Yongke Mu

Temporal and azimuthal dependence of sound propagation in shallow water with internal waves

The short time scale (minutes) and azimuthal dependence of sound wave propagation in shallow water regions due to internal waves is examined. Results from the shallow water acoustics in random media (SWARM-95) experiment are presented that reflect these dependencies. Time-dependent internal waves are modeled using the dnoidal solution to the nonlinear internal wave equations, so that the effects of both temporal and spatial variability can be assessed. A full wave parabolic equation model is used to simulate broadband acoustic propagation. It is shown that the short term temporal variability and the azimuthal dependence of the sound field are strongly correlated to the internal wave field.

Signal variability in shallow-water sound channels

Coherence of broad-band acoustic waves for mid-to-high frequencies (0.6-18 kHz) is obtained for a very shallow-water (15-m-deep) waveguide over a wide band of environmental conditions and for a source-receiver range of 387 m. Temporal behavior is sampled at two different rates: one that resolves at fractions of a second over intermittent periods of 40 s and another that resolves at 10 min over periods of several days. Spatial behavior is sampled in the vertical by hydrophones with spacings of the order of meters. The effect of environmental variability on coherence, in particular, soundspeed fluctuations in the water column and wind-induced modulations of the air-sea interface, is noted as a function of acoustic frequency and ray path. Analysis of the acoustic fluctuations over short time scales more accurately resolves the temporal decorrelation of the received signal due to sea surface waves. The vertical sampling of the received signal permits an analysis of arrival-angle fluctuations. The dependence of coherence on the number of surface bounces is studied by comparing arrivals that have zero, one, two, and three surface bounces.

From geology to geoacoustics—Evaluation of Biot–Stoll sound speed and attenuation for shallow water acoustics

A procedure for estimating acoustic wave velocity and attenuation in ocean sediment using a minimum amount of geological and geotechnical data is demonstrated. First, the Biot–Stoll theory is presented. Next, various asymptotic formulae for the attenuation coefficient are derived for high, low, and intermediate frequencies. These expressions clearly isolate the effects of intergranular Coulomb friction and fluid viscous dissipation on the attenuation of shear and compressional waves. Under the constraint of a minimum amount of geological and geotechnical information, a sequence of empirical equations is compiled to convert basic data, such as blow count number from a Standard Penetration Test or shipboard density, into sediment geoacoustic properties. As a demonstration, two well-known field cases, the Atlantic Generating Station (AGS) site and the Atlantic Margin Coring (AMCOR 6010) site, are examined. By incorporating the uncertainty involved in the data collection, the estimated geoacoustical parameter...

Parameter uncertainty analysis on acoustic response in fluid filled poroelastic media

This paper examines the effect of input parameter uncertainty in the prediction of acoustic response in ocean sediment. Due to the relatively large number of physical parameters in the model, and the difficulty of their direct measurement in the ocean environment, uncertainty in the predicted acoustic response is expected. Utilizing a second order Taylor series expansion, this paper demonstrates a methodology for estimating the output uncertainty in terms of statistical moments from the input uncertainty. The methodology is then tied to a practical procedure that uses engineering measurements and empirical relations to determine parameters such as shear modulus, bulk modulus, and permeability, from porosity. A related issue of pore size distribution and its effect on acoustic response is also investigated.

Frequency dependent transmission loss related to modal properties in layered nonlinear internal wave environments

Broadband data were taken during the SWARM 95 experiment from airgun and linear frequency modulated sources while nonlinear internal waves were crossing two different propagation tracks. In a previous study, the geotime behavior of the depth‐averaged received intensity was examined, and amplitude modulations were associated with the passage of internal waves. Additionally, a time‐frequency analysis of the data exhibited periodic changes in the modal structure of the signals. Though some discrepancies occurred, both of these effects were observed in parabolic equation simulations of the SWARM environment. To address the discrepancies in received intensity levels, various physical mechanisms will be investigated, for example geoacoustic attenuation and ocean thermocline variability. Effects on the time‐frequency representations of signals passing through internal wave packets will be documented and categorized. To understand features of frequency dependent transmission loss, relationships between the eigenv...

