Brian Sperry

Vertical array receptions of the Heard Island transmissions

A long, vertical line array was deployed off Monterey, California during the Heard Island Feasibility Test to measure the modal content of the received signals. The array contained 32, equally spaced hydrophones spanning from 345 to 1740‐m depth. The multichannel data were recorded through a tether to the R/V Point Sur. The measurements had very low signal to noise ratios and indicated the cw transmission losses were approximately 140 dB for a source/receiver range of 17 000 km. Modal content was analyzed using (i) the modal extent versus depth, (ii) frequency‐vertical wave‐number spectra, (iii) modal beamforming and (iv) least squares fitting. All led to the conclusion that the modal population is surprisingly rich. There was strong evidence of population up to at least mode seven in the data.

SOUND TRANSMISSION AND SPATIAL COHERENCE IN SELECTED SHALLOW-WATER AREAS: MEASUREMENTS AND THEORY

Experiments from several shallow-water areas are summarized. Coherent sound transmission results, particularly wavenumber spectra, are compared to range-dependent calculations that use oceanographic and geophysical characteristics from measurements and archives as bounded inputs to the propagation codes. In general excellent agreement was obtained between the measured and calculated results for both narrowband and broadband transmissions between 50 Hz and 1 kHz to ranges of 40 km. A relative signal gain (RSG) method for the estimation of horizontal coherence length was applied to measured RSG results and yielded coherence lengths on the order of 30λ at 400 Hz at distances of 40 km. Perturbation theory was applied to the shallow-water waveguide under the condition of adiabatic normal modes and expressions were derived for the phase structure function that was simplified by the use of Gaussian correlation functions. These analytical results, along with estimates of the variances of the environmental variables permitted the estimation of the coherence function and the RSG. The calculated coherence function and RSG were found to be consistent with measured RSG and replica correlation results. The fluctuations in the oceanic water volume were found to be the dominant factor in the loss of coherence.

Three-dimensional propagation effects near the mid-Atlantic Bight shelf break (L)

Significant three-dimensional (3-D) environmental variability exists in the vicinity of the shelf break along the mid-Atlantic Bight. This study examines the influence of azimuthal coupling due to this variability on acoustic propagation in this region. Numerical studies employing a 3-D ray code, a hybrid ray-mode code, and a 3-D parabolic equation model are used to study the significance of azimuthal coupling on various propagation paths. These paths include up-slope, slant-slope, and cross-slope propagation. The numerical analysis suggests that, for the propagation ranges less than 60 km examined, the influence of azimuthal coupling is negligible compared to the inherent uncertainty in the environment itself.

Preliminary Acoustic and Oceanographic Observations from the Winter Primer Experiment.

Abstract : A joint acoustics and physical oceanography experiment was conducted in the winter of 1997 on the shellbreak and continental slope south of New England in the Middle Atlantic Bight. This experiment, Primer4, provided a seasonal contrast to the previous summer Primer3 experiment and had the same goals and tasks: to study the thermohaline variability and structure of the shellbreak front and its effects on acoustic propagation. To accomplish the linked oceanographic and acoustic objectives of this experiment, a combination of measurements were made. Seasoar hydrography, shipboard ADCP measurements, Satellite IR sea surface temperature field observations, and AXBT drops were employed to study the larger scale oceanographic fields. To study the finer scale, which includes internal waves, a number of rapid-sampling thermistor strings and current meters, including a moored, upward looking ADCP, were deployed. The acoustics components consisted of three 400 Hz tomography transceivers, a 224 Hz source and two hydrophone arrays. To study the geoacoustic parameters in the bottom a number of SUS charges were also deployed. The field setup was approximately the same for both the summer 1996 and winter 1997 experiments; however the weather conditions and the thermal structure of the mixed layer were radically different. This report is dedicated to the data from the Winter 1997 Primer4 experiment.

Consistency and accuracy of perturbative inversion methods for group travel time data

Perturbation theory for ocean acoustic modal group speed responses to small environmental changes is investigated with regard to its applicability to ocean acoustic tomography. Assuming adiabaticity, the inverse problem for each vertical eigenmode is an integral equation whose kernel involves the eigenfunction and its frequency derivative. A proof is given for the equivalence of two dissimilar forms of the integral equation. Numerical examples are given for the inversion kernel for four types of sound-speed profiles, and then the parameter range (amplitude and scale size) in which perturbation theory is accurate is examined. It is found that the range of validity is determined not only by the amplitude of the perturbations, but also by their vertical scale size.

