Michael D. Vera

The effect of bottom interaction on transmissions from the North Pacific Acoustic Laboratory Kauai source

Acoustic signals transmitted from a 75-Hz broadband source near Kauai as part of the North Pacific Acoustic Laboratory (NPAL) experiment were recorded on an array of receivers near California at a range of 3890 km, and on a vertical line array at a range of 3336 km in the Gulf of Alaska. Because the source is approximately 2 m above the seafloor, and the bottom depth at the receivers near California is approximately 1800 m, acoustic interaction with the bathymetry complicates the identification of the recorded arrivals with those present in numerical simulations of the experiment. Ray methods were used to categorize acoustic energy according to interactions with the sea bottom and surface and to examine the significance of seafloor geometry. A modal decomposition was also used to examine the role of range-dependent bathymetry and to associate the effects on the acoustic field with seafloor features at specific ranges. Parabolic-equation simulations were performed in order to investigate the sensitivity of...

Comparison between ocean-acoustic fluctuations in parabolic-equation simulations and estimates from integral approximations.

Line-integral approximations to the acoustic path integral have been used to estimate the magnitude of the fluctuations in an acoustic signal traveling through an ocean filled with internal waves. These approximations for the root-mean-square (rms) fluctuation and the bias of travel time, rms fluctuation in a vertical arrival angle, and the spreading of the acoustic pulse are compared here to estimates from simulations that use the parabolic equation (PE). PE propagations at 250 Hz with a maximum range of 1000 km were performed. The model environment consisted of one of two sound-speed profiles perturbed by internal waves conforming to the Garrett-Munk (GM) spectral model with strengths of 0.5, 1, and 2 times the GM reference energy level. Integral-approximation (IA) estimates of rms travel-time fluctuations were within statistical uncertainty at 1000 km for the SLICE89 profile, and in disagreement by between 20% and 60% for the Canonical profile. Bias estimates were accurate for the first few hundred kilometers of propagation, but became a strong function of time front ID beyond, with some agreeing with the PE results and others very much larger. The IA structure functions of travel time with depth are predicted to be quadratic with the form theta(2)vc0(-2)deltaz(2), where deltaz is vertical separation, c0 is a reference sound speed, and thetav is the rms fluctuation in an arrival angle. At 1000 km, the PE results were close to quadratic at small deltaz, with values of thetav in disagreement with those of the integral approximation by factors of order 2. Pulse spreads in the PE results were much smaller than predicted by the IA estimates. Results imply that acoustic tomography of internal waves at ranges up to 1000 km can use the IA estimate of travel-time variance with reasonable reliability.

Examples and Applications In Long-Range Ocean Acoustics

Acoustic energy propagates effectively to long ranges in the ocean interior because of the physical properties of the marine environment. Sound propagation in the ocean is relevant to a variety of studies in communication, climatology and marine biology. Examples drawn from ocean acoustics, therefore, are compelling to students with a variety of interests. The dependence of sound speed on depth results in a waveguide that permits the detection of acoustic energy at ranges, in some experiments, of thousands of kilometres. This effect serves as an illustration of Snells law with a continuously variable index of refraction. Acoustic tomography also offers a means for imaging the oceans thermal structure, because of the dependence of sound speed on temperature. The ability to perform acoustic thermometry for large transects of the ocean provides an effective means of studying climate change. This application in an area of substantial popular attention allows for an effective introduction to concepts in ray propagation. Aspects of computational ocean acoustics can be productive classroom examples in courses ranging from introductory physics to upper-division mathematical methods courses.

Horizontal coherence in the NPAL experiment

Acoustic transmissions from a broadband source near Kauai with a center frequency of 75 Hz were recorded on a two‐dimensional receiver array at a range of 3900 km as part of the North Pacific Acoustic Laboratory (NPAL) experiment. The receiver array consisted of five vertical line arrays (VLAs), with separations transverse to the propagation path ranging from a few hundred meters to a few kilometers. The coherences derived from the data have been compared to two different numerical predictions based on sound‐speed perturbations due to internal waves. An approximation to the acoustic path integral yields a prediction for the length scale of horizontal coherence. Parabolic‐equation simulations of propagation from the source to each VLA through multiple realizations of a random internal‐wave field provide another estimate of the coherence at each horizontal separation. [Work supported by ONR.] a)J. A. Colosi, B. D. Cornuelle, B. D. Dushaw, M. A. Dzieciuch, B. M. Howe, J. A. Mercer, R. C. Spindel, and P. F. W...

Complex-density, equivalent-fluid modeling of acoustic interaction with the seafloor

Acoustic interaction with the seafloor can generate both compressional and elastic shear waves in the solid. Accurate models of shear propagation are often computationally expensive and difficult to apply to long-range propagation. When the sound field in the water is of primary interest, an equivalent-fluid model of the seafloor, with parameters chosen to match the reflection coefficient of the actual elastic solid, can sufficiently characterize the effect of the bottom on energy in the water. The effective density of the seafloor material in this approach can be a complex number. Prior methods for generating equivalent fluids were intended for low shear speeds and low grazing angles. Recent developments in the technique were intended to extend its validity to higher shear speeds and a wider range of angles. These efforts were initially motivated by the need to simulate bottom-interacting arrivals for the broadband Kauai source in the North Pacific Acoustic Laboratory experiment at megameter ranges. The ...

Comparison of ocean-acoustic horizontal coherence predicted by path-integral approximations and parabolic-equation simulation results

A line-integral approximation to the acoustic path integral has been used to generate predictions for the characteristic length scale of horizontal, cross-range coherence in long-range ocean-acoustic propagation. These estimates utilize a single range-independent sound-speed profile and the mean variance, as a function of depth, of fractional sound-speed perturbations due to internal waves. The length scales predicted by the integral approximation have been compared to the values generated by parabolic-equation simulations through multiple realizations of Garrett-Munk internal waves. One of the simulation environments approximates the Slice89 experiment; transmissions from a 250-Hz source were simulated in a deep-water transect to a maximum range of 1000km. The second environment corresponds to one of the propagation paths in the North Pacific Acoustic Laboratory (NPAL) experiment. The source in this experiment was bottom-mounted near Kauai, Hawaii and the relevant receiver consisted of five vertical line...

Numerical simulations of broadband acoustic propagation through a sound-speed field impacted by oil

Because its sound‐speed characteristics differ substantially from seawater, the presence of oil in the water column might be expected to impact acoustic propagation in the ocean environment. Empirical expressions are employed for the sound speed in oil; it depends on the API (a measure of density), temperature, and pressure. Environmental data taken in the vicinity of the Deepwater Horizon site taken during the 2007 Littoral Acoustic Demonstration Center experiment are used to generate profiles of temperature, pressure, and sound speed. Different values of API and different distributions for the weighting fraction of the oil on the total sound speed are employed in numerical simulations. The computed, received acoustic fields for different values and locations of oil concentration are compared to the case where no oil is present in several source‐receiver ranges. These forward models are intended to examine the feasibility of acoustically tracking oil as it travels from a source upward through the water c...

Modeling the acoustic receptions at the NPAL array from the Kauai source

Acoustic transmissions from a 75‐Hz source near Kauai to a vertical line array near California were recorded as part of the North Pacific Acoustic Laboratory (NPAL) experiment. Extensive environmental measurements were also performed as part of the experiment and were intended to ensure correspondence between numerical simulations and the data. Despite the availability of this information, the process of identifying the recorded arrivals with predictions has not been a simple one. Since the source is near the seafloor at about 800 m depth, and the depth at the receiver is approximately 1800 m, acoustic interaction with the bathymetry has been explored as a possible complication. Ray simulations that allow for specular reflection from the bottom indicate that fully‐refracted and bottom‐interacting paths can reach the receiver range (about 3900 km) at similar travel times. The simultaneous presence of both kinds of acoustic energy could contribute to the identification difficulties. A series of parabolic‐eq...

Inverting for the properties of an elastic seafloor using complex‐density equivalent fluids.

Receptions were recorded from a bottom‐mounted broadband source (with a center frequency of 75 Hz) located near Kauai as part of the basin acoustic seamount scattering experiment (BASSEX). Travel times for arrivals from this source have been modeled at basin‐scale ranges using complex‐density equivalent fluids for the elastic seafloor material. The collection of acoustic data at shorter ranges as part of BASSEX allows for a more detailed examination of the accuracy attainable by an equivalent‐fluid representation of the seafloor. The use of equivalent fluids is intended to depict the conversion of acoustic energy into shear waves. The performance and stability of propagation models can be substantially improved if equivalent fluids can be used to accurately characterize bottom interaction. Simulations have been performed in order to search for the equivalent fluid that best explains received data at a range of a few kilometers. The reflection coefficient of this equivalent fluid can then be matched to par...

Geoacoustic inversion over a basalt seafloor.

Geoacoustic inversion from data just off a volcanic island (Kauai) poses significant challenges. One of the significant challenges is to separate the effects of seafloor roughness and shear wave conversion in the basement. Lava flow fields are known to have significant roughness on several scales relevant to low‐frequency acoustic propagation (75 Hz). Basalt is known to have a very high‐compressional speed of 3000 m/s and a variable shear speed ranging from 500 to 2000 m/s. The combination of shear and roughness can lead to significant ambiguity while performing a geoacoustic inversion because both mechanisms can lead to attenuation of the coherent field. During the BASSEX 2004 experiment up to 20 transmissions from the fixed NPAL source (75 Hz, m‐seq with a 50‐Hz bandwidth) were received on a towed horizontal line array at many orientations and ranges from the source. In this paper we look at several short‐range transmissions and attempt to sort out the scattering effects of roughness with the attenuatio...