Shihong Zhou

Internal wave characteristics at the ASIAEX site in the east China Sea

The characteristics of internal waves (IWs) observed during the Asian Sea International Acoustics Experiment 2001 in the East China Sea are presented in this paper. Temperature data from a 17-element thermistor chain exhibit clear IW features in shallow water. Large-amplitude oscillations, up to 35 m, are noted due to the semi-diurnal internal tides. High-frequency (HF) and narrow-bandwidth IW trains around 6 c/h ride on semi-diurnal IWs. The spectrum of vertical displacement of the IWs, calculated from the thermistor chain data, falls as /spl omega//sup -1.6/ in the frequency band of 0.1-4 c/h. For higher frequencies (>6 c/h), the spectrum falls as /spl omega//sup -3.0/. Vertical coherence of the IWs for both semi-diurnal internal tides and HF IWs is analyzed. Comparisons of our observations with other data, obtained from SWARM95, the Barents Sea, and the Gulf of Mexico, display some common characteristics of shallow-water IWs.

Fluctuating modal interference characteristics in shallow water with a seasonal thermocline

In temporal and spatial array signal processing, especially for a large-aperture horizontal line arrays (HLA), degradation of the horizontal longitudinal correlation of signals among different hydrophones due to multipath dispersion commonly limits the array gain. It is therefore important to reduce or cancel the dispersion effects between hydrophone signals. Modal filtering is one of the available approaches to utilize the interference structure of sound fields and to improve the signal processing performance. In this paper, modal filtering based on a warping transform from a nonlinear time-frequency signal to a linear one is used to separate modes from a broadband signal. Then, the contributions of modes to interference striation are determined in order to explain the difference of the waveguide invariant between modes. The variability of waveguide invariants in fluctuating shallow water with a strong seasonal thermocline and internal waves is examined experimentally. The experiment was performed with a...

A Waveguide-Invariant-Based Warping Operator and Its Application to Passive Source Range Estimation

A formula for the instantaneous phase of the cross-correlation function of two different modes using the relationship between the horizontal wavenumber difference and frequency described by the waveguide invariant is deduced in this paper. Based on the formula, a waveguide-invariant-based warping operator suitable for both reflected and refracted modes in shallow water at low frequency is presented, providing an effective tool to filter the cross-correlation function of modes from the signal autocorrelation function. Using the phase of the filtered cross-correlation component in the frequency domain, a passive source ranging method on a single hydrophone is proposed. Simulated and experimental data using impulsive signals verify the validity of the derived warping operator and source ranging method.

The cooperative array performance experiment: A joint China‐US vector field experiment.

The cooperative array performance experiment (CAPEx) was performed in Lake Washington near Seattle in September 2009. Acoustic transmissions in the 1.5–4 kHz band were recorded simultaneously on two vertical arrays: one a conventional 32‐element pressure‐sensor array and the other an 8‐element array that measured both pressure and the three orthogonal components of acoustic particle velocity at each element. The present talk is an overview of both the data collected and the hardware used during CAPEx. Data were collected on the stationary arrays for both stationary‐ and towed‐source scenarios in water 60 m deep. The source‐receiver range varied between 10 m and 4 km. The data collected at short range demonstrate the relationship between the pressure and particle velocity fields. At more distant ranges, the particle velocity data are used to estimate the bearing to the source. Experimental results are compared to predictions generated using numerical models. [Work supported by ONR.]

Frequency warping transform of the vertical energy flux and its use for passive source ranging

The frequency warping transform of the vertical energy flux and a passive impulsive source ranging method with a single acoustic vector sensor are presented in this paper. The cross-correlation component of two different modes in the vertical energy flux is transformed into separable impulsive sequence. With a guide source, the source range is extracted from the relative delay time of the impulsive sequence. Comparing with warping the pressure autocorrelation function, there is no need to delete the modal autocorrelation component for warping the vertical energy flux, especially for its real part. Besides, the source ranging result based on the time delay of the warped vertical energy flux is much better for closer range. The frequency warping transform of the vertical energy flux and the passive source ranging method are verified by experimental data.

Characteristics of very-low-frequency pulse acoustic fields measured by vector sensor and ocean bottom seismometer in shallow water

In the marine environment, the effects of the seafloor and subbottom elastic media on the propagation of very low frequency (VLF) sound wave should be considered. In order to understand the mechanism of seismoacoustic field propagation, a very-low-frequency sound propagation experiment was conducted in shallow water using seafloor-located acoustic vector sensor and ocean bottom seismometers (OBS) in December 2017. The water depth is about 100 m. Pulse sources detanoted at depths of 7 m and 50 m were employed to emit the VLF signals. Comparing the signals received by OBS with that of the acoustic vector sensor, a narrow-band wave package group at VLF had been observed, which was excited by the interaction of acoustic waves in water with the layered shear seabed. In this paper, the pulse signals are used to analyze the characteristics of sound propagation and spatial correlation, amplitude and phase structure of separated normal modes based on warping transform, horizontal and vertical acoustic energy flow at different ranges. The time arrival structure and time expansion of the elastic-bottom-induced wave package group and its relationship with source distance and depth are also analyzed.

Analysis of vector sound field in the direct-arrival zone in deep water

Vector sensors have attracted extensive interest for their abilities to measure the scalar acoustic pressure field and particle velocity at a single location in space. During a deep-water experiment held in the South China Sea in 2014, a vector sensor placed at the depth of 3146 m received the experimental signals launched from a towed transducer at 140 m depth. In the direct-arrival zone of the sound field generated by the towed source, the propagation properties of the vector sound field are studied based on the ray theory. It is shown that both the energies of the horizontal particle velocity and the vertical particle velocity are closely related to the grazing angle, which varies greatly with the horizontal distance between the source and the receiver. Then a source-range-estimation method is proposed based on the energy difference between the horizontal particle velocity and the vertical particle velocity. The proposed method is tested by experimental data.