Measurements of low-frequency transmission loss in the Trans-Arctic Acoustic Propagation experiment

Both CW signals and phase-coded sequences of maximal length transmitted over 2600 km in the Arctic Ocean during the Trans-Arctic Acoustic Propagation (TAP) experiment have been processed to measure transmission losses of the low-frequency (19.6, 17.6, and 21.6 Hz) signals. The acoustic source level of 195 dB re I uPa was precisely controlled in the transmission ice camp "Turpan", north of Svalbard, by measuring a sound pressure vertical profile at a fixed distance of 100 m from the source. All TAP signals were received at the ice camp "SIMI" in the Beaufort Sea using vertical and horizontal arrays. Due to a relative drift of the ice camps, the signals were received at different ranges, which provided a two dimensional sound level spatial distribution. This measured sound pressure distribution was compared to the sound field predicted theoretically. A good agreement has been obtained for the signal levels and the interference structure. The pulse compression technique allowed the authors to recover a mode arrival structure of the phase-coded signals. Modes from 1 to 3 were clearly separated in the time domain, which provided the mode amplitudes and propagation time. The results correspond with the theoretical prediction based on up-to-date ice scattering models. The measured transmission losses at 20 Hz are significantly less (by approximately 10 dB) than the estimations made from the existent empirical formulae.