Broadband acoustic variability due to internal solitary waves

A shallow water (70–90‐meter‐deep) broadband acoustic experiment with a source‐receiver range of 15–18 km in an azimuthally dependent environment was designed as a part of the SWARM’95 field study. Temporal behavior of the water column was sampled every minute from two sources placed above and below the thermocline transmitting signals over a period of several hours while the water column was measured for the passage of internal waves. Spatial behavior was sampled in the vertical plane by hydrophones with spacing on the order of meters. Coherence of broadband acoustic waves for frequencies (20–300 Hz) is examined for the waveguide over different environmental conditions. The effect of environmental variability on coherence, in particular the sound‐speed fluctuations in the water column due to the internal solitary waves, is noted as a function of acoustic frequency and azimuth. Analysis of the acoustic fluctuations over short time scales (10–15 minutes) may resolve the temporal decorrelation of the receiv...

Current tomography in coastal regions

Reciprocal acoustic transmissions have proved successful for tomography in deep water. However, relatively little effort has been focused on shallow‐water environments where sound propagation and scattering takes place often over very short ranges in both the ocean (due to volume and surface) and the seabed. In these regions the effect of depth variations on sound speed is negligible and the salinity and temperature are dominant causes of volume fluctuations. Nevertheless, the methodology can still be utilized to determine the current. In a well‐calibrated high‐frequency (0.6–18 kHz) acoustic experiment, the range averaged horizontal current in two different directions is determined based on the differential travel times recorded from three reciprocal source–receiver stations. These tripod stations were arranged in a triangular configuration. Data were obtained in nearly isovelocity condition with slight variations in the salinity profile during several tide cycles. The feasibility of obtaining range aver...

On acoustic wave field response to internal waves in shallow water for variable source position

Acoustic variability is examined using a subset of the swarm\’95 experiment data in our earlier work [J. Acoust. Soc. Am. 108, 2577 (2000)]. Acoustic signals are highly modulated both temporally and spatially due to the presence of internal waves. In addition to energy fluctuations, a large level change in transmission loss was found in the airgun data due to source position changes. The mechanism that contributes to acoustic modulations is mainly due to acoustic mode coupling induced by internal wave propagation. The acoustic modes are decomposed from the data. Mode decorrelation time is examined for different propagation tracks. Fluctuations of the modal correlation indicate an acoustic recorrelation at a longer lag time, which also indicate acoustic intensity variations due to internal waves. Source location was found to be the major factor for high‐energy level variations. It is found that the airgun is depth position sensitive. The acoustic intensity and frequency variations due to source position ch...

Frequency analysis of broadband acoustic signals propagating through nonlinear internal waves

Nonlinear internal waves along the ocean thermocline are known to affect modal compositions and other properties of acoustic signals. Such nonlinear waves were observed during the SWARM 95 Orthogonal Propagation Experiment, where extensive data were collected from two broadband sources. One source was a 30‐s linear frequency modulated sweep signal, and the other was a 0.1‐s airgun shot. Both sources were repeated every minute and received by two vertical linear arrays several kilometers away. Sound speed profiles were measured near the source and the receiver arrays simultaneously. The influences of internal waves on the broadband frequency behavior of received and simulated signals are described. Features of energy spectral density and of transmission loss versus frequency plots are used to detect resonant frequencies where anomalous loss occurs. A time‐frequency analysis of received signals at several ranges is performed to determine modal propagation characteristics, including modal transitions of the ...

Analysis of internal wave interactions with broadband acoustic signals propagating along the shelf during the SWARM’95 experiment

A multi‐institution experiment was conducted in 1995 on the New Jersey continental shelf to assess the Shallow Water Acoustic propagation in Random Media (SWARM’95). Two major source/receiver geometries were established. One was parallel to the continental shelf edge and the other was perpendicular. The orthogonal propagation consisted of two broadband sources transmitting signals perpendicular to the direction of internal wave field. The first signal was a transient airgun source (0.1‐s duration), while the second was a linear frequency modulated sweep (30‐s duration). Placed above and below the thermocline, these sources were transmitting signals every minute for a few hours at one location. Here we present a 2‐h segment of these observations during 4 August 1995 when nonlinear internal waves were present. The transmissions were received by two vertical line arrays at different ranges and angles from the source. The dependence on the azimuth between the internal waves and the acoustic transmissions was ...