Chirp Sonar Survey Research Articles

Geoacoustic inversion of 3.5-kHz towed receiver data from the KOREX-17

The Korea Reverberation Experiment 2017 (KOREX-17) was conducted in a shallow water located in the south of the Geoje island, Korea. During the experiment, sound propagation measurements were made using a 3.5-kHz CW signal along different 2 tracks to a distance of ~10 km from a bottom-mounted source, ARMS (Autonomous Reverberation Measurement System). The signals were received by a SRH (Self Recording Hydrophone), which was towed at a depth of ~20 m during the measurements. The sound speed profile was almost iso-velocity, and the sediment at the site was mainly composed of silt having a mean grain size of 6 phi. Since the sound propagation in shallow water is greatly influenced by sound interaction with the sediment, geoacoustic inversion was tried using a genetic algorithm based matched field processing in which the measured acoustic pressure field was compared to the simulated field predicted by a parabolic-equation based propagation model (RAM). The results are compared to the geoacoustic parameters obtained by the empirical relationship to mean grain size and a sediment layering information obtained by a chirp sonar survey. [Work supported by Agency for Defense Development, Korea (UD170014DD).]The Korea Reverberation Experiment 2017 (KOREX-17) was conducted in a shallow water located in the south of the Geoje island, Korea. During the experiment, sound propagation measurements were made using a 3.5-kHz CW signal along different 2 tracks to a distance of ~10 km from a bottom-mounted source, ARMS (Autonomous Reverberation Measurement System). The signals were received by a SRH (Self Recording Hydrophone), which was towed at a depth of ~20 m during the measurements. The sound speed profile was almost iso-velocity, and the sediment at the site was mainly composed of silt having a mean grain size of 6 phi. Since the sound propagation in shallow water is greatly influenced by sound interaction with the sediment, geoacoustic inversion was tried using a genetic algorithm based matched field processing in which the measured acoustic pressure field was compared to the simulated field predicted by a parabolic-equation based propagation model (RAM). The results are compared to the geoacoustic parameters ob...

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.]

Measurements of mid-frequency bottom interacting signals in Jinhae bay located on the southern coast of Korea

Acoustic bottom interacting measurements were conducted in shallow water (nominal water depth of 60 m) in Jinhae bay, southern coast of Korea, using 4 to 12 kHz CW signals in May 2015 and 2016. The surficial sediment at a site is mainly composed of silty clay with a mean grain size of 8 phi. Since the seafloor is relatively soft, the bottom-bounced path was very weak compared to direct and sea-surface-bounced paths. On the other hand, a strong arrival reflected from the sub-sediment layer was received after the bottom-bounced arrival especially at lower frequencies. The arrival time difference between the arrival reflected from water-sediment interface and that reflected from the second interface is used to estimate the sound speed in the surficial sediment layer. In addition, the bottom loss as a function of grazing angle are estimated using the bottom-bounded path. Finally, the results are compared to the sedimentary structure imaged by chirp sonar survey. [Work supported by Agency for Defense Development, Korea.]

Estimation of sound speed and attenuation in mud sediments using combustive sound source signals measured on the New England continental shelf

Acoustical measurements were made in coincidence with an environmental survey for future sediment characterization experiments in the mud patch region of the New England continental shelf from July 22 to August 3, 2015. One vertical line array (VLA) containing six hydrophones and 15 environmental sensors was deployed in 75-m water at the center of the survey (40.477o, −70.604o) in order to record noise data and the acoustic signals generated by the combustive sound source (CSS) [McNeese et al., J. Acoust. Soc. Am., 2014]. Fifteen CSS shots detonated at various ranges (2.5 km and 14.5 km) and depths (5 m, 10 m, and 20 m) were recorded by the VLA. The sub-bottom layering structure with a mud layer overlaying a sand bottom was obtained from the CHIRP sonar survey. This paper utilizes the modal dispersion characteristics to invert the sound speed in the bottom. Then, the sound attenuation in mud as a function of frequency is estimated using the transmission loss (TL) data. Measured modal dispersion curves are compared with modeled dispersion curves based on the inverted bottom acoustic parameters. [Work supported by ONR.]

