This paper describes the signal processing scheme used in sources and receivers for the remote acoustic measurement of the oceanic sound velocity field. Ocean sound velocity varies in response to interior dynamic processes so that a continuous sound field measurement is expected to provide a unique view of physical oceanographic phenomena. The basic signal processing technique uses maximal length shift register sequences with frequency-domain (nonpower of 2 FFT) matched filtering.

An underwater acoustic pulse compression system

Continuous acoustic transmission (133 Hz, 60-ms resolution) between a bottom-mounted source near Oahu, Hawaii, and a bottom-mounted receiver at 4000-km range near the coast of northern California was recorded to learn how to measure precisely the travel time so that basin-scale fluctuations in the Pacific can be detected. Daily incoherent averages of some of the multipaths exhibited stability during this period. The standard deviation of the travel time of the resolved peaks in the daily incoherent averages is about 30 ms. An acoustic method, based on cross-correlation, is derived to estimate the change in the average acoustic phase (travel time) to a precision of about 0.018 cycles (135 mu s) every 2 min. Travel-time estimates based on the cross-correlator reduce the aberrations due to internal waves by about 19 dB in comparison with CW transmissions. The new travel-time estimator is applied to the measurements to examine some of the fluctuations of the Pacific. >

Ocean acoustic tomography: estimating the acoustic travel time with phase

Reviews the development of the theory and experiment for deep and shallow water since 1963. Theoretical approaches use rays, modes and a wide variety of other concepts. The classic case has a constant depth profile of sound velocity, but for long ranges one must allow for profiles varying both with range and with the transverse direction. Experimentally the transmission in deep water can be very good, especially for low frequencies in the SOFAR channel, where what little attenuation there is has been demonstrated to arise from a boron relaxation process. For shallow water the largest uncertainties at low frequencies can come from the unknown nature of the bottom structure. The study of variability has been helped by the existence of fixed sites, showing the occurrence of fluctuations with a great variety of mechanisms and time scales. For most of these the key is multipath interference, e.g. this comes into the acoustic effects of internal waves in the deep sea, and also into the spatial variability in deep channels.

https://iopscience.iop.org/article/10.1088/0034-4885/42/2/003/pdf

Guided acoustic waves in the ocean

Fluctuations in the Florida Current, summer 1970

Acoustic continuous-wave measurements are presented, which were carried out during June 1978 in the North Sea at a water depth of 30 m. The source was operated at a frequency of 295 Hz and the signal was recorded by two hydrophones at a distance of 1100 m south and east, respectively, from the source. The observed fluctuations of the acoustic amplitude and phase are analysed for periods from 10 min up to 2 hours and are compared with simultaneously measured currents. A theory is developed with the objective of relating the acoustic fluctuations to motions of the sea. For the considered periods and the density stratification during the experiment it is assumed that the observed currents are due to internal waves. The theoretical results agree well with the measurements.

Acoustic remote sensing of internal waves in shallow water

Sound waves of 420 Hz continuously transmitted across the Florida Current from a fixed source to fixed hydrophones are used to investigate relationships between temporal fluctuations of the waves and changes of Florida Current transport and water‐column temperatures. The amplitude and phase of the received waves show variations with periods in the range from seconds to weeks and longer‐term trends. Characteristically, variations of acoustic phase correspond to changes of transport and temperature, but fluctuations of acoustic amplitude are noiselike and not readily identifiable. Long period changes of transport and temperature result in slowly changing phases of the multipath arrivals and varying patterns of interference and amplitude fading. Spectra of the amplitude fluctuations show components over a wide frequency range which mask components corresponding to transport and temperature changes. The above characteristics also appear in calculations with ray‐propagation models in which sound speed versus depth was varied with time in accordance with measured variations. Since the models have plane parallel boundaries, characteristic features of the variations appear to be independent of surface and bottom irregularities. This raises questions of modeling amplitude fluctuations from transport and temperature changes obtained from acoustic phase measurements. The paper summarizes relevant observations and analyses.

K. Yacoub

Papers

Fixed‐System Studies of Underwater Acoustic Propagation