Wave‐theory modeling of oceanic T‐phase coupling at continental margins and seamounts

The role of seismo‐acoustic seabed scattering as a mechanism for coupling of seismic energy into oceanic teleseismic waves or T‐phases is investigated using a new versatile modeling capability for seismo‐acoustic propagation in laterally inhomogeneous or range‐dependent ocean waveguides. The Virtual Source Approach (VISA) uses a local Rayleigh–Kirchhoff approximation to handle the transmission and reflection of plane waves at the vertical interfaces separating horizontally ocean stratified sectors. Combined with the wavenumber integration approach which inherently computes the plane‐wave decomposition of the seismo‐acoustic field in stratified fluid‐elastic waveguides, this approach provides a robust approximation to the seismo‐acoustic coupling phenomena in shallow and deep ocean waveguides. The VISA approach has been implemented in the OASES seismo‐acoustic modeling framework and used to investigate the role of seismo‐acoustic conversion and scattering by seabed topography and roughness in generating oc...

Mode excitation for a source on a rough, elastic, sloping bottom

In the ATOC program, the source was sited on a slope at a depth of the SOFAR axis so that the signal would excite the axial modes efficiently for long‐range propagation. One of the important issues was the elevation of the source off the seafloor. Buoying it on a mooring into the water column presented significantly more engineering challenges than simply resting it on a platform on the seafloor. Here, the impact upon the mode excitation spectrum of the source depth on a rough, elastic, sloping bottom is examined using the new range‐dependent OASES code. This code matches vertical wave numbers across range‐independent sectors, which may include interface roughness, as it steps out in range. It has been very successful in modeling the related problem of the effect of epicenter depth for T‐phase excitation on a sloping bottom [Sperry et al., J. Acoust. Soc. Am. 100, 2641(A) (1996)]. A Munk sound‐speed profile is assumed with a minimum at 1000 m and a sloping bottom out to a range of 30 km after which the bo...

Measurement and analysis of the propagation of sound from the continental slope to the continental shelf

Carried out jointly by the Woods Hole Oceanographic Institution (WHOI), Naval Postgraduate School (NPS), Harvard University, and the University of Rhode Island (URI), the measurement program of an integrated acoustic‐oceanographic field study called Shelfbreak PRIMER took place in the Middle Atlantic Bight. One of the goals of Shelfbreak PRIMER is to characterize and understand the propagation of sound from the continental slope to the continental shelf, including the effects of shelfbreak frontal processes, seasonal stratification, and topographic variations. The field work included two intensive 3‐week experiments, one in July 1996 (summer) and the other one in February 1997 (winter). In particular, each of the two experiments employed a suite of acoustic and oceanographic sensors including several transceivers/sources and two vertical hydrophone arrays (VLAs) straddling the shelfbreak front and a SeaSoar that provided several high‐resolution, three‐dimensional surveys of the frontal region. The results...

Possible mechanisms for T‐phase generation

Recent extensions to the wave‐number‐integration approach [Schmidt et al., J. Acoust. Soc. Am. 98, 465–472 (1995) and H. Schmidt, J. Acoust. Soc. Am. 97, 3316(A) (1995)] have made it possible to apply spectral methods to stepwise range‐dependent elastic problems. Using these methods, it is possible to study the potential mechanisms of T‐phase generation in oceanic waveguides. Such a study has applications in the prediction of tsunamigenisis from T‐phase observations. The general environment considered is a canonical sound‐speed profile and sloping bottom. The seismoacoustic field is determined via the above spectral methods, and then a modal decomposition is applied. The initial modal excitation and subsequent energy exchange from coupling is examined. The energy that becomes trapped in the water column is indicative of what might be observed on a distant hydrophone. The inclusion of elastic effects in these range‐dependent scenarios is likely to be critical in order to fully model the T‐phase generation....

Focusing effects due to solitons: 3D Gaussian beam modeling

As is well known, the SOFAR channel in deep water reduces spherical spreading, to cylindrical spreading, allowing sound to propagate to enormous distances. Similarly, soliton packets can produce channels in shallow water, forming acoustic corridors, their walls consecutive solitons. As the solitons pass over a propagation path they can generate dramatic focusing and defocusing effects. Acoustic modeling of such phenomena is challenging in that 3D (horizontal refraction) effects are clearly important (Nx2D approaches fail here). The geotime evolution is equally important—we must model a series of frozen oceans as the soliton packet passes by. Gaussian beam tracing models are ideally suited for such 4D modeling. We have developed a MATLAB Gaussian beam‐tracing model to address these problems. It includes capabilities for a variety of useful beam options, from ‘‘geometric beams’’ to the most formal beam theory based on paraxial approximations. The latter is implemented using a novel ‘‘reduced delta‐matrix fo...