Observations of ocean spice and isopycnal tilt sound-speed structures in the mixed layer and upper ocean and their impacts on acoustic propagation.

A 400-m deep and 970 km long conductivity, temperature, depth section in the Northeast Pacific Ocean is decomposed into sound-speed variations associated with tilting isopycnals and ocean spice. The vertical distribution of sound-speed variance from these two processes shows significant fluctuations in the mixed layer (ML) and transition layer (TRL) below. Acoustic simulations at 400 and 1000 Hz are conducted with the decomposed fields to quantify their relative impact on upper ocean propagation for source locations in the ML and TRL. The low frequency simulations show that localized scattering processes dominate the propagation while higher frequencies experience more diffuse scattering. For propagation in the ML, spice generates the most loss while tilt can reduce loss when combined with spice. Statistics further show that energy can couple into and out of the ML duct depending on source depth and frequency.

Observations of the space/time scales of Beaufort sea acoustic duct variability and their impact on transmission loss via the mode interaction parameter.

The Beaufort duct (BD) is a subsurface sound channel in the western Arctic Ocean formed by cold Pacific Winter Water (PWW) sandwiched between warmer Pacific Summer Water (PSW) and Atlantic Water (AW). Sound waves can be trapped in this duct and travel long distances without experiencing lossy surface/ice interactions. This study analyzes BD vertical and temporal variability using moored oceanographic measurements from two yearlong acoustic transmission experiments (2016-2017 and 2019-2020). The focus is on BD normal mode propagation through observed ocean features, such as eddies and spicy intrusions, where direct numerical simulations and the mode interaction parameter (MIP) are used to quantify ducted mode coupling strength. The observations show strong PSW sound speed variability, weak variability in the PWW, and moderate variability in the AW, with typical time scales from days to weeks. For several hundreds Hertz propagation, the BD modes are relatively stable, except for rare episodes of strong sound speed perturbations. The MIP identifies a resonance condition such that the likelihood of coupling is greatest when there is significant sound speed variability in the horizontal wave number band 1/11

Beaufort Sea observations of 11 to 12.5 kHz surface pulse reflections near 50 degree grazing angle from 2016 to 2017.

Sea-surface acoustic scattering is investigated using observations from the 2016-2017 Canada Basin Acoustic Propagation Experiment. The motions of the low-frequency acoustic source and/or receiver moorings were measured using long-baseline acoustic navigation systems in which the signals transmitted once per hour by the mooring instruments triggered high-frequency replies from the bottom-mounted transponders. The moorings recorded these replies, giving the direct path and single-bounce surface-reflected arrivals, which have grazing angles near 50°. The reflected signals are used here to quantify the surface scattering statistics in an opportunistic effort to infer the changing ice characteristics as a function of time and space. Five scattering epochs are identified: (1) open water, (2) initial ice formation, (3) ice solidification, (4) ice thickening, and (5) ice melting. Significant changes in the ice scattering observables are seen using the arrival angle, moment of reflected intensity and its probability density function, and pulse time spread. The largest changes took place during the formation, solidification, and melting. The statistical characteristics across the experimental region are similar, suggesting consistent ice properties. To place the results in some physical context, they are interpreted qualitatively using notions of the partial and fully saturated wave fields, a Kirchhoff-like approximation for the rough surface, and a thin elastic layer reflection coefficient model.

Acoustic scattering comparison of Kirchhoff approximation to Rayleigh-Fourier method for sinusoidal surface waves at low grazing angles.

The Fourier series method for implementing the Rayleigh hypothesis [Rayleigh-Fourier method (RFM)] is used as a reference solution to assess the Kirchhoff approximation of the Helmholtz integral [Helmholtz-Kirchhoff approximation (HKA)] for modeling broadband scatter from sinusoidal surfaces at low grazing angles. The HKA is a valuable solution because it has an eigen-ray interpretation without unbounded caustic amplitudes and discontinuous shadow zones. Plane wave studies of the HKA, however, show it becomes inaccurate at low grazing angles. This study quantifies how this limitation manifests with increasing transmission distance for time domain scattering simulations. Scattering results are compared over a complete surface wave cycle with parameters modeling sea surface-swell. The HKA agrees reasonably well with the RFM in point source calculations for limited extensions of transmission distances beyond where plane wave comparisons begin to diverge. Past these distances, HKA solutions begin to show significant over-prediction of the acoustic amplitude around late arrivals. This over-prediction is frequency dependent and eigen-ray interference offers an explanation of this behavior. Further extending the transmission range leads to a significant HKA error, and a range is found at which flat surface reflections have less error.

Calibration of vertical array tilt using snapping shrimp sound.

Snapping shrimp are the dominant biological source of high-frequency (>2 kHz) ambient noise in warm coastal waters. In a recent experiment, the highly impulsive signals produced by shrimp snaps were recorded continually by a large-aperture vertical array (56 m) that was bottom-moored in 100-m deep shallow water. Assuming the array vertical, initial localization of individual snaps based on wavefront curvature along the array indicated that all snaps came from either above or beneath the flat seabed. By constraining all snaps to originate from the seabed, several hundred snaps within a radius of 500 m from the array over a 20-s window were detected successfully and localized in the three-dimensional space of time-of-arrival, range, and array tilt. Since the estimated array tilt for each snap is a projection of the absolute array tilt onto the nominal array-snap plane, the maximal tilt in the range and tilt domain corresponds to the absolute array tilt. Both simulations and data demonstrate that snapping shrimp can be exploited as a source of opportunity for calibration of vertical array tilt.

Observations of scatter from surface reflectors with Doppler sensitive probe signals.

Previous analyses of surface scatter from the at-sea KAM11 experiment were made with linear frequency modulated waveforms that emphasized a single slope direction for arrivals in time-varying impulse response estimates. This analysis of Doppler sensitive waveform transmissions, made in the same geometry, resolves additional scatter arrivals with opposite slope. The different Doppler shifts in scatter observations are related to dispersed, naturally occurring, sea surface features that reflect the transmitted waveform to the receiver. The positions of these surface reflectors are estimated from the delay and Doppler shift of the observed arrivals without needing a receiving array with high spatial resolution.