Indra Jaya

Shallow‐water acoustic/geoacoustic experiments at the New Jersey Atlantic Generating Station site

Shallow‐water acoustic experiments were conducted at the Atlantic Generating Station site on the New Jersey Continental Shelf as a part of a long‐term study of the shallow‐water physics in this region. A vertical hydrophone array was used to assess the dispersion characteristics of the broadband acoustic field. A geoacoustic data set including bottom impedance profiles were constructed based on previous measurements of geological parameters from sediment core data. Experimental results show that the acoustic wave‐field dispersion is dependent on the azimuth angle. This dependency may be attributed to several factors relating to topography, range, and depth features.

Deterministic and stochastic analyses of acoustic plane‐wave reflection from inhomogeneous porous seafloor

The propagator matrix technique for the acoustic plane‐wave reflection in layered, porous, anisotropic seafloor is combined with Monte Carlo simulation to investigate the layering effect of the sediment. Statistical data from a geologically known site are processed and used as a benchmark. Realizations of sediment property profiles are generated using a spectral technique. A deterministic analysis demonstrates that the amplitude and phase of reflection coefficient is highly dependent on the variation of porosity. In a stochastic analysis, the effect of randomness is examined. It is found that the fluctuation of the sediment physical properties around a mean profile has a significant influence on the seabed reflectivity, particularly for the incident angles near and above the critical value. Frequency behavior of reflectivity is strongly dependent on the correlation length of medium inhomogeneities. This behavior is attributed to the interaction among different waves that propagate in the porous sediment l...

Time‐frequency energy distribution analysis of a shallow water acoustics experiment at the AGS site

The short time Fourier transform (STFT), wavelet transform (WT), and Wigner distribution (WD) are used to characterize the time‐frequency distribution of air gun signatures in azimuth and range. The signatures were obtained in a shallow water experiment conducted at the AGS site. The distributions are explained in relation to the acoustic interaction with the seafloor. Accurate descriptions of these distributions are important for better understanding of the acoustic field in shallow water. By means of a combination of the time‐frequency distribution and empirical orthogonal functions (EOF), the effects of higher‐order modes are studied and localized on the features of the received time series. Numerical simulations using the SNAP model are conducted to interpret some of the observed features. Comparisons between the simulated and measured time series indicate that the higher‐order modes are more sensitive to the fine layer structure of the seafloor physical properties. a)Present address: Office of Naval ...

Estimation of stochastic wave‐number variations at the Atlantic Generating Station site and comparison with acoustic data.

The estimation of horizontal wave‐number variations in a stochastic shallow‐water environment at the Atlantic Generating Station (AGS) site is constructed. A method using empirical orthogonal functions has been developed [Longfritz et al., J. Acoust. Soc. Am. 97, 3316 (A) (1995)] to relate wave‐number fluctuations to environmental variations. First, the AGS site is modeled as a stochastic shallow channel with a range‐dependent layered sediment bottom. The layer interface depths and intralayer sound speeds are treated as random variables. Then, the estimation procedure is applied to a particular propagation track, where geoacoustic profiles and acoustics measurements are both available. Environmental profiles along the track are treated as a sample from a stochastic ensemble. Estimates are obtained for both the range variance of the wave numbers and the particular deviations associated with each profile in the sample. Comparisons are made between results from the estimation procedure and simulations from t...

Mode coupling and mode stripping in propagation across the range‐dependent Atlantic Generating Station (AGS) site.

The complicated patterns of modal structure and attenuation observed in sound propagation through a range‐dependent shallow‐water environment are used to investigate and visualize the mechanisms of mode coupling and stripping. Broadband experiments have been carried out at the geologically known AGS site in the past few years. This presentation reports on a recent experiment which provides significantly more acoustic data along different propagation tracks and, consequently, permits high resolution of physical processes in range. A mode‐filtering technique is used to construct modal structure from the data, while modal attenuation is obtained from spectral ratios. The evolution of modal structure is determined as a function of frequency and range. This is examined in view of environmental parameters along a selected propagation track. Numericalsimulations are carried out using interpolations of the known geoacoustic environment and employing the KRAKEN normal mode and RAM parabolic equation models. Result...

