Observed limiting cases of horizontal field coherence and array performance in a time-varying internal wavefield

Abstract

Using a moored source and horizontal/vertical line array combination, horizontal coherence properties of high signal to noise ratio (≥20dB) 100–1600Hz signals have been measured. Internal waves in the area of the measurement created moving episodic sound-speed anomaly structures, influencing coherence length. Measured horizontal coherence scales for 100Hz ranged from 5 to 20 acoustic wavelengths, and were inversely related to the sound-speed anomaly strength. Horizontal field properties were compared with fields computed using modal decompositions of the vertical signals. The comparison allows azimuthal field coherence properties to be studied apart from normal-mode interference effects.

Acoustic Ducting, Reflection, Refraction, and Dispersion by Curved Nonlinear Internal Waves in Shallow Water

“…It is seen that these numbers have an average of about 25 wavelengths, in good agreement with the work of Carey [25]. Also, during periods of strong internal wave activity, this correlation length decreases significantly, in agreement with what was found numerically by Finette and Oba [4] and previously found in the limited analysis of Collis et al [23]. This lends further credence to the possibility that ducting was one of the mechanisms strongly affecting the acoustic signal.…”

“…17(e), we display the power in each of the received normal modes, obtained by mode filtering at the VLA located at the end of the HLA. Mode analysis procedures are described in [23]. This plot shows that internal wave events are generally accompanied by evidence of mode coupling, i.e., some energy is lost by mode 1 and is coupled into the higher order modes.…”

“…The HLA was oriented close to north/south on the seafloor with the VLA at the southern end. Sound transmitted along this path was analyzed by Collis et al [23] for horizontal coherence study and further information on the experimental equipment can be found in that paper.…”

Scales of time and space variability of sound fields reflecting obliquely from underwater slopes

“…Recent experiments with horizontal arrays have provided measurements of horizontal field coherence and horizontal array gain. (Orr et al, 2004;Collis et al, 2008;Duda et al, 2012) domains are very different. Likewise, incoherent beam power along the synthetic arrays differs between the two source positions.…”

Coherence length as a figure of merit in multireference near-field acoustical holography

“…From these data, the coherence length, L C , is determined. Although the term "coherence length" is used in other studies, such as in optical holography applications 15 and underwater acoustics, 16 here it is defined as the spatial distance L C along the array over which c 2 z m ;z drops from unity to some desired threshold. In this paper, a coherence threshold of c 2 z m ;z ¼ 0.5 is used.…”