Zhaohui Peng

Reverberation vertical coherence and sea-bottom geoacoustic inversion in shallow water

Optimal array-processing techniques in the ocean often require knowledge of the spatial coherence of the reverberation. A mathematical model is derived for the reverberation vertical coherence (RVC) in shallow water (SW). A method for analysis of RVC data is introduced. Measured reverberation cross-correlation coefficients as a function of time and frequency, obtained during the Asian Seas International Acoustic Experiment (ASIAEX) in the East China Sea, are reported. SW reverberation from a single shot provides a continuous spatial sampling of the surrounding sound field up to several tens of kilometers and holds valuable information on the geoacoustic properties of the sea floor over this distance. SW reverberation data can, therefore, be used as the basis for a quick and inexpensive method for geoacoustic inversion and has the obvious advantage that acquiring the data in situ requires only a single platform. This paper considers the use of the vertical coherence of the reverberation as the starting point for such an inversion. Sound speed and attenuation in the sea bottom at the ASIAEX site are obtained over a frequency range of 100-1500 Hz by finding values that provide the best match between the measured and predicted RVC.

Sea surface effect on shallow-water reverberation

Wideband reverberation measurements were made at a fixed location in the East China Sea on 3 and 5 June 2001 using the same measurement system. Sound-speed profiles were similar during both measurements. Wind speed (W) and rms surface-wave height (σ) changed from 2.74m∕s and 0.10m on 3 June to 7.45m∕s and 0.33m on 5 June. Thus, these measurements offer an opportunity to evaluate sea-surface effects on reverberation vertical coherence (RVC), RVC-inverted bottom acoustic parameters, and reverberation level (RL) in shallow water. The two sets of RVC and RL data, in a frequency range of 100–2500Hz, show differences that are the apparent effects of the surface roughness. With increasing sea state, the RVC increases and the RL decreases. The effective bottom losses, inverted from the RVC data, correspond to the variation of sea state. This additional loss gives a physical explanation of the characteristics of both the measured RVC and RL. These preliminary findings show the importance of surface effects in shal...

On shallow-water bottom reverberation frequency dependence

Determinations of bottom scattering strength in the decade below 1 kHz under downward refracting conditions have been made using acoustic reverberation and transmission data from the 2001 East China Sea Asian Seas International Acoustic Experiment (ASIAEX). The measurements were performed using explosive sources and receiving hydrophones in ship-suspended vertical-line arrays. The focus of this paper has been the dependence of bottom scattering strength on the frequency and characterization of the uncertainties associated with the extraction of scattering strength from reverberation. The derived bottom scattering strength gradually rises with frequency from 100-300 Hz and then more rapidly above 300 Hz. A potential explanation suggests that the frequency variation results from two scattering mechanisms, rough layer scattering at the low end of the band and sediment near-surface volume scattering at the high end. The spatial extrapolation of these results is explored by comparing them with similarly derived scattering strengths using data obtained under the Navys Harsh Environments Program at a somewhat separated site (56 km) under environmental conditions similar to those during ASIAEX. In the ASIAEX analysis, it has been found that the largest source of uncertainty in the scattering-strength frequency dependence arises from persistence of finite-amplitude effects associated with the source signal

Integrating the energy flux method of shallow‐water reverberation with physics‐based seabed scattering models.

The energy flux (angular spectrum) method for shallow‐water (SW) reverberation was first presented 3 decades ago in archived Chinese journals. These included closed form expressions for reverberation in SW with a down refraction or isovelocity profile. [Zhou, Acta Acust. 5, 86–99 (1980); Chin. J. Acoust. 1, 54–63 (1982)]. More papers on SW reverberation using this method have recently been published in JASA and IEEE JOE. However, most of these papers are based on semiempirical seabed scattering models, which do not have a physical basis. In the past 30 years, one of the major accomplishments in ocean acoustics is the improvement in our understanding of seabed scattering, resulting from a significant effort of both at‐sea measurement and theoretical modeling. [Jackson and Richardson, High‐Frequency Seafloor Acoustics, (2007)]. Benefiting from this accomplishment, this paper integrates the energy flux model of SW reverberation with the physics‐based seabed scattering models in the angular domain. This integ...

