Analysis of acoustic propagation in the region of the New England continental shelfbreak

Abstract

During July and August of 1996, a large acoustics/physical oceanography experiment was fielded in the Mid-Atlantic Bight, south of Nantucket Island, MA. Known as the Shelfbreak Front PRIMER Experiment, the study combined acoustic data from a moored array of sources and receivers with very high resolution physical oceano graphic measurements. This thesis addresses two of the primary goals of the exper iment, explaining the properties of acoustic propagation in the region, and tomo graphic inversion of the acoustic data. In addition, this thesis develops a new method for predicting acoustic coherence in such regions.Receptions from two 400 Hz tomography sources, transmitting from the continen tal slope onto the shelf, are analyzed. This data, along with forward propagation modeling utilizing SeaSoar thermohaline measurements, reveal that both the shelfbreak front and tidally-generated soliton packets produce stronger coupling between the acoustic waveguide modes than expected. Arrival time wander and signal spread show variability attributable to the presence of a shelf water meander, changes in frontal configuration, and variability in the soliton field. The highly-coupled nature of the acoustic mode propagation prevents detailed tomographic inversion. Instead, methods based on only the wander of the mode arrivals are used to estimate pathaveraged temperatures and internal tide "strength".The modal phase structure function is introduced as a useful proxy for acoustic coherence, and is related via an integral transform to the environmental sound speed correlation function. Advantages of the method are its flexibility and division of the problem into independent contributions, such as from the water column and seabed.

Characteristics of acoustic propagation to the eastern vertical line array receiver during the summer 1996 New England shelfbreak PRIMER experiment

“…If this were to come entirely from a change in average water temperature along the acoustic travel path, it would require a of 1.3 C, assuming an average mode-1 group velocity of 1480 m/s. This is found to be in good agreement with the findings from a simplified acoustic inversion for range-and depth-averaged water temperature [17] that is compared to both SeaSoar and thermistor records.…”

“…12 shows the signal spread, as measured by the IQR, for mode 1 for the SE source receptions. Higher modes were seen to have progressively more spread (not shown here), some of which is due to increasing amounts of modal filtration leakage ("crosstalk") from the aperture-limited mode filtering [17]. The values shown in Fig.…”

“…It is well known that a certain amount of travel-time variability will result from tidal activity on the continental shelf. For instance, calculations by Headrick et al and Sperry [5], [17], among others, have shown how tidally driven solibores can introduce tidal period wander and mode coupling: however, there are also many other direct means by which the tides can influence the acoustics. These include tidal currents (both barotropic and baroclinic), horizontal advection of water masses by the tides, and the simple barotropic change in water depth.…”

“…To estimate the effects of horizontal refraction for the PRIMER experiment, the "horizontal ray/vertical mode" theory developed by Weinberg and Burridge [23] was applied using the USGS bathymetry for the region and a 3-D sound speed field constructed from SeaSoar data. In the worst case scenario, the differences in mode travel times between straight-line propagation and the horizontally refracted path were less than a millisecond [17]. Straight-line propagation from source to receiver can therefore be assumed, given the smallness of this effect.…”

Tidal Analysis Near the Shelfbreak in the Middle Atlantic Bight

“…The work resulted in one Ph.D. thesis (Sperry, 1999), as well as two manuscripts which have been submitted to refereed journals (Lynch et al, 2000, andSperry et al, 2001). …”