Modeling transmission loss over stratified muddy sediments based on chirp sonar imagery

The Office of Naval Research is sponsoring a shallow water acoustics experiment offshore New Jersey where the sediment consists nominally of a layer of mud over sand, and where the layer thickness is about 10 m. A preliminary chirp sonar survey shows that there exists fine layering within the mud layer. Nearby core data also reveal the existence of fine layers within the mud. One of the interesting acoustic questions is what impact such fine layering within the muddy sediments has on acoustic propagation. Based on the chirp sonar imagery, various plausible models of sediment layering and material composition are proposed and the transmission loss is estimated for those models for the purpose of assisting field experiment planning.

Estimating Geoacoustic Properties of Surficial Sediments in the North Mien-Hua Canyon Region With a Chirp Sonar Profiler

Acoustic reflection coefficients and attenuation rolloff rates of the seabed near North Mien-Hua Canyon, northeast of Taiwan, were measured from chirp sonar echoes during the Quantifying, Predicting, and Exploiting (QPE) Uncertainty Initiative Experiment in 2009. Using these measurements and the Biot theory with a fluid approximation, the depth-averaged sound speed, density, and medium attenuation of the surficial sediment layer were estimated. The sediment types in the chirp sonar survey area vary from fine sand to coarse silt. To capture this spatial variability, the 1-D geoacoustic parameter estimates along the chirp sonar track were interpolated onto a 2-D grid using an objective mapping technique. The 2-D maps of surficial sediment properties, along with interpolation errors, can be further applied to underwater sound propagation models in the experimental area.

Ice sheet retreat dynamics inferred from glacial morphology of the central Pine Island Bay Trough, West Antarctica

Pine Island Glacier drains portions of the West Antarctic Ice Sheet into the Amundsen Sea. During the Last Glacial Maximum the glacier extended nearly 500 km from its present location onto the outer continental shelf. Unusually restricted sea-ice cover during the austral summer of 2010 allowed for a systematic multibeam swath-bathymetric and chirp sonar survey of the mid-shelf section of Pine Island Trough. The mapped glacial landforms reveal new information about the paleo-Pine Island Ice Stream's dynamic retreat from the mid-shelf area and confirm previous suggestion of a retreat in distinct steps. The periods of grounding line stability during the overall retreat phase are marked by sediment accumulations, i.e. grounding zone wedges. These wedges are here mapped in sufficient detail to characterize spatial dimensions and estimate the volume of deposited sediment. Considering a range of sediment flux rates from the paleo-Pine Island Ice Stream we estimate that the largest and most clearly defined grounding zone wedge, located at about 73°S in the surveyed area, took between 600 and 2000 years to form. The ice stream retreated landward of this wedge before 12.3 cal ka BP. The swath-bathymetric imagery of landforms in Pine Island Trough includes glacial features that suggest that retreat between periods of grounding line stability may be associated with episodes of ice shelf break-up. The depths of grounding line wedges decrease in a landward direction, from 740 to 670 m, and record elevation of the grounding line as it stepped landward. In all, the grounding line elevation varied by only ∼80 m over a distance of just over 100 km, implying a low ice sheet profile during retreat. Finally, we revisited seismic reflection profile NB9902, acquired along Pine Island Trough in 1999, in combination with the newly acquired swath-bathymetric imagery from 2010. Together these data show that the ice stream paused during its retreat to form grounding zone wedges at an area in central Pine Island Trough where a high in dipping bedrock strata exists and the glacial trough is narrow, forming a bathymetric “bottle neck”.

Littoral and deep-seafloor sediment characterization from recent chirp sonar surveys

Sediment sound-speed and attenuation were estimated from the chirp sonar reflection data that were collected during recent surveys in the New Jersey Shelf and northern Gulf of Mexico. Physical properties of the sediment were also estimated using the Biot–Stoll model outlined in [Schock, IEEE J. Ocean. Eng. 29(4) (2004)]. In addition, independent measurements of sound-speed and attenuation were also conducted using acoustic probes and light-bulb implosions. Results from independent measurements were in agreement with those of chirp sonar data. Measured sound speed and attenuation values are typical of silty-sand sediments in the New Jersey Shelf and silty-clayey sediments in the northern Gulf of Mexico. Finally, frequency-dependence of sound-speed and attenuation was estimated within a wide frequency band using the data from co-located light-bulb implosion (0.5–4 kHz), chirp-sonar, (2–12 kHz), and sediment acoustic-probe (5–120 kHz) measurements. Observed small frequency-dependence of sound-speed and linear frequency-dependence of attenuation are in agreement with those predicted by an extended Biot–Stoll model. [Work supported by ONR.]

