Acoustic fluctuations in the open ocean—A measurement using a fixed refracted path

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Abstract
Amplitude and phase (transit time) fluctuations in pulses sent between a fixed transmitter and a fixed receiver over a wholly refracted 17.2‐km path were recorded for 144.5 h. The sites were the southwest flank of Cobb Seamount and a lesser peak 17.2 km away, both at 1000 m depth. Eight‐cycle pulses at 4166 Hz and 16‐cycle pulses at 8333 Hz were sent alternately every 15.7 sec and received at three receivers located at 0, 5, and 15 m along a horizontal arm located perpendicular to the transmission path. Power spectra have been computed from the time series of phase and amplitude at a single receiver and the phase difference between two receivers. Extreme care was used in the analysis of the data to ensure that the time series obtained represented a single path. The power spectra of the phase data exhibit dominant tidal peaks at 24, 12.4, and 6.2 h; the power spectra of the amplitudes show less evidence of the tides. Between the inertial frequency and the Väisälä frequency, the power spectra of the phase, amplitude and phase difference fall off at approximately ω−3, ω−1, and ω−1/2, respectively. The power spectra of the phase differences for the 15‐, 10‐, and 5‐m spacings scale according to a plane wave arrival. The phase‐difference variance is below theoretical predictions, and the amplitude variance is above theoretical predictions. The high‐amplitude variance is tentatively identified as ’’micro’’ multipath interference. Evidence is presented to show that the oceanographic regime of the Cobb Seamount area is typical of the open ocean environment. Subject Classification: [43]30.20, [43]30.25.

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