Measured wave-front fluctuations in 1000-km pulse propagation in the Pacific Ocean

Abstract

A 1000-km acoustical transmission experiment has been carried out in the North Pacific, with pulses broadcast between a moored broadband source (250-Hz center frequency) and a moored sparse vertical line of receivers. Two data records are reported: a period of 9 days at a pulse rate of one per hour, and a 21-h period on the seventh day at six per hour. Many wave-front segments were observed at each hydrophone depth, and arrival times were tracked and studied as functions of time and depth. Arrivals within the final section of the pulse are not trackable in time or space at the chosen sampling rates, however. Broadband fluctuations, which are uncorrelated over 10-min sampling and 60-m vertical spacing, are observed with about 40 (ms)2 variance. The variance of all other fluctuations (denoted as low-frequency) is comparable or smaller than the broadband value; this low-frequency variance can be separated into two parts: a wave-front segment displacement (with vertical correlation length greater than 1 km) that varies substantially between rays with different ray identifiers, and a distortion (with vertical correlation length between 60 m and 1 km) of about 2 (ms)2 variance. The low-frequency variance may be explained as the effect of internal waves, including internal tides. The variance of the broadband fluctuations is reduced somewhat but not eliminated if only high-intensity peaks are selected; this selection does not affect the statistics of the low-frequency fluctuations.

A test of basin-scale acoustic thermometry using a large-aperture vertical array at 3250-km range in the eastern North Pacific Ocean

“…Narrowband analyses showed a surprisingly rich modal population. The experiment that most closely resembles the one described here occurred during July 1989, when broadband signals were transmitted from a moored 250-Hz source to a 3-km-long vertical receiving array 1000 km distant in the north central Pacific Ocean ͑Howe et al, 1991;Duda et al, 1992;Cornuelle et al, 1993;Worcester et al, 1994͒. This experiment showed that energy confined near the sound-channel axis is significantly scattered by small-scale oceanic variability, reducing the vertical resolution that can be obtained using tomographic methods ͑Colosi et al, 1994͒.…”

Ray chaos and ray clustering in an ocean waveguide

“…Depending on the form of the background profile, coherent ray clusters may appear in earlier, middle and later portions of a timefront. In our opinion, a stability in earlier portions of the wavefront to be measured in the field experiments [10,11] could be explained by peculiarities of the respective background sound-speed profile.…”

“…When the sound waves propagate over long distances, an effective means for monitoring the medium is based on the effect of spatial variations of the sound speed on the signal arrival times, one of the main measurable characteristic in longbase acoustical experiments. Extensive field measurements, that have been carried out in recent years [10,11], showed smearing of timefront segments in the rear of the sound pulse. Hardly resolvable microfolds in the late-arriving portions of the timefront, to be observable in field experiments, can be reasonably explained by the ray's sensitivity to initial conditions.…”

“…The sensitivity of chaotic rays to initial conditions and small variations of the environmental parameters causes a smearing of some timefront segments (representing time arrivals in the time-depth plane) that has been really observed in the field experiments [10,11]. Ray chaos seems to pose restrictions on the ray perturbation theory based applications to the tomography.…”

Shallow Water Internal Waves and Associated Acoustic Intensity Fluctuations

“…Studies conducted over the past several years established internal waves as a dominant factor in causing fluctuations in the travel time of acoustic pulses sent over long distance through the ocean [1][2][3] . The internal waves are considered to be a limiting factor in the propagation of acoustic energy in the frequency range 50 Hz to 20 kHz and the effects are manifested as amplitude and phase variations in the acoustic signal 1 .…”