Evidence of mud diapirism and coral colonies in the ionian sea (central mediterranean) from high resolution chirp sonar survey

A chirp sonar survey in the Ionian Sea investigated the Calabrian margin, the Calabrian accretionary wedge, the Taranto Trench and the Apulian foreland. Shallow tectonics structures have been related to deeper ones, recognised on CROP seismic profiles. The identified echo characters have been compared with those described in the modern literature and have been related to different kinds of sediments, on the basis of core samples. Based on echo character and morphology we have recognised: 1) A widespread presence of mounds, up to 50 m high, occurring on the Apulian plateau as isolated mounds in the deepest zones (1600-800 m) and in groups in the shallower ones (800-600 m); they have been interpreted as coral mounds, according to a recent discovery of living deep water coral colonies in this zone. 2) Some mud diapirs, isolated or in groups of two or three elements, widespread in the whole study area. In analogy of what has been observed on the Mediterranean Ridge, their presence suggests the activity of deep tectonic structures (thrusts and faults) and a reduced thickness (or absence) of Messinian evaporites in this part of the Ionian Sea.

Estimating sediment speed and attenuation with sub‐bottom reflections

An inversion method based on sub‐bottom reflection is investigated using LFM data collected by a Vertical Linear Array from the SW06 experiment. The distance between the LFM source and the array is about 230m. After extracting the impulse response with matched filter from the received signal, the sub‐bottom reflections were found to be strong. The chirp sonar survey nearby the experiment site showed that there is a prominent shallow sub‐bottom R reflector with about 20 m depth (based on 1500 m/s). The relative arrival time and amplitude of the sub‐bottom reflection signals were used to estimate the sound speed and the attenuation of the sediment. Because the direct arrivals in the water were influenced strongly by the internal wave, the first bottom reflection was applied as the reference path to calculate the relative arrival time and absorption loss in sediment. The sediment attenuation was estimated by assuming that it had linear frequency dependence. The estimated values of the sediment sound speed and attenuation were compared with matched field geoacoustic inversion results published by other research group.

Geoacoustic parameters inversion by ambient noise in South China Sea experiment of ASIAEX

Knowing bottom geoacoustic parameters is of great importance for using sonar systems effectively in shallow water. This study is based upon the vertical line array (VLA) in South China Sea experiment of the Asian Seas International Acoustic Experiment (ASIAEX), dated from May 3, 2001 to May 16, 2001, in which the ambient noise was also measured in shallow water. Beamforming is used to calculate the vertical directionality of ambient noise, but limited by the array element spacing, the major analysis frequency range is 100 to 400 Hz. Comparison between forward modeling (normal mode code) and experimental results show the energy ratio of up-looking and down-looking beams, which can be referred to the energy reflection coefficient of the bottom-potentially a function of angle and frequency. From the reflection coefficient, critical parameters of the sediments such as sound speed, density, and attenuation coefficient can be obtained. Finally, this result is verified by on-site chirp sonar surveys during the experiment, and also by other inversion investigation in this area using other sound source.

Inversion of range-dependent geoacoustic properties in South China Sea ASIAEx01 experimental site

Matched-field inversion of range-dependent geoacoustic properties is studied using broadband (50–200 Hz, 240–260 Hz, and 550–600 Hz) acoustic data collected in the South China Sea during the ASIAEx01 experiment. Range-dependent sediment sound-speed and attenuation profiles are inverted using a global optimization scheme based on genetic algorithms to minimize an objective function defined by the Bartlett processor output. For the forward model, an efficient coupled normal mode model is used to calculate broadband acoustic fields incorporating range-dependent bathymetry and sediment layers thickness obtained by chirp sonar surveys. Inversions were performed starting from a range-independent region with a relatively short source/receiver distance. Additional geoacoustic profiles were inverted by incorporating the previously inverted profiles as the source/receiver distance was increased. The results obtained at three different frequency bands are in good agreement, especially when the range-dependency of bathymetry and sediment layers thickness is included in the inversion. [Work supported by ONR.]

