Jerald W. Caruthers

The Kauai Experiment

The Kauai Experiment was conducted from June 24 to July 9, 2003 to provide a comprehensive study of acoustic propagation in the 8–50 kHz band for diverse applications. Particular sub‐projects were incorporated in the overall experiment 1) to study the basic propagation physics of forward‐scattered high‐frequency (HF) signals including time/angle variability, 2) to relate environmental conditions to underwater acoustic modem performance including a variety of modulation schemes such as MFSK, DSSS, QAM, passive‐phase conjugation, 3) to demonstrate HF acoustic tomography using Pacific Missile Range Facility assets and show the value of assimilating tomographic data in an ocean circulation model, and 4) to examine the possibility of improving multibeam accuracy using tomographic data. To achieve these goals, extensive environmental and acoustic measurements were made yielding over 2 terabytes of data showing both the short scale (seconds) and long scale (diurnal) variations. Interestingly, the area turned out...

An iterative approach for approximating bubble distributions from attenuation measurements

A precise theory exists, based on an integral equation, by which acoustic signal attenuation versus frequency, due to a known bubble-density distribution versus bubble radius, may be calculated. Lacking a simple inversion scheme for the integral equation, an approximation which accounts only for attenuation due to resonant bubbles is available (and often applied) to calculate a bubble distribution. An iterative approach for improving on that resonant bubble approximation is presented here. That new approach is based on alternating calculations and corrections between attenuation data and the bubble distribution presumed to have produced it. This iterative technique is tested, first, on two simulated data sets of bubble distributions. It is then applied to attenuation data measured as a function of frequency from 39 to 244 kHz during the Scripps Pier Experiment [Caruthers et al., Proc. 16th Int. Cong. on Acoust., pp. 697–698 (1998)]. The results of the simulations demonstrate the validity of the method by ...

Acoustic attenuation in very shallow water due to the presence of bubbles in rip currents

An experiment was performed just off the research pier at the Scripps Institute of Oceanography to determine the acoustic effects of small bubbles in very shallow water (∼6 m depth). The distance offshore was ∼300 m. The propagation lengths were 2–10 m, and the frequency range was from 39 to 244 kHz. During the experiment, rip currents passed through the field of measurement instruments. These rip currents were laden with bubbles created in the surf between the instruments and the shore. The effects of these rip currents on the spatial distributions of the resulting acoustic attenuation are discussed. From the attenuation data, the bubble distributions are calculated using a new iterative approach [Caruthers et al., in press, J. Acoust. Soc. Am.] that is based on the well-known resonant bubble approximation. Calculated bubble distributions varied from an essentially uniform lack of bubbles during quiescent periods to highly inhomogeneous and dense bubbly regions within rip events. Such observed distributi...

Attenuation Measurements Across Surface-Ship Wakes and Computed Bubble Distributions and Void Fractions

A surface ships wake is composed of several hydrodynamic phenomena. A large part of that wake contains a mixture of air bubbles of various sizes in turbulent water. Eventually, as the wake ages, the turbulence subsides and bubbles begin to rise at rates that are determined by their sizes. These bubbles of various sizes and concentrations control the propagation of acoustic signals inside and across a wake. To further our understanding of these phenomena, a series of three continuous-wave (CW)-pulsed signals were transmitted across a wake as the wake aged. Each transmission contained a set of four 0.5-ms-long pulses. The 12 pulses ranged over frequencies from 30 to 140 kHz in 10-kHz steps. The acoustic attenuations across wakes that were due to varying bubble-size densities within the wakes were determined experimentally. From those data, estimates of the bubble densities as functions of the speed of the wake-generating ship, the wakes age, and acoustic frequency were calculated. From the bubble-density results, power-law fits and void fractions are calculated. The attenuation measurements were taken at 7.5-m intervals behind the wake-generating ship and continued for about 2 km. The experiment was run for wakes generated at ship speeds of 12- and 15-kn wakes, and the 15-kn run was repeated for consistence determination. The bubble densities were observed to have power-law forms with varying parameters with the strongest, for early ages, having an exponent of -3.6 and a void fraction of 4 x 10-7 , and with both diminishing for older wakes, as might be expected.

The partition wavenumber in acoustic backscattering from a two-scale rough surface described by a power-law spectrum

Scattering from the ocean bottom is often assumed to be controlled by two spatial scales: the larger scale associated with reflections from plane facets, and the smaller one associated with diffuse scattering from height variations. Choosing the wavenumber for this partitioning has proven to be important but troublesome. For this work, scattering data are simulated using Helmholtz-Kirchhoff or physical optics theory and selected input geomorphology. These data are inverted to provide rms slope of facets and rms heights of small-scale roughness using a simple two-scale roughness model introduced previously (J. W. Caruthers and J. C. Novarini, IEEE J. Oceanic Eng., vol. 18, pp. 100-106, 1993). Bottom relief is described by power spectra of the power law form, and the bottom is assumed to be impenetrable. The work introduces a new criterion for effecting this partition based on setting a roughness parameter equal to unity. The criterion is shown to be valid for the cases analyzed based on the ability of the inversion model to recover the input geomorphology. >

Near‐field acoustic scattering from simulated two‐dimensional, wind‐driven sea surfaces

The Helmholtz integral has been evaluated numerically in the near field for simulated two‐dimensional (2‐D), randomly rough surfaces and point sources. Surfaces chosen for simulation were wind‐driven sea surfaces having a Pierson–Moskowitz directional spectrum (wind speeds of 8 and 11 m/s). Estimates of the complex scattered field along with higher‐order statistical moments are calculated as ensemble averages. Predictions via this approach are compared over the frequency range from 0.2–2.0 kHz with analytic and numerical solutions obtained for the specular direction after applying the Fraunhofer approximation to both the source and receiver. Results indicate that the transition between far field and near field as well as from smooth to rough surfaces occurs at frequencies in reasonable agreement with predictions based on simple estimates. The technique is also used to show that the true 2‐D nature of the sea surface is important in the analysis of near‐field scattering from it. Differences between the out‐of‐specular scattering for up/down and cross‐wind cases are shown to be significant.

Observation of high-frequency sound propagation in shallow water with bubbles due to storm and surf

An experiment was performed off the shore of Panama City, FL, to measure the spatial and temporal coherence of high-frequency signals that were transmitted between fixed towers. Transmission was along paths