Jee Woong Choi

Measurement and simulation of the channel intensity impulse response for a site in the East China Sea

A model for the channel intensity impulse response Ic(t) is presented that is generally applicable for source-receiver ranges less than ten water depths. The separate impulse response functions from each arrival, such as the single surface bounce or surface-to-bottom bounce, are modeled using bistatic scattering concepts and are incoherently summed for the total response function. The expression Ic(t) is equivalent to a time-averaged response and embodies the boundary scattering and reflection physics corresponding to the center frequency at which computations are made. To compare with observations, Ic(t) is convolved with representations of the 8- and 16-kHz continuous wave (CW) pulses, and an 8–16-kHz frequency-modulated (FM) pulse, that were used in the Asian Sea International Acoustics Experiment conducted in the East China Sea (depth 105m). For the FM case the computation frequency is 12kHz, the center frequency of the FM pulse. It is found that six primary arrivals dominate the response for ranges l...

Mid-to-high-frequency bottom loss in the east China Sea

Bottom-loss measurements made in the East China Sea in May-June 2001 as part of the Asian Sea International Acoustics Experiment as a function of frequency (2-20 kHz) and seabed grazing angle (15/spl deg/-24/spl deg/) are presented. The measurements are interpreted as estimates of the modulus of the plane wave reflection coefficient and data are compared to predicted values using a reflection coefficient model, based on a two-layered sediment for which the sound speed in the surficial sediment layer is allowed to vary as a linear k/sup 2/ profile, where k is acoustic wave number. The region below this layer is modeled as a half-space with constant density and sound speed. The reflection coefficient model is driven by eight geoacoustic parameters; these are estimated from the data by minimizing the weighted squared error between the data and the model predictions for a candidate set of parameters. The parameter estimates for the sediment layer are thickness, 0.9/spl plusmn/0.5 m; density, 2.0/spl plusmn/0.1 g/cm/sup 3/; and attenuation, 0.25/spl plusmn/0.05 dB/m/kHz, with sediment layer sound speed increasing from 1557/spl plusmn/4 m/s at the water-sediment interface to 1625/spl plusmn/35 m/s at a depth of 0.9 m. The parameter estimates for the half-space are density, 2.0/spl plusmn/0.1 g/cm/sup 3/; attenuation, 0.25/spl plusmn/0.15 dB/m/kHz; and sound speed, 1635/spl plusmn/52 m/s. Variances for these estimates are derived using the Bootstrap method. This parameter set produced model curves that agreed reasonably well with the observations of bottom loss over the entire frequency range and is consistent with the range of independently measured geoacoustic variables. Since this mid-to-high-frequency data set does not provide detailed information about the sediment structure for depths beyond about 3 m, the geoacoustic parameter set is more properly viewed as description of the sediment layer and sediments in the underlying 2 m. Similarly, a self-consistent construction of a geoacoustic model for the East China Sea should necessarily amalgamate the mid-to-high-frequency results given here with results obtained at lower frequencies.

Properties of the acoustic intensity vector field in a shallow water waveguide.

Acoustic intensity is a vector quantity described by collocated measurements of acoustic pressure and particle velocity. In an ocean waveguide, the interaction among multipath arrivals of propagating wavefronts manifests unique behavior in the acoustic intensity. The instantaneous intensity, or energy flux, contains two components: a propagating and non-propagating energy flux. The instantaneous intensity is described by the time-dependent complex intensity, where the propagating and non-propagating energy fluxes are modulated by the active and reactive intensity envelopes, respectively. Properties of complex intensity are observed in data collected on a vertical line array during the transverse acoustic variability experiment (TAVEX) that took place in August of 2008, 17 km northeast of the Ieodo ocean research station in the East China Sea, 63 m depth. Parabolic equation (PE) simulations of the TAVEX waveguide supplement the experimental data set and provide a detailed analysis of the spatial structure of the complex intensity. A normalized intensity quantity, the pressure-intensity index, is used to describe features of the complex intensity which have a functional relationship between range and frequency, related to the waveguide invariant. The waveguide invariant is used to describe the spatial structure of intensity in the TAVEX waveguide using data taken at discrete ranges.

First-order and zeroth-order head waves, their sequence, and implications for geoacoustic inversion

The relation between the head wave and an arrival often called a ground wave is analyzed with parabolic equation-based simulations, and an interpretation of such a ground wave as a head wave sequence is presented. For a Pekeris waveguide the envelope of the spectrum of the ground wave arrival corresponds to the spectrum of a single head wave, and spectral peaks correspond to odd multiples of the mode-1 cutoff frequency. This head wave is first order in its ray series classification and its amplitude spectrum goes as ∣S(f)∣∕f, where S(f) is the source amplitude spectrum. Basic variations from a Pekeris waveguide are considered; isospeed layers or a positive sound speed gradient in the seabed can each give rise to arrivals that are zeroth order in ray series classification and higher amplitude. For a sound speed gradient there is either a low-amplitude interference head wave whose properties are akin to a first-order head wave, or a high-amplitude interference head wave or non-interfering refracted wave who...

