Observed intensity and horizontal field coherence variability of low‐frequency pulse transmissions on the continental shelf.

Abstract

Rapid coastal environmental evolution leads to highly variable acoustic fields. To quantify such variability, one component of the Shallow‐Water 2006 (SW06) program on the shelf east of New Jersey was time series measurement of sound transmitted from fixed sources to joined horizontal and vertical line arrays. Transmission paths were both cross‐shelf and along‐shelf (across and along dominant internal‐wave crests). Data were collected for over one month. Intensity time series of 100–400‐Hz pulses was found to have strong variability at periods from hours to over a day, consistent with long‐wavelength internal‐tide effects. Such effects can arise from adiabatic mode and/or coupled mode propagation. Separation of fluctuations into slow and rapid contributions allows calculation of a time‐varying horizontal coherence‐length statistic. For along‐wave crest transmission, this was highly variable, typified by values ranging from a few acoustic wavelengths to over 40 wavelengths, typically 10–25. The slow coherence‐length fluctuations had signatures of periodic (tidal) mode‐refraction episodes (with short scale) during active intervals, caused by internal‐wave ducting. Conditions were more steady at other times. Across‐crest transmissions showed shorter than expected scale lengths of tens of wavelengths with more subtle tidal dependence. [Work supported by the Office of Naval Research.]