Analysis of time series data in the East China Sea generated from explosive sources

Time series data collected on the APL–UW/URI VLA in the East China Sea as part of the Asian Sea International Acoustics Experiment are analyzed for the information they contain on the characteristics of the seabed. Sound generated by explosive sources deployed by the IOA propagates in a shallow water wave guide under downward refracting conditions, making the received field at the VLA sensitive to the structure of the seabed. A broadband normal mode approach is used to model the measured time series in the 10–500 Hz band. The complex multipath arrival pattern as a function of source–receiver range and source depth allows one to infer certain characteristics of the seabed without the aid of an inversion approach. A finer specification of the seabed, including the determination of the statistics of the geoacoustic parameters is achieved by a simulated annealing inversion methodology. The sensitivity of the acoustic propagation to the elastic properties of the seabed is also investigated along with the natur...

Modeling of reverberation in the East China Sea

Reverberant time series recorded in the East China Sea component of the Asian Seas International Acoustics Experiment are analyzed with the aid of a model of acoustic scattering from inhomogeneities in the seabed. Wideband sources deployed by the IOA were used to produce the time series, which were recorded on a thirty‐two element VLA also deployed by the IOA. The model employs perturbation theory with the framework of a normal mode approach to evaluate scattering by fluctuations in the sound speed and density in the sediment volume as well as of the roughness of the water–sediment interface. The relative importance of surface and volume scattering at low frequencies in shallow‐water environments, such as the East China Sea experimental location, is considered. Also, by incorporating this model in an environmental inversion scheme, information about the distribution of inhomogeneities may be gained. [Work supported by ONR.]

Separation of scattering mechanisms for the Asian Sea International Acoustics Experiment East China Sea reverberation measurements

Low‐frequency monostatic reverberation data collected in the East China Sea on a VLA are analyzed to infer bottom scattering strength characteristics. The VLA was deployed and the data collected by researchers from the Institute of Ocean Acoustics, Beijing, China. Reverberation data originating from ranges from 3–15 km are analyzed using coherent array processing methods to determine scattering strengths as a function of frequency and angle at the scatterer. Subaperture processing is used to separate sea surface and bottom contributions and to gain physical insight into the scattering mechanisms responsible for the observed reverberation level. Modeled transmission loss obtained from analyses of measured forward propagation data is employed within the framework of the subaperture signal processing to enable the extraction of the scattering strength. [Work supported by ONR.]

Ocean reverberation: Modeling, measurements and inversions

Research on ocean reverberation has practical and scientific significance. Much progress has been made in the past three decades to improve our understanding of reverberation. However, there remain important unanswered questions and a real scarcity of high-quality basic research data sets. New progress on the reverberation modeling and the low-frequency (LF) seabed scattering characterization in shallow water (SW) requires three essential conditions: 1). A reliable reverberation model using a physics-based seabed scattering function, 2). Carefully calibrated broadband reverberation data, and 3). A ground truth about the seabed geoacoustic model. Some related work on these topics is introduced in this paper. The energy flux method for SW reverberation is briefly introduced. Integration of this method with physics-based seabed scattering models directly and intuitively results in a general expression for SW reverberation. A simple relationship between the classic scattering cross-section and the modal scatt...

A comparison of East China Sea low frequency bottom scattering strength determinations

Low frequency shallow water bottom scattering strength determinations conventionally involve accounting for the two‐way transmission from source to scattering region to receiver and correcting for the size of the contributing bottom scattering area. Experiment uncertainties in transmission loss, bottom homogeneity and isotropy, contributions from volume, and ocean surface backscatter, can all contaminate the accuracy and robustness of such determinations. Bottom scattering strengths were determined in octave bands from 50–800 Hz reverberation data using Institute of Acoustics sources and the receiving array on Shi‐Yan III and from transmission data obtained on the APL‐UW/URI receiving array on R/V Melville during East China Sea ASIAEX in 2001. Seabed geoacoustic parameters were inferred from the forward data, which were then used to model transmission and extract scattering strength from the reverberation data. The ASIAEX scattering strengths were compared with other measurements of bottom scattering stre...

Analysis of sound propagation data taken in the East China Sea

As a part of the Asian Seas International Acoustic Experiment (ASIAEX2001), sound propagation data from wideband (explosive) sources were recorded in the East China Sea by using a 32‐element suspended array. The propagation was measured as a function of range in two perpendicular tracks (one up to 60 km) and as a function of azimuth for a fixed range of 30 km. Supporting environmental data, obtained from a 17‐element thermister chain, XBT, and CTD, showed very complex variation in the water column. In this paper, transmission loss (TL) as a function of range, frequency and azimuth is briefly introduced. Seabottom acoustic parameters such as density, velocity, and attenuation are inverted from the sound propagation data. Then, these parameters plus internal wave data are used as inputs to PE and normal‐mode codes qualitatively to explain observed strong fluctuations in sound propagation. The inversion techniques used for estimating seabottom parameters, including spatial mode filtering and dispersion analy...