Matched-field inversion of seabed geoacoustic properties complemented by chirp sonar surveys

Matched-field inversion of range-dependent seabed parameters is studied using broadband (525–725 Hz) acoustic data collected on the New Jersey Shelf during the SWARM95 experiment. An efficient global optimization scheme is used to minimize an objective function defined by the Bartlett processor output in both beam and mode space. With the input from chirp sonar bathymetry and sediment thickness data, a broadband acoustic field is calculated by a coupled normal mode model and range-dependent geoacoustic properties are inverted. An independent chirp sonar inversion of acoustic impedance profiles was also performed along the same track by using chirp sonar data. The inversion results indicate good agreement between two inversion methods and show their effectiveness if they are used as complementary to each other. The effect of a strongly range-dependent water column on the inversion performance and possible broadband inversion using single hydrophone is also studied. [Work supported by ONR.]

Performance of one-dimensional chirp sonar inversion in complex environments

Chirp sonar surveys can provide subbottom images with cm-scale vertical resolution and tens of meters of penetration into the sediment. Recently, chirp sonar reflection amplitude data have been used for the inversion of subbottom attenuation and impedance profiles [e.g., Panda et al., J. Acoust. Soc. Am. 96, 3022–3035 (1994)]. Furthermore, by making use of physics-based sediment models and correlation between certain parameters, real-time estimation of sediment properties such as density, porosity, and sound-speed profiles has become possible [Turgut and Wolf, J. Acoust. Soc. Am. 98, 2972(A) (1995)]. In this work, effects of fine layering and lateral inhomogeneities on the performance of chirp sonar inversion have been studied. Simulated reflection amplitude data were obtained by using a 2-D pseudospectral method for a poro-viscoacoustic sediment model based on the Biot theory. Limits of one-dimensional inversion were discussed in this framework and several recommendations were made to improve the data collection methods in more complex environments. [Work supported by ONR.]

Acoustic field propagation through a shallow water waveguide dominated by internal waves

The continental shelf off the New Jersey coast is a site of intense internal-wave activity. The space–time variability of an acoustic field in this shallow-water region was studied using oceanographic measurements along a 42-km track from the SWARM95 [shallow-water acoustics in a random medium] experiment. A sound–speed model based on the KdV equation was developed to include the nonlinear contribution of the internal wave field, with parameters estimated from towed yo-yo ctd data, ship radar images of internal-wavesurface expression and acoustic backscatter images of subsurface internal-wave structure. The linear internal-wave contribution was modeled by an ADCP-derived displacement power spectrum. Bathymetry and sub-bottom parameters were previously estimated from wave-field inversions using chirp sonar surveys. Simulation results are presented for acoustic propagation at frequencies of 224 and 400 Hz using a wide-angle parabolic equation method to compute realizations of the acoustic field. Results include estimates of the intensity fluctuations and coherence for two vertical arrays located at 33 and 42 km from the moored sources. [Work supported by ONR.]

Acoustic intensity fluctuations induced by internal waves in a shallow-water waveguide

The SWARM (Shallow Water Acoustics in a Random Medium) experiment performed during July–August 1995 made simultaneous acoustic and oceanographic measurements along a 45-km track on the continental shelf off the New Jersey coast. The area is a site of intense internal wave activity, primarily in the form of soliton packets that perturb the sound-speed profile in range, depth, and time. Incorporating environmental parameters estimated from SWARM measurements, computer simulation results are presented for acoustic propagation through this environment at frequencies of 224 and 400 Hz. A wide-angle parabolic equation method was used to compute multiple realizations of the acoustic field. The simulations employed bathymetry and sub-bottom acoustic parameters estimated from chirp sonar surveys. Oceanographic parameters were obtained from thermistor strings and yo-yo CTDs in conjunction with satellite images, which allowed for estimation of internal wave-field parameters that were used to compute sound-speed distributions. Numerical results include estimates of the full-field and modal-intensity fluctuations, as well as the spatial coherence at two vertical arrays located at 33 and 45 km from the moored sources. [Work supported by NRL and ONR. The authors wish to acknowledge the contribution of the SWARM group to this work.]