Frequency dependence of sound propagation in shallow water: Theory and experiments

Experimental observations of broadband acoustic propagation in a known geological region of the Atlantic Generating Station (AGS) site [Badiey et al., J. Acoust. Soc. Am. 96, 3593–3604 (1994)] has prompted new approaches to understanding frequency‐dependent behavior in shallow‐water regions. First, recent acoustic observations and detailed geological borehole measurements are reviewed, along with a three‐dimensional model of the geoacoustic data that have been developed using the kriging method. Parabolic equation modeling, including range‐dependent sound speed and attenuation, is performed for both cw and broadband signals in this region. This is accompanied by normal mode investigations in which trapped modes in the layered media and range‐dependent mode coupling are examined. A modal‐based theory is presented to explain quantitatively the interference patterns observed in the experimental data (transmission loss versus frequency) in terms of waveguide parameters. It is shown how layered shallow‐water w...

Mode filtering of broadband signals for range‐dependent shallow‐water environments

A mode filtering technique is described and applied to broadband signatures obtained from recent shallow‐water acoustic experiments [Badiey et al., J. Acoust. Soc. Am. 96, 3593–3604 (1994)]. The technique uses a combination of a wavelet transform and singular value decomposition. The order of the modes is arranged by means of their respective energy levels. Each energy level is directly associated with the eigenvalues of the decomposed signal. The progressions of modal structures in time and frequency are given for various azimuthal propagation paths. Evolution of modal structure with frequency is examined by measuring the extent of the frequency band for each propagation path. It is shown that the pattern of the modal structure becomes more complicated for higher modes. Furthermore, from the time‐frequency analysis, it can be seen that the existence of a given mode can vary with time and that the medium filters the broadband signal. The mode shape is shown as a function of time, frequency, azimuth, sound...

A shallow water acoustic/geoacoustic experiment at the New Jersey Atlantic Generating Station Site

Shallow water acoustic experiments were conducted at the Atlantic Generating Station (AGS) Site on the New Jersey Continental Shelf as a part of a long‐term study of the shallow water physics in this region. A vertical hydrophone array was used to assess the dispersion characteristics of a broadband acoustic field. A geoacoustic data set and bottom impedance profiles were constructed based on previous measurements of geological parameters from sediment core data. Experimental results show that the acoustic wave field dispersion is dependent on the azimuth. This dependency may be attributed to several factors relating to topography and range and depth features. Empirical orthogonal functions (EOF) were used to decompose the acoustic normal modes. A preliminary discussion of simulation results and the sediment layering effects on the features of the acoustic normal modes and the corresponding group and phase velocities associated with each mode will be given in connection with the experiment. a)Present address: Office of Naval Research, Ocean Acoustics program Code 324OA, 800 North Quincy St., Arlington, VA 22217‐5000.

Parametric study and sensitivity analysis of plane‐wave reflection from poroelastic seafloor

Acoustic wave reflection from seafloor is very sensitive to the sediment properties. Assuming a plane wave reflected from an homogeneous but anisotropic seafloor, a parametric study is conducted for seabed properties such as shear and bulk modulus, permeability, porosity, etc. It is demonstrated from the numerical solution by the propagator matrix method that the porosity is the most sensitive parameters that affects the reflection coefficient and bottom loss. The effect is most pronounced at low grazing angles. The numerical evidence is further supported by an asymptotic analysis of the analytical expressions of sediment wave velocity and the reflection coefficient at the water–sediment interface, for homogeneous, isotropic material. The numerical study is then extended to inhomogeneous sediment and the sensitivity to different layering patterns is examined. a)Present address: Office of Naval Res., Ocean Acoust. Prog. Code 324 OA, 800 N. Quincy St., Arlington, VA 22217.

Stochastic analysis of a three‐dimensional seismoacoustic field

Based on geological information existing for a site on the New Jersey continental shelf [Badiey et al., J. Acoust. Soc. Am. 90, 2372 (A) (1991)], a three‐dimensional geoacoustic data set was generated using various widely accepted empirical relations. A statistical analysis is performed on the acoustic properties which reveals the random, inhomogeneous nature of seabed sediment. The statistical structure in both the horizontal and the vertical directions are quantified in terms of mean, standard deviation, and correlation length. The data set is then used in a theoretical modeling by both the deterministic and stochastic techniques. The deterministic technique utilizes an anisotropic, inhomogeneous, poroelastic model. The sensitivity of the various physical parameters to the seismoacoustic responses is examined. For stochastic analysis, the Monte Carlo technique is applied. Utilizing the statistical moments obtained from analyzing the field data, a large number of random realizations of seabed acoustic pr...