same bandwidth and source track to estimate continuous wave source levels that were measured independently with a reference hydrophone on the tow body.

Horizontal coherence of sound propagation in the presence of internal waves on the New Jersey continental shelf

The knowledge of spatial coherence of sound field is required for sonar application and array design. In shallow water waveguides, the spatial and temporal evolution of the temperature field induced by internal waves will cause signal fluctuations and variations in spatial coherence. During the Shallow Water Acoustic Experiment 2006, the three dimensional temperature field of internal waves was measured and reconstructed while acoustic signals were simultaneously transmitted between various sources and an L-array. These internal wave events have been used to investigate the internal wave effect on temporal coherence and vertical structures of acoustic normal modes [Wan and Badiey, J. Acoust. Soc. Am. 135, 2014]. In this paper, the horizontal structures of acoustic normal modes are examined. The arrival angles of normal modes are analyzed. The horizontal coherence of sound field is obtained at different frequencies for these internal wave events. The relationship between horizontal coherence and internal wave parameters, such as propagation direction, speed, and amplitude, is discussed. [Work supported by ONR322OA.]

Maximum entropy inference of seabed attenuation parameters using ship radiatedbroadband noise

The use of ambient noise as a means of inferring physical properties of the ocean is of significant interest in ocean acoustics. This study employs the received acoustic field generated by the passage of the R/V Knorr on the New Jersey continental shelf to test a new approach that estimates the probability distributions of both the aspect dependent source levels and the parameters that represent the geoacoustic structure of the seabed. Since the source levels and the environmental parameters have an intrinsic ambiguity, both classes of parameters must be included in the model hypothesis space as random variables. The statistics of the error function, needed to uniquely specify the likelihood function, are estimated with a maximum entropy approach by creating a data ensemble that includes samples from time periods where the ship-receiver geometry is dominated by stern, bow, and port aspect. This method has its origins from the observation that Bayes’ rule is symmetric with the interchange of the data space and the hypothesis space. [Research supported by ONR 32 OA.]

Sei whale localization and tracking using a moored, combined horizontal and vertical line array near the New Jersey continental shelf

In 2006, a multidisciplinary experiment was conducted in the Mid-Atlantic continental shelf off the New Jersey coast. During a 2 day period in mid-September 2006, more than 200, unconfirmed but identifiable, sei whale (Balaenoptera borealis) calls were collected on a moored, combined horizontal and vertical line hydrophone array. Sei whale movements were tracked over long distances (up to tens of kilometers) using a normal mode back propagation method. This approach uses low-frequency, broadband passive sei whale call receptions from a single-station, two-dimensional hydrophone array to perform long distance localization and tracking by exploiting the dispersive nature of propagating acoustic modes in a shallow water environment. Source depth information and the source signal can also be determined from the localization application. This passive whale tracking, combined with the intensive oceanography measurements performed during the experiment, was also used to examine sei whale movements in relation to...

Inversion of acoustical data from the "Shallow Water 06" experiment by statistical signal characterization.

This paper presents an application to validate an acoustic signal characterization scheme for ocean acoustic tomography and geoacoustic inversions proposed by Taroudakis, Tzagkarakis, and Tsakalides [J. Acoust. Soc. Am. 119, 1396-1405 (2006)] using data from an experiment at sea. The data were collected during the Shallow water '06 (SW06) experiment off the New Jersey Continental Shelf and the inversion results (sea-bed geoacoustic parameters and source range) are compared with those reported from the same data by Bonnel and Chapman [J. Acoust. Soc. Am. 130(2), EL101-EL107 (2011)]. The comparison and the signal reconstruction using estimated values of the model parameters are satisfactory indicating that the new signal characterization method is useful for practical applications of acoustical oceanography.

Cell geometry records the physiological responses of coccolithophores to Paleogene climate change

The physiology and ecology of the calcifying marine phytoplankton group coccolithophores are expected to be susceptible to future changes in environmental conditions forced by increasing atmospheric carbon dioxide and temperature (e.g., Fabry et al., 2008). A key trait of the physiological state of a phytoplankton cell is cell size (e.g., Finkel et al., 2007), which we have captured from exquisitely preserved fossil coccospheres in biometric measurements of coccolith length, coccosphere size and number of coccoliths per coccosphere (Fig. 1). Our exquisitely preserved calcareous microfossils are obtained from sites where post-depositional alteration of the calcite is virtually absent. Typically these sites contain hemipelagic, clay-rich lithologies with high sedimentation rates that rapidly encapsulate calcareous microfossils, isolating them within an impermeable matrix (Bown et al., 2008). These sediments contain a remarkably high diversity of fossil coccolithophores and an unusually high abundance of intact coccospheres, where the original spherical-subspherical cell covering of coccoliths is completely preserved, instead of the typical disarticulation into loose coccoliths. The pristine preservation of fragile and taxonomically important central-area structures in coccoliths combined with the preservation of very small (<3μm)coccolithsenablesustostudycoccospheregeometry across a wide variety of Paleogene taxa at species level. We present material from Tanzania (Nicholas et al., 2006), New Jersey (Miller et al., 1998) and Californian (John et al., 2008) continental shelf environments, where the shallow water depth minimized water-column disaggregation and dissolution and the shelf setting promoted high sedimentation rates. The thick sequence of clay-rich oozes at deep-sea Labrador Sea site ODP 647A (Firth, 1989) has also yielded diverse coccospheres that illustrate a high-latitude North Atlantic Eocene assemblage. Intact fossil coccospheres can be interpreted on a cellular level through analogy with the cellular geometry of their modern counterparts (Gibbs et al., 2013). This facilitates an unprecedented insight into the cellular physiology of cells that are millions of years old and its link to population-level responses to climate change. Our biometric data (following Fig. 1; Gibbs et al., 2013 and Henderiks, 2008) reveals the cellular growth and calcification response of more than 30 coccolithophore species to long-term variability in sea-surface conditions during the Paleogene greenhouse interval, between 66 and 34 million years ago. Cell geometry from abundant coccospheres of the common Paleogene genera

