Linus Chiu

EXPERIMENTAL EVIDENCE OF HORIZONTAL REFRACTION BY NONLINEAR INTERNAL WAVES OF ELEVATION IN SHALLOW WATER IN THE SOUTH CHINA SEA: 3D VERSUS Nx2D ACOUSTIC PROPAGATION MODELING

A joint Taiwanese-U.S. field experiment was conducted in the South China Sea (SCS), entitled the South China Sea Oceanic Processes Experiment (Taiwan)/Non-Linear Internal Waves Initiative (US) (SCOPE/NLIWI), the ocean acoustics portion of which occurred during April 12–22, 2007. The acoustics objective was to quantify the temporal and spatial variability in acoustic propagation characteristics on the continental shelf in the presence of locally-generated and trans-basin nonlinear internal waves (NLIW). Broadband (400 Hz center frequency) m-sequence signals transmitted nearly continuously by a source moored near the seabed were received by vertical line arrays at 3 and 6 km range. The acoustic transect was oriented approximately parallel to the wave fronts of the shoaling trans-basin NLIWs which had crossed the deep basin from their origin in the Luzon Strait. The acoustic propagation variability due to strong vertical and horizontal refraction induced by the very large NLIWs creates an extremely complex acoustic field as a function of time and space. Experimental data and numerical acoustic propagation modeling results are presented to (1) examine and estimate the contribution of internal wave induced horizontal refraction to the received acoustic field; and (2) to quantify the range of propagation angles relative to the internal wave fronts within which strong horizontal refraction occurs and 3D propagation models are required to accurately predict the range- and depth-dependent acoustic propagation.

Enhanced acoustic mode coupling resulting from an internal solitary wave approaching the shelfbreak in the South China Sea

Internal waves and bathymetric variation create time- and space-dependent alterations in the ocean acoustic waveguide, and cause subsequent coupling of acoustic energy between propagating normal modes. In this paper, the criterion for adiabatic invariance is extended to the case of an internal solitary wave (ISW) encountering a sloping bathymetry (i.e., continental shelfbreak). Predictions based on the extended criterion for adiabatic invariance are compared to experimental observations from the Asian Seas International Acoustics Experiment. Using a mode 1 starter field, results demonstrate time-dependent coupling of mode 1 energy to higher adjacent modes, followed by abrupt coupling of mode 5-7 energy to nonadjacent modes 8-20, produces enhanced mode coupling and higher received levels downrange of the oceanographic and bathymetric features. Numerical simulations demonstrate that increasing ISW amplitude and seafloor slope enhance the coupling of energy to adjacent and nonadjacent modes. This enhanced coupling is the direct result of the simultaneous influence of the ISW and its proximity to the shelfbreak, and, compared to the individual effect of the ISW or shelfbreak, has the capacity to scatter 2-4 times the amount of acoustic energy from below the thermocline into the upper water column beyond the shelfbreak in realistic environments.

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.

Acoustic mode coupling due to subaqueous sand dunes in the South China Sea

The large subaqueous sand dunes on the upper continental slope of the South China Sea are expected to couple acoustic propagating normal modes. In this letter, the criterion of adiabatic invariance is extended to the case of a waveguide possessing bedforms. Using the extended criterion to examine mode propagation over the bedforms observed in the sand dune field in 2012, results demonstrate that bedforms increase mode coupling strength such that the criterion for adiabatic propagation is exceeded for waveguides with small bedform amplitude to water depth ratios; increasing bedform amplitude enhances mode coupling. Numerical simulations confirm the extended criterion parameterization.

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.

Design and implementation of OFDM acoustic modem for underwater communication

This paper is aimed to develop a low-complexity receiver by treating Doppler rate (phase offset) as nuisance parameter in the hybrid channel estimation. For slowly changing channels, two simple adaptive equalization schemes, based on least squares (LS) and minimum mean-squared error (MMSE) principles, are proposed. For fast-changing channels, we adopt pilot-based channel estimation for equalization. We show the bit-error-rate (BER) performance of the OFDM detectors over a set of measured channel impulse responses in the northeastern sea of Taiwan. Simulation results reveal that adaptive equalizers and pilot-based equalizer can perform quite well in calm and fast-changing channel conditions respectively.

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.

Radial and azimuthal acoustic propagation effects of the continental slope and sand dunes in the northeastern South China Sea

A sound source, transmitting a 1.5-2.0 kHz chirp signal periodically, was towed at a depth of 50 m along a circular track that has a radius of 3 km centered on a vertical hydrophone array moored on the upper slope of the northeastern South China Sea during the Sand Dunes Acoustic Propagation Experiment in 2014. The largest amplitude of these sand dunes was close to 20 m with horizontal length scales between 200 and 400 m. Two-dimensional (2-D) and three-dimensional (3-D) underwater acoustic propagation models, namely FOR3D with the Nx2D option and FOR3D with the fully 3D option, were employed to simulate the acoustic propagation over the continental slope, with and without the sand dunes, from the towed source to the vertical hydrophone array. Environmental inputs to the models were measured bathymetry and sound speed profiles, obtained from multibeam echo sounding surveys and moored oceanographic sensors, respectively. Simulation results pertaining to the 2-D and 3-D propagation effects in relation to th...

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...