Precursor arrivals in the Yellow Sea, their distinction from first-order head waves, and their geoacoustic inversion

Measurements made as part of the 1996 Yellow Sea experiment at location 37 degrees N, 124 degrees E, undertaken by China and the U.S. are analyzed. Signals generated by explosive sources were received by a 60-m-length vertical line array deployed in waters 75 m deep. Evidence is presented that precursor arrivals measured at ranges less than 1 km are refracted waves that are zeroth order in their ray series classification, and this directly points to the existence of a gradient in sediment sound speed. In contrast, first-order head waves, which are much weaker in amplitude, would exist only if this gradient were absent. It is found that the energy spectrum of precursor arrivals agrees well with a zeroth-order model, i.e., it is proportional to the source amplitude spectrum, S(f), where f is frequency, rather than a first-order model, which would have it proportional to S(f)/f. From travel time analysis the sediment sound speed just below the water-sediment interface is estimated to be 1573 m/s with a gradient of 1.1 s(-1), and from analysis of the energy spectrum of the precursor arrivals the sediment attenuation is estimated to be 0.08 dB/m/kHz over the frequency range 150-420 Hz. The results apply to a nominal sediment depth of 100 m.

High-frequency bistatic sea-floor scattering from sandy ripple bottom

To obtain the bistatic scattering function on the sandy ripple bottom, high-frequency bistatic sea-floor scattering measurements were made in the shallow waters off the east coast of Korea. A sand ripple field was present at the site, with wavelength generally in the 10-20-cm range. The mean ripple orientation relative to the direction of wave propagation was estimated to be roughly 20/spl deg/-30/spl deg/. Field experiments were made to measure forward (in-plane) and out-of-plane scattering from the ripple bottom. The measured scattering strengths were compared to the predictions of the APL-UW bistatic scattering model. Overall, forward-scattering strength measurements showed favorable comparison with the model predictions. The global scattering characteristics for the ripple bottom gave an augmented out-of-plane scattering.

Overview of the KIOST-HYU Joint Experiment for Acoustic Propagation in Shallow Water Geological Environment

This paper presents an overview of the geological environment investigation and underwater acoustic measurements for the purpose of “Study on the Relationship between the Geological Environment and Acoustic Propagation in Shallow Water”, which are jointly carried out by KIOST (Korea Institute of Ocean Science & Technology) and Hanyang University in the western shallow water off the Taean peninsula in the Yellow Sea in April-May 2013. The experimental site was made up of various sediment types and bedforms due to the strong tidal currents and coastal geomorphological characteristics. The geological characteristics of the study area were intensively investigated using multi-beam echo sounder, sub-bottom profiler, sparker system and grab sampler. Acoustic measurements with a wide range of research topics in a frequency range of 20~16,000 Hz: 1) low frequency sound propagation, 2) mid-frequency bottom loss, 3) spatial coherence analysis of ambient noise, and 4) midfrequency bottom backscattering were performed using lowand mid-frequency sound sources and vertical line array. This paper summarizes the topics that motivated the experiment, methodologies of the acoustic measurements, and acoustic data analysis based on the measured geological characteristics, and describes summary results of the geological, meteorological, and oceanographic conditions found during the experiments.

Measurements of Monostatic Bottom Backscattering Strengths in Shallow Water of the Yellow Sea

ABSTRACT: Measurements of bottom backscattering strengths in a frequency range of 6-14 kHz were made on the shallow water off the southern Gyeonggi Bay in Yellow Sea in May 2013, as part of the KIOST-HYU joint acoustics experiment. Geological surveys for the experimental area were performed using multi-beam echo sounder, sparker system, and grab sampling to investigate the bottom topography, sub-bottom profile and composition of surficial sediment, respectively. In this paper, the backscattering strengths as a function of grazing angle (in range of 28° ~ 69°) were estimated and compared to the predictions obtained by Lambert’s law and APL-UW scattering model. Finally, the effects of geoacoustic parameters corresponding to the experimental area on the backscattering strengths are discussed.Keywords: Bottom backscattering, Reverberation, Bottom roughness, Seafloor volume scattering PACS numbers: 43.30.Gv, 43.30.Hw†Corresponding author: Jee Woong Choi (choijw@hanyang.ac.kr) Department of Marine Sciences and Convergent Technology, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan15588, Republic of Korea(Tel: 82-31-400-5531, Fax: 82-31-406-6255)

Spectral properties of the interference head wave

The interference head wave propagating through a sediment with a linear sound speed gradient is studied as a function of the parameter zeta, which is itself a function of acoustic frequency f, sediment sound speed and its gradient, and range. For zeta<1 the amplitude spectrum of the interference head waves goes as |S(f)|/f, where S(f) is the source spectrum. For increasing zeta beyond unity a more complicated modulation of S(f) ensues, which is explained by a channel transfer function H1(f), constructed analytically from a summation of terms involving zeroth-order refracted waves (referred to as a ray approach). For zeta greater than or approximately equal to 2 this summation compares well with a wave theory result for the interference head wave involving a fluid-fluid boundary. The amplitude spectrum of the interference head wave in the absence of sediment attenuation is |S(f)| x |H1(f)| and it is essential to know these to obtain an estimate of sediment attenuation from field observations. Examples of |S(f)| x |H1(f)| are presented for which H1(f) is computed directly using the ray approach and indirectly using the parabolic wave equation. A brief discussion on the application of these results towards the inversion of sediment attenuation is given.

Measurement of acoustic particle motion in shallow water and its application to geoacoustic inversion.

Within an underwater acoustic waveguide, the interference among multipath arrivals causes a phase difference in orthogonal components of the particle velocity. When two components of the particle velocity are not in phase, the fluid particles follow an elliptical trajectory. This property of the acoustic field can be readily detected by a vector sensor. A non-dimensional vector quantity, the degree of circularity, is used to quantify how much the trajectory resembles a circle. In this paper, vector sensor measurements collected during the 2013 Target and Reverberation Experiment are used to demonstrate the effect of multipath interference on the degree of circularity. Finally, geoacoustic properties representing the sandy sediment at the experimental site are inverted by minimization of a cost function, which quantifies the deviation between the measured and modeled degree of circularity.