Andrea Y. Chang

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.

THREE-DIMENSIONAL ACOUSTIC SIMULATION OF AN ACOUSTIC REFRACTION BY A NONLINEAR INTERNAL WAVE IN A WEDGE BATHYMETRY

Nonlinear internal wave (NIW) results in three dimensional acoustic effect such as ducting and whispering gallery effects in acoustic propagation. Acoustic energy restricted within internal wave crests (crest–crest) on the shelf constitutes the ducting effect, and energy confined along the crest when the source is located upslope from the NIW crest is known as the whispering gallery effect. Numerical experiments are presented in this paper for the study of 3D acoustic effects caused by both internal wave and wedge-bathymetry. 3D effects are predicted by Wide-Angle-FOR3D and the modal contents are calculated by MOS3DPEF. Following are the case studies detailing differences between 2D and 3D calculation, and the joint effect of propagating internal waves with upslope-bathymetry. Modeled time series of transmission Loss reveal that internal wave induces the oceanic waveguide and concentrate acoustic energy along the wave front. By modeling larger calculation ranges (20 km) and deeper deploying sources, the changing of the growth and decline of acoustic energy and lower acoustic mode amplitude by range, along the front of internal wave can be observed in this paper.

Modeling of shallow water ambient noise based on adiabatic mode theory

State-of-the-Art noise prediction has been used to model noise emissions from industrial facilities, shipping activity in underwater environment to estimate noise level contours beyond the source area, and to determine specific impacts at potentially sensitive receptors, including marine mammals, etc. This paper presents the ambient noise modeling using adiabatic mode theory as acoustic propagation model to evaluate the noise statistics including temporal coherence and spatial variability offshore western Taiwan. In this model, the ocean model (Taiwan Coastal Ocean Nowcast/Forecast System, TCONFS, formulated on the basis of the Princeton Ocean Model) generating time varying/spatial dependent temperature profiles for water column variability, and geo-acoustic database are used as environmental inputs to this model. The modeling results demonstrate the temporal/spatial variability induced by time varying ocean model output, manifested by measured data. (This work is supported by National Science Council of Taiwan).

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.

Acoustic propagation in the South China Sea: Internal waves and sand dunes

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 the ONR 322OA-funded NLIWI Acoustics field experiment. The dunes’ formation mechanism is hypothesized to be the internal solitary waves (ISW) which generate from tidal forcing on the Luzon Ridge on the east side of the SCS, propagate west across the deep basin with amplitudes regularly exceeding 125 m, and dissipate large amounts of energy via turbulent interaction with the continental slope, suspending and redistributing the bottom sediment. These internal waves and sand dunes are important acoustical features, as it is expected that they will cause significant anomalies in the acoustical field. Data analysis and modeling are presented to quantify the degree to which these features impact broadband (850-1200 Hz) signals propagating along an acoustic transect oriented perpendicular to the internal wave fronts and sand dune crests.

Observations of acoustic propagation and geoacoustic inversion affected by subaqueous sand dunes in the South China Sea

The large subaqueous sand dunes are expected to affect underwater acoustic propagation. Very large subaqueous sand dunes on the upper continental slope of the northern SCS were discovered in water depths of 160 m to 600 m, which composed of fine to medium sand. In this talk, serial acoustic experiments conducted by Taiwan and United States in the South China Sea during 2012-2014 are overviewed and mid-frequency propagation data/model as well as mid-frequency geo-acoustics are aimed. For mid-frequency propagation, results demonstrate that subaqueous sand dune bedforms fluctuate the distinguishable and dispersive mid-frequency acoustical channel; causing the least distinct arrival patterns in the sand dune area. Numerical simulations using broadband modeling given the adequate initial field and Pade term confirm the observations in the experimental data. This talk also presents experiment results of normal incidence survey tracks, and the errors in reflection coefficient estimation and the resulting sedimen...

Acoustic experiments over the South China Sea upper-slope sand dunes

A series of experiments were methodically carried out between 2012 and 2014 to study the impact of large underwater sand dunes and the combined impact of these sand dunes and large-amplitude nonlinear internal waves on sound propagation over the upper continental slope including anisotropic propagation characteristics and focussing/defocussing scattering phenomena. The spatial distribution and scales of the sand dunes were first mapped by two multibeam echo sounding (MBES) surveys in 2012 and 2013. The 2013 experiment also provided some coring and initial acoustic transmission data to give useful knowledge of the geoacoustic properties of the sand dunes based on forward propagation modeling and least-squares fitting to the measured levels. The 2014 experiment was more comprehensive, entailing the deployment of autonomous mobile sources, a towed source and a moored source transmitting signals in different frequency bands and different geometries to a vertical line hydrophone array. Our data analysis result...

Resonant interaction of acoustic waves with subaqueous bedforms: Sand dunes in the South China Sea

The large subaqueous sand dunes in the South China Sea are expected to produce the coupling of energy between acoustic normal modes. In this letter, resonant interaction between acoustic propagating modes and subaqueous bedforms are numerically investigated as a function of bedform wavelength, acoustic frequency and bedform packet length. The results demonstrate that bedform wavelength impacts acoustic mode coupling behavior, with the principal transfer of energy occurring between acoustic modes whose eigenvalue difference is equal to the peak value in the bedform wavenumber spectrum. The observed effect of wavelength is greater than that of acoustic frequency and bedform packet length.

Three-dimensional acoustic propagation effect in subaqueous sand dune field

Very large subaqueous sand dunes are discovered on the upper continental slope of the northern South China Sea in water depth of 160–600 m, which composed of fine to medium sand. The amplitude and the crest-to-crest wavelength of sand dunes are about 5–15 m and 200–400 m, respectively. This topographic feature could causes strong acoustic scattering, mode coupling, and out-of- plane propagation effects, which consequently result in sound energy redistribution within ocean waveguide. This research focus on the three-dimensional propagation effects (e.g., horizontal refraction) induced by the sand dunes in the South China Sea, which are expected as the angle of propagation relative to the bedform crests decreases. The three-dimensional propagation effects are studied by numerical modeling and model-data comparison. For numerical modeling, the in-situ topographic data of subaqueous sand dune and sound speed profiles were inputted to calculate the acoustic fields, which were further decomposed into mode field...

Normal incidence measurement in a subaqueous sand dune field in the South China Sea

Regions with subaqueous sand dunes have been discovered on the upper continental slope of the northern South China Sea. These large subaqueous sand dunes are expected to cause errors in the measurement of normal incidence reflection. This letter presents experiment results of two normal incidence survey tracks conducted in 2013, and the errors in reflection coefficient estimation and the resulting sediment properties induced by sand dune bedforms. The results demonstrate that the reflected energy is focused and scattered by different parts of sand dune bedforms and that they produce significant variation in the estimated reflection coefficients and the inverted geoacoustic properties.