Journal of Computational Acoustics

Single-mode excitation is a powerful tool for studying many oceanographic processes. Meanwhile, the complex time-variant ocean environment poses a great challenge for single-mode excitation because Green's function matrix of sound field changes quickly. In their previous work, the authors built a system to excite single-mode in a relative simple environment. The purpose of this paper is to study the feasibility of single-mode excitation in rapidly time-variant ocean environment. An improved recursive algorithm is presented to adapt for time-variant environment where the single-mode can be excited within very short time by this algorithm. A typical time-variant shallow water environment is simulated, and results of the single-mode excitation in this environment are presented.

Statistical problems encountered in the study of the influence of random inhomogeneities in layered shallow water on the propagation of sound signal is considered. The study is carried out by the example of two-layer models of the sea — a stochastic Pekeris waveguide and a waveguide with a regular refraction in the water layer, which describes the presence of the thermocline. The results were obtained by statistical simulation without approximations and assumptions. In the middle frequency range for actual parameters of sound speed fluctuations in shallow sea with a loss penetrable bottom, the specific features of acoustic field statistical moments behavior have been discovered. They did not get adequate attention in the scientific literature.

Internal waves in shallow-water cause variations in sound speed profiles and lead to acoustic travel-time perturbations. In summer 2007, a combined acoustics/physical oceanography experiment was performed to study both the acoustical properties and the ocean dynamics of the Yellow Sea. The internal waves were recorded by the thermistor arrays. The receiving hydrophone array is enabled to monitor the acoustic travel-time fluctuations over the internal wave activities. It is shown that the activity of high frequency internal waves (having 3–6 min period) dominated the travel time perturbation. In this paper, we compare the data of high frequency internal wave with acoustic travel-time perturbation data and analyze the correlation between them. A simple relation between the modal travel-time perturbation and the displacement of the thermocline is developed which might be useful for monitoring purposes.

Addressed is the physics of sound propagation through a shallow water waveguide whose seabed has a random rough surface. The basic coupled integral equations for the modal amplitudes can be split into forward and backward going coupled integral equations. Numerical solutions of the coupled equations are obtained for multiple roughness realizations from which ensemble averages can be obtained for the modal intensities and cross-mode coherence as a function of range. Sample calculations illustrate some of the basic characteristics of the average cross-mode coherence and intensity with and without seabed attenuation and for different source depths on range scales of 1500 acoustic wavelengths.

In shallow-water acoustic waveguide, the bottom of the sea (as the boundary condition that determines the Green's function of the sound propagation) plays an extremely important role in the sound field in water medium, especially at low frequency. Commonly the bottom is characterized by geoacoustic (GA) parameters and nearly all the existing numerical codes for sound field prediction use the GA parameters as the input of the bottom. However, the GA parameters are model based (needs a specified layered bottom model to mount the parameters in) which in practical applications may cause a series of problems such as model mismatching, extra uncertainty for field prediction and time consuming in MFP inverting. When the two bottom reflective parameters (P, Q) are used instead of GA parameters, which are model-free and directly determine the effect of the bottom on the sound field respectively in phase and strength, those problems will be solved naturally. In this paper, for range-independent waveguide using (P, Q) directly as input to construct the Green's function in terms of normal mode field is fully developed. For range-dependent waveguide, we propose an effective-equivalence based mapping between the bottom reflective parameters (P, Q) and the effective half-space (EHS) GA parameters to make (P, Q) as input parameters indirectly. Then all the existed numerical codes can still be used for the calculation of sound field without any modification except using P and Q as the input parameters of the bottom. The accuracy of this mapping is verified for both range-independent and range-dependent waveguide. At last the uncertainty analysis in GA space as well as in PQ space is briefly discussed to reveal the "extra uncertainty" due to the inter-coupling among the GA parameters.

To investigate statistical problem of moderate to low-frequency sound scattering on the two-dimensional (2D) random inhomogeneities of a shallow sea with horizontal boundaries (bottom and surface) and loss local-mode approach has been utilized. An explicit-form solution by quadrature for modal amplitudes as a function of distance is represented. In adiabatic approximation asymptotical estimations have been performed, which demonstrate how medium weak fluctuations influence the local eigenvalues and statistical characteristics of a field. It is shown that general effects previously established while studying the random layered problem keep the force. Spatial scales of statistical influence and laws of dependences are determined. Also it follows from the analysis of the explicit-form solution for modal amplitudes that in this shallow-sea model with horizontal boundaries and in the absence of regular variations of sound speed the coupling of modes is insignificant. General statistical effects are described well within the framework of adiabatic approximation.

Offshore wind farms are the main project on Taiwan's western coast. Since the underwater noise generated by piling poses a threat to marine mammals, the issue of the detrimental impact of noise on Sousa Chinensis has drawn considerable attention. To avoid behavioral disturbances and injury from pile driving noise, we propose the Underwater Noise Impact Region Alert System (UNIRAS) to estimate the acoustic field at any depth and distance from the piling sources. The system can be illustrated as four components: environment databases, an acoustic propagation model, source modeling and alert region (AR) prediction. Inputs are derived from the Taiwan Coastal Ocean Nowcast/Forecast System (TCONFS), which generates underwater sound speed profiles with temporal and spatial variation, along with geoacoustic and bathymetry databases that are imported as environmental inputs. Adiabatic mode theory is used to simulate the piling noise propagation in shallow water and the impulsive noise emanating from the source is evaluated via the finite element method. With the auditory threshold of cetacean being set as the criterion level, the system can demonstrate the modeling outputs and predict the noise impact region, and these results are useful for planning how to station the guard boats to prevent dolphins from entering the noise impact region.