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

Abstract

Broadband acoustic signals were transmitted during November 1994 from a 75-Hz source suspended near the depth of the sound-channel axis to a 700-m long vertical receiving array approximately 3250 km distant in the eastern North Pacific Ocean. The early part of the arrival pattern consists of raylike wave fronts that are resolvable, identifiable, and stable. The later part of the arrival pattern does not contain identifiable raylike arrivals, due to scattering from internal-wave-induced sound-speed fluctuations. The observed ray travel times differ from ray predictions based on the sound-speed field constructed using nearly concurrent temperature and salinity measurements by more than a priori variability estimates, suggesting that the equation used to compute sound speed requires refinement. The range-averaged ocean sound speed can be determined with an uncertainty of about 0.05 m/s from the observed ray travel times together with the time at which the near-axial acoustic reception ends, used as a surrogate for the group delay of adiabatic mode 1. The change in temperature over six days can be estimated with an uncertainty of about 0.006°C. The sensitivity of the travel times to ocean variability is concentrated near the ocean surface and at the corresponding conjugate depths, because all of the resolved ray arrivals have upper turning depths within a few hundred meters of the surface.

Ray chaos and ray clustering in an ocean waveguide

“…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 equations (11) are, in general, nonintegrable and are known to have chaotic solutions even under a periodic perturbation H 1 [2,3,4,15,22].…”

“…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.…”

“…The ray chaos studies have been especially encouraged by the field experiments [11] where acoustical signals with 75 Hz center frequency and 37.5 Hz bandwidth, transmitted near the sound channel axis in the eastern North Pacific Ocean, have been recorded with a vertical receiving array between depths of 900 m and 1600 m at a range of 3250 km. The measurements have shown a clear contrast between well-resolved earlier portions of the received wavefronts, corresponding to steep rays with large values of the action variable, and smearing rear segments of the wavefronts corresponding to near-axial rays with small actions.…”

Manifestation of scarring in a driven system with wave chaos

“…One of the most intriguing object is the underwater acoustic (UWA) and the long range sound propagation in the ocean [13,14,15]. Application of methods of quantum chaos to the wave chaos proves to be efficient in many publications [16,17,18,19,20].…”