Three-dimensional spatial coherence measurements: Vertical, longitudinal/transverse horizontal coherence.

In the Shallow Water ’06 experiment, two L-shape arrays were deployed. Two horizontal components of these arrays were laid on the sea bottom. One was exactly perpendicular to the direction of sound propagation. The other was exactly parallel to the direction of sound propagation. This configuration offered an opportunity to directly measure the vertical, longitudinal horizontal, and transverse horizontal coherence. The results of spatial coherence were averaged over different pairs of hydrophones and over a frequency bandwidth of 100 Hz. The vertical coherence showed receiver depth dependence. When the source and the receivers were located below the thermocline, both the vertical and longitudinal horizontal coherence lengths (in units of wavelength) increased with increasing range and frequency. The longitudinal horizontal coherence length was much larger than the vertical coherence length. These results were similar to the predictions by Smith’s model [J. Acoust. Soc. Am. 60, 305–310 (1976)] with a frequency dependent bottom reflection loss [Zhou, Chin. Phys. 1, 494–504 (1981)]. The transverse horizontal coherence length/wavelength decreased with frequency. When the source depth was within the thermocline, the transverse horizontal coherence lengths exhibited weak range dependence in the 100–300 Hz range and was larger than 40 wavelengths. [Work supported by ONR.]