Coherence measurements of acoustic normal modes during one month of internal wave events on the New Jersey continental shelf

During the Shallow Water Acoustic Experiment 2006 (SW06) conducted on the New Jersey continental shelf, the three-dimensional (3D) temperature field for one month of internal wave (IW) events has been reconstructed by Badiey et al. [J. Acoust. Soc. Am. El. 134(5), 4035 (2013)]. The aforementioned IW events, with the angle between the acoustic track and the IW front varying from -8° to 83°, were measured while simultaneously acoustic signals were transmitted from fixed sources at an along-shelf distance of about 20 km with frequencies at 87.5–112.5 Hz (m-sequence), 175–225 Hz (m-sequence), 270–330 Hz (chirp), and 470–530 Hz (chirp), respectively. The acoustic signals were recorded by an L-shaped hydrophone array moored inside the area with IW measurements. The main goal of this paper is to analyze the coherence of acoustic normal modes accompany with the simultaneously measured IW events. The coherence of acoustic normal modes decomposed from the measured acoustic field is obtained as a function of frequency and IW parameters, such as the IW propagation direction, amplitude, coherence length, etc. The relationship between the modal coherence and IW parameters is discussed. [Work supported by ONR322OA.]

Low frequency sound attenuation measurements in marine sediments

This paper reports measurements of sound attenuation in marine sediments from two locations on the outer New Jersey continental shelf. At one site the sediment in the top 20 m is primarily sandy clay, while the other site includes a thin (3–5 m), over-lying layer of sand at the sea floor. The attenuation was inverted from close range, broadband data from light bulb implosions deployed at stations at the sites. The inversion method made use of the time-frequency dispersion information in signals received at single hydrophones. The signals were first processed by time warping to resolve the propagating modes at relatively close ranges (50–80 water depths). The inversion is carried out in two stages. The first stage inverted the sound speed and density by modeling the modal group velocities, and these estimates were used in the second stage to invert the attenuation from the modal amplitude ratios. The results provide estimates of low-frequency sound attenuation that can be compared to predictions from different models of sound propagation to assess the frequency dependence in the band from 100–500 Hz.

Acoustic normal mode fluctuations due to internal waves in the Shallow Water 2006 experiment

The Shallow Water 2006 (SW06) experiment was a large-scale acoustic experiment conducted on the New Jersey continental shelf in the summer of 2006 with substantial investment from the Office of Naval Research. The main goal of this experiment was to find a detailed understanding of the waveguide during the propagation of broadband acoustic signals in the presence of internal waves (IW). One month of IW events recorded during this experiment has been reconstructed with the aids of densely deployed thermistor strings and ship-borne X-Band radars. The comprehensive IW measurements enabled the study of the correlation between the IW-induced time-varying environment and the acoustic normal mode fluctuations. During this experiment, an L-shaped hydrophone array was moored inside the area with IW measurements. Acoustic sources transmitting for 7.5 min every 30 min starting on the hour were deployed at the shallow end of the across-shelf path and the outer end of the along-shelf path respectively. The acoustic modal fluctuations in modal arrival time, intensity, temporal coherence, and spatial coherence during the aforementioned IW events are analyzed. The relationship between the modal behavior and IW parameters is investigated and possible IW inversion schemes using acoustic measurements are discussed. [Work supported by ONR322OA.]

Geoacoustic inversion of noise radiated by surface ships

Addressed is the issue of determining physical properties of the seabed in shallow water using ambient noise from surface ships of opportunity. Both the amplitude and phase of the source spectrum are unknown. The coherent modal interference structure in the received spectra is sensitive to parameters such as the sound speed in the seabed, the speed of the ship, the source-receiver range, the water depth, etc. Such sensitivity allows in some cases for information on these parameters to be extracted from a statistical information approach such as maximum entropy. In shallow water, the rate of decay with propagation range of the received incoherent levels is sensitive to the intrinsic seabed attenuation. For ship of opportunity data, estimating values for the attenuation is complicated by the ambiguity existing between source level and attenuation. This work explores various methods that attempt to resolve the source level and attenuation ambiguity using experimental data taken in the Gulf of Mexico and the New Jersey continental shelf. One approach is to use short-range data to give a prior distribution for the source levels and to then estimate the marginal distributions of the attenuation with longer-range data. [Work supported by ONR.]

Time reversal of modal arrivals for broadband signals in horizontally stratified shallow water due to internal waves

Shallow water waveguides in the presence of internal waves can make a time-varying, horizontally stratified medium that can significantly affect broadband pulse propagation. In this paper, we analyze a unique data set obtained during the passage of an internal wave event in a region where a broadband sound source (270 to 330 Hz) was received by an L-shaped hydrophone array about 20 km away in 80 meters on the New Jersey continental shelf. During the time 20:30 and 22:07 GMT, on 17 August 2006, an approaching IW affected the dispersion characteristic of the broadband LFM chirp signal while passing an acoustic track in shallow water waveguide. Modal behavior is examined before and after the internal wave front crossed the source-receiver track. While dispersion characteristics of the signals changed, modal arrival time reversal occurred. The corresponding group and phase velocities that signals experienced during this phenomenon are analyzed using the theory of horizontal rays and vertical modes. These results have motivated theoretical and modeling studies of the waveguide behavior since it was first reported in 2010. [Work supported by ONR322OA.]