Assessing the horizontal refraction of ocean acoustic tomography signals using high-resolution ocean state estimates

Abstract

The analysis of signals for acoustic tomography sent between a source and a receiver most often uses the unrefracted geodesic path, an approximation that is justified from theoretical considerations, relying on estimates of horizontal gradients of sound speed, or on simple theoretical models. To quantify the effects of horizontal refraction caused by a realistic ocean environment, horizontal refractions of long-range signals were computed using global ocean state estimates for 2004 from the Estimating the Circulation and Climate of the Ocean (ECCO2) project. Basin-scale paths in the eastern North Pacific Ocean and regional-scale paths in the Philippine Sea were used as examples. At O(5 Mm) basin scales, refracted geodesic and geodesic paths differed by only about 5 km. Gyre-scale features had the greatest refractive influence, but the precise refractive effects depended on the path geometry with respect to oceanographic features. Refraction decreased travel times by 5-10 ms and changed azimuthal angles by about 0.2°. At O(500 km) regional scales, paths deviated from the geodesic by only 250 m, and travel times deviated by less than 0.5 ms. Such effects are of little consequence in the analysis of tomographic data. Refraction details depend only slightly on mode number and frequency.

Seismic Ocean Thermometry of the Kuroshio Extension Region

“…If the excitation is substantially narrower, sharp fronts and internal waves can still produce such variations and suppress the waveform coherence. (d) Mesoscale anomalies and shifts in the Kuroshio Extension cause lateral deflections of the T-wave paths (e.g., Dushaw, 2014;Heaney & Campbell, 2016;Munk, 1980) that change between the repeating events. Sufficiently large deflections could produce lensing and thus changes in the received waveforms.…”

“…The T ‐wave excitation is understood to be confined to a narrow section of the trench, maybe a few tens of kilometers wide (de Groot‐Hedlin & Orcutt, 1999; Okal, 2008). Horizontal and vertical refraction by sound speed anomalies due to mesoscale eddies and other transients (e.g., Dushaw, 2014; Munk, 1980) could change the waveforms between repeating events. Interaction with bathymetry, especially the numerous tall seamounts in the region, can further induce mode coupling and reduce the SNR and CC.…”

WIGWAM Reverberation Revisited

“…Travel times of acoustic signals over 5 Mm ranges in the Pacific Ocean vary by less than 0:5 s over decadal time scales (Dushaw et al, 2009(Dushaw et al, , 2013. Horizontal refraction of those signals, from either oceanographic or topographic features, are mostly inconsequential (Munk and Zachariasen, 1991;Dushaw, 2014;Dushaw and Menemenlis, 2014), certainly inconsequential for the simple calculation employed here. Finally, the acoustic signals are dominated by propagation confined near the sound channel axis.…”

Surprises in Physical Oceanography: Contributions from Ocean Acoustic Tomography

“…In the central North Pacific, ray travel times over 1000 km are about 600 s, while the enormous heat (Dushaw et al 1993b). The effects of horizontal refraction and variations of salinity on tomographic measurements are negligible (Voronovich et al 2005;Dushaw 2014;Morawitz et al 1996;Dushaw et al 2009Dushaw et al , 2016b. Sound speed and temperature may be used interchangeably.…”