Temporally stable bottom-interacting upslope sound propagation south of Malta

The influence of dynamical ocean features on seabed-interacting acoustic transmissions was investigated southeast of Malta in the Mediterranean Sea. Waveforms at 610-890 Hz were transmitted to mimic impulse propagation over two fixed paths of 25 and 45 km length. Area conditions included topographically trapped diurnal waves, stochastic internal waves, a 60-m-thick surface mixed layer, and weak mesoscale flows. Many stable arrivals with transmission loss of 57 to 66 dB were observed on the short path. The matched-filter recovered impulse response showed a high degree of temporal coherence over the 5-d-long experiment. Travel times of tracked individual signal peaks showed root mean square variation of 3 ms. Much of this reported travel-time variability results from slowly fluctuating dominance between micromultipaths, rather than time variation of a single steady ray. Spectral analysis of the travel times provided weak evidence of a peak at the diurnal tidal frequency, with no reliable evidence of semidiurnal-band oscillations of arrival peak levels or times. Most rays interact with the seabed, and model simulations with different bottom attenuation coefficients support intuition that seabed properties determine a base-state transmission loss and impulse response in the area, only slightly modified by water column variations.