Broadband Acoustic Propagation Research Articles

Frequency dependent effects of environmental parameters on the broadband acoustic wave propagation in shallow water waveguides

Acoustic wave intensity plays a crucial role in underwater acoustics, particularly in shallow water regions in which the signal suffers substantial energy fluctuations. The physical properties of the environment significantly impact the intensity of broadband acoustic waves in shallow water waveguides. The accuracy of the intensity is vital for the reliability of any inverse algorithm used to obtain the physical properties of the waveguide boundaries. In this paper, we explore the effects of sound speed fluctuations in the water column on broadband acoustic propagation across different frequencies. Our observations reveal that at higher frequencies the sound intensity is more sensitive to oceanographic variability. This poses challenges for geo-acoustic inversions when parameters in the water column are not well-known, particularly at higher frequencies. To validate our findings, we present simulations using normal modes and parabolic equation with examples from recent experimental observations conducted on the New England Mud Patch Area and the continental shelf region of the United States during 2017, 2021, and 2022. These experiments provide valuable insight into the intricate behavior of acoustic waves in shallow water environments at different frequency bands. [Work supported by ONR (Grant No. N00014-21-1-2760).]

Effect of oceanography on broadband acoustic wave propagation in a range dependent shallow water environment

To investigate the effects of oceanographic variability on broadband acoustic wave propagation in shallow water environments, two experiments were carried out on the New England continental shelf region in 2017 and 2022, respectively. Towed sources were used to transmit chirp waveforms over the frequency ranges of 0.5—1.25 kHz and 1.5 – 4.0 kHz on the same acoustic tracks. The broadband acoustic transmissions were received by vertical acoustic arrays with multiple hydrophones. In 2017, the sound speed in the water column was isovelocity while in 2022 the presence of a duct at the depth of 20 m and upward refracting conditions reduced the interaction of acoustic transmissions with the seafloor compared to the experiment in 2017. Experimental observations show the importance of the water column on bottom interacting broadband waveforms causing challenges for the inversion algorithms. In this paper, we present acoustic and oceanographic results showing how the variability of sound speed could change the characteristics of the broadband transmission. Modeled results versus experimental data are discussed. [Work supported by ONR.]

Broadband acoustic wave propagation in temporally variable range dependent shallow water environment

Broadband acoustic wave propagation in shallow water regions involves interactions with the sea surface, sea bottom, and the water column. The topic of shallow water acoustics in the presence of variable water column has been of interest in recent years. Experimental observations have shown the importance of the effects due to the boundaries and the sound speed profile in the water column. However, more work needs to be done to establish ground truth in numerical modeling for temporally variable water column in a range dependent environment. In this paper we examine the broadband propagation on the shelf as well as the range dependent shelf break regions to depict how temporal variability of sound speed could change the dispersion characteristics of the broadband signal. Comparison between modeled and experimental data are provided. [Work supported by ONR OA program.]

Low Frequency Broad Band Acoustic Propagation in Andaman Sea

During November 2017, an active source seismic survey was performed in Andaman sea to study the acoustic propagation characteristics. High power low frequency acoustic signals generated by 20-air gun array onboard ORV Samudra Ratnagar were recorded from INS Sagardhwani at four different depths within 8 km ranges in shallow and deep waters. Low frequency sound levels were estimated using root mean square and power spectral values. Amplitude levels were analysed with respect to arrival time variation with frequency and is presented.

Broadband acoustic propagation in a seagrass meadow throughout a diurnal cycle

Broadband acoustic propagation in a seagrass meadow throughout a diurnal cycle

Effects of azimuthal dependent sediment layer structure on broadband acoustic propagation during Seabed Characterization Experiment in 2017

Seabed physical properties profoundly affect acoustic energy upon interaction. The inherent structural composition of the sediment column interacting with acoustic waves has been a topic of research for decades. Besides the intrinsic physical properties that cause signal attenuation, one of the features that various sediment structures share when interacting with acoustic signals, is causing azimuthal dependence on the broadband acoustic wave propagation. This phenomenon studied in late 1990's [Badiey et al., JASA, 1997a, b] was revisited during the Seabed Characterization Experiment in 2017 (SBCE 2017). The sediment layer structure was obtained from the Chirp Sonar survey collected in the summer of 2015 during pilot study. These data plus CTD and bathymetric measurements are used to construct environmental input along radial tracks for acoustic field computations and range-varying wave number spectral calculations to support SBCE 2017 data analysis. The simulation results show azimuthal variability of acoustic normal modes similar to the experimental data documented in earlier studies [Badiey et al. JASA, 1997 and 2017]. Current results indicate that azimuthal variability of sediment physical properties is one of the causes of acoustic variability in this region. [Work supported by ONR 321OA.]

Broadband acoustic wave propagation and scattering in Delaware Bay Estuary

Physical oceanography of estuaries is complex due to many factors including fresh water river discharge, oceanic salt water intrusion, variable ebb and flood tides, and surface winds. In these confined environments, high frequency (1–25 kHz) broadband acoustic waves are shown to through large intensity fluctuations with substantial energy scattering due to their interaction with the sea surface, sea bottom, and refraction due to the stratified water column. In this paper, we present results from a series of experiments that were conducted in the Delaware Bay estuary where fixed source-receiver configurations allowed calibrated long-term acoustic and oceanographic measurements to be conducted while atmospheric conditions such as wind speed and direction were carefully measured concurrently. It is shown that intensity fading due to oceanographic features and tide induced sound speed profile refraction in stratified water column occurs regularly and periodically. Wind generated surface gravity water waves in...

Data analysis and modeling of broadband acoustic propagation perpendicular to internal wave fronts and sand dune crests in the South China Sea

Very large subaqueous sand dunes were discovered on the upper continental slope of the northeastern South China Sea (SCS) in the spring of 2007 during an ONR 322OA-funded field experiment which was designed to study the large transbasin internal solitary waves (ISW) that are generated by tidal forcing in the Luzon Strait. These internal waves and sand dunes are important acoustical features, as it is expected that they will cause significant anomalies in the acoustical field. In the spring of 2014, a broadband source was deployed to transmit 850-1150 Hz LFM signals to receivers on a mooring in the center of the sand dune field. The acoustic transect was oriented perpendicular to the dune crests, ISW fronts and isobaths. Data analysis and extended modeling are presented to quantify the degree to which these features impact the propagation of broadband signals in the 100-2000 Hz band as a function of source depth and frequency.

Broadband acoustic wave propagation in three-dimensional shallow waveguide with variable sound speed profile and boundary roughness

Propagation of broadband acoustic signals in shallow water environment is a complex four-dimensional problem that needs to be addressed with input from the spatial and temporal physical parameters of waveguides with rough boundaries. To construct a numerical model of this four-dimensional problem, methods such as the Parabolic Equation (PE), Horizontal Rays and Vertical Modes, 3D Ray Method, and Nx2D PE have been utilized in recent years. However, data/model comparison still remains a challenge and accurate comparison between measured and modeled acoustic fields in the waveguide is badly needed. Lack of environmental input to use for modeling is one reason, but with proper sampling of environment it may be overcome by novel experimental design based on the knowledge of waveguide physics. Acoustic frequency can also be utilized as one of the key parameters to simplify the problem and adopt strategies in conducting calibrated experiments. In this paper, we provide a broad view of recent advancements in three-dimensional acoustic wave propagation in shallow water waveguides in the presence of variable volumetric and boundary conditions. The effect of broadband acoustic wave center frequency and bandwidth with respect to the scale of environmental variability is also discussed. [Work supported by ONR 322 OA.]

Modal analysis of broadband signal arrival on Beaufort shelf

Using previously reported temperature and salinity data collected over the past decade, time evolving sound speed profiles were constructed and used (Prog. Oceanography, 127, pp.1-20, 2014) as input to a parabolic equation model. Modeling of broadband acoustic signal propagation in the Arctic shelf-basin region during ice free sea surface season was reported recently (J. Acoust. Soc. Am. 136(4), Pt. 2, 2317, 2014). It was shown that when broadband acoustic signals propagate from deep water to shallow water, the modal dispersion is changed. The change is mostly affected by the water thermocline behavior in time and space and by source depth. The modal arrival structure can be influenced by a number of factors including the degree of Pacific warm water intrusion, upwelling of the warm saline Atlantic water into the cold surface layer, the range-dependence of the water layers over the shelf break, and the depth of the source. In this paper, we analyze the range-dependent propagation in terms of adiabatic and coupled mode regimes in order to explain the model results. [Work supported by ONR.]

Modeling three dimensional environment and broadband acoustic propagation in Arctic shelf-basin region

Rapid climate change over the last decade has created a renewed interest in the nature of underwater sound propagation in the Arctic Ocean. Changes in the oceanography and surface boundary conditions are expected to cause measurable changes in the propagation and scattering of low frequency sound. Recent measurements of a high-resolution three-dimensional (3-D) sound speed structure in a 50 x 50 km2 region in an open-water shelf-basin region of the Beaufort Sea offer a unique and rare opportunity to study the effects of a complex oceanography on the acoustic field as it propagates from the deep basin onto the continental shelf. The raw oceanography data were analyzed and processed to create a 3-D sound speed field for the water column in the basin-slope-shelf area. Recent advances in both 2-D and 3-D acoustic modeling capability allow one to study the effects of the range- and azimuth-dependent water column layers on the frequency-dependent acoustic modal structure. Of particular interest is the nature of the 3-D and mode-coupling effects on the frequency response induced by the oceanography. The results will likely be useful in designing acoustic experiments with serious logistical constraints in the rapidly changing Arctic Ocean.

Measured three-dimensional effects of mode-1 and mode-2 nonlinear internal waves on broadband acoustic wave propagation in shallow water

Horizontal shadowing effects and frequency shifts of acoustic intensity level curves were measured with a bottomed horizontal array in the East China Sea during the summer of 2008. Low-frequency acoustic pulses were transmitted by two fixed sources at 33 km (270–330 Hz LFM) and 20km (450–550 Hz LFM) range. Strong shadowing effects were observed when mode-1 nonlinear internal wave fronts were nearly parallel to the acoustic propagation path. Numerical studies indicated that shadowing effects are more complex for mode-2 nonlinear internal waves due to acoustic-mode dependent focusing and defocusing. These effects were further analyzed using 3-D PE simulations for more dynamic mode-2 nonlinear internal waves observed on the US New Jersey Shelf. The shadowing was less pronounced for internal waves with curved wave-fronts and small amplitudes. However, regular and irregular frequency shifts were still present. The experimental observations coupled with 3-D PE simulations suggest that nonlinear internal waves may be sensed and characterized via low-frequency acoustic signals. [Work supported by the Office of Naval Research.]

The Influence of Positional Parameters on the Effect of a Noise-Jammer Based on Broadband Acoustic Propagation Model

In a long time, the specialists in underwater acoustic countermeasure field were used to get an approximate evaluation of the transmission loss of noise signals by spreading loss equation of spherical wave, which led to results with a low accurateness. Based on normal mode theories and discrete Fourier transformation (DFT), derived the composition and process of the broadband acoustic propagation model. Then the transmission loss could be calculated via this propagation model, which could get a relatively more accurate result. The influence of three key positional parameters, the orientation, distance and depth of the noise-jammer were studied in this paper. The method and conclusions in this paper can provide a reliable foundation for tactical research on noise-jammers.

Broadband propagation and photoacoustic time reversal imaging using k-space methods

Numerical, time domain, models of broadband acoustic propagation using k-space methods will be described and applied to the problem of image reconstruction in photoacoustic imaging. k-space methods are a subset of time-stepping pseudospectral methods, which use FFTs to calculate field gradients, with an additional correction to make the solutions exact in the homogeneous case. Furthermore, they are closely related to a number of methods in the mathematical, engineering and physical sciences literature, including non-standard finite difference schemes, exponential integrators, wavenumber domain reverse time migration, and wave propagator models developed to solve the time-dependent Schroedinger equation. These connections will be discussed and used to illuminate the advantages of the k-space approach for large scale modelling of broadband acoustic waves. To illustrate the method, a fluid-based k-space model will be applied to the inverse acoustic initial value problem encountered in tomographic photoacoustic imaging. It is well known that this can be solved, in the linear case, using a time-reversed numerical model. Extensions from this basic case to absorbing and non-linear media will be explored.

Numerical simulations of broadband acoustic propagation through a sound‐speed field impacted by oil.

Because its soundspeed characteristics differ substantially from seawater,the presence of oil in the water column might be expected to impact acousticpropagation in the ocean environment. Empirical expressions are employed forthe sound speed in oil; it depends on the API (a measure of density), temperature,and pressure. Environmental data taken in the vicinity of the Deepwater Horizonsite taken during the 2007 Littoral Acoustic Demonstration Center experimentare used to generate profiles of temperature, pressure, and sound speed. Differentvalues of API and different distributions for the weighting fraction of theoil on the total sound speed are employed in numerical simulations. The computed,received acoustic fields for different values and locations of oil concentrationare compared to the case where no oil is present in several sourcereceiverranges. These forward models are intended to examine the feasibility of acousticallytracking oil as it travels from a source upward through the water column,or in locating oil suspended in the water column. These computations can aidin identifying the limitations on a realistic experimental scenario.

Boundary roughness and the effect of internal waves on signal coherence for shallow water transmission.

Broadband acoustic propagation experiments at three shallow sites allow for comparison of coherency of individual surface‐reflected bottom‐reflected modes of propagation. There appears to be a dependence of the correlation parameters of times and length on frequency and mode number that cannot be attributed to internal waves alone and likely depends on bottom and surface roughness. At low frequencies, f<100 Hz, during periods of quiescence internal waves, all modes of propagation have equally long coherence parameters. The coherency decrease equally for all modes as internal wave activity increases. For higher frequencies, 200 Hz <f<1000 Hz, the coherence parameters depend on mode number, with the lower order modes always more coherent than successive higher order modes. At still higher frequencies, f>1000 Hz, identifiable modes are not always observed; instead there is a continuum of arriving pulse energy with very low coherency even with minimal internal waves. Apparently, the randomizing effect of internal waves depends on bottom and surface roughness and frequency. At low frequencies, the boundaries appear flat and internal waves have a minimal effect. At the highest frequencies, phase coherence is already degraded by boundary roughness so that the slightest of internal wave activity completely randomizes the signals.

Measurement and Analysis of Broadband Acoustic Propagation in Very Shallow Water

The results of an experiment conducted to understand the frequency dependence of acoustic propagation in very shallow water using a ship as an acoustic source are presented and the possible causes of the features appearing in the results are discussed. The result shows large propagation losses at a frequency of approximately 140 Hz. The results of numerical simulation including the shear property and layered structure of the bottom imply that the features might be caused by the shear property of the lower sediment that lies below the thin upper fluid sediment.

The application of k-space acoustic propagation models to biomedical photoacoustics

k−space models for broadband acoustic pulse propagation differ from pseudo−spectral time domain (PSTD) models in their treatment of the time step. By replacing a finite−difference scheme with a propagator, exact for homogeneous media, larger time steps can be taken without loss of accuracy or stability and without introducing dispersion. Three k−space models for modeling photoacoustically generated (PA) pulses are described here. A very simple, exact, model of PA propagation in a homogeneous fluid is used to introduce the k−space propagator, and two models of propagation in heterogeneous media, originally designed for modeling scattering in soft tissue, are adapted for use in photoacoustics [Mast et al., IEEE Trans. UFFC 48, 341−354 (2001); Tabei et al., J. Acoust. Soc. Am. 111, 53−63 (2002)]. Our motivation for describing these models comes from biomedical PA imaging, in which one of the current limitations is the assumption that soft tissue has a uniform sound speed. Efficient, accurate, and simple−to−encode forward models such as these are very useful for studying the effects of the heterogeneities encountered in practice. They may also be useful in designing PA imaging schemes that can account for acoustic heterogeneities. [This work was funded by the EPSRC, UK]

Features of the broadband acoustic propagation in very shallow water

In shallow water, acoustic propagation is severely influenced by the property and structure of the bottom. We present the results of acoustic measurement conducted in a very shallow water of about 11‐m depth to characterize the features of broadband acoustic propagation. In the measurements using both of the ship and projectors as sources, the signal was received at four hydrophones, two of them were located in the water and another two were located in the sediment. Broadband propagation losses derived from ship noise revealed the frequency band that has high losses around 120–140 Hz. Analysis results on the possible causes of high losses at that frequency band are presented.

Single- and multi-channel underwater acoustic communication channel capacity: A computational study

Acoustic communication channel capacity determines the maximum data rate that can be supported by an acoustic channel for a given source power and source/receiver configuration. In this paper, broadband acoustic propagation modeling is applied to estimate the channel capacity for a time-invariant shallow-water waveguide for a single source-receiver pair and for vertical source and receiver arrays. Without bandwidth constraints, estimated single-input, single-output (SISO) capacities approach 10 megabitss at 1 km range, but beyond 2 km range they decay at a rate consistent with previous estimates by Peloquin and Leinhos (unpublished, 1997), which were based on a sonar equation calculation. Channel capacities subject to source bandwidth constraints are approximately 30-90% lower than for the unconstrained case, and exhibit a significant wind speed dependence. Channel capacity is investigated for single-input, multi-output (SIMO) and multi-input, multi-output (MIMO) systems, both for finite arrays and in the limit of a dense array spanning the entire water column. The limiting values of the SIMO and MIMO channel capacities for the modeled environment are found to be about four times higher and up to 200-400 times higher, respectively, than for the SISO case. Implications for underwater acoustic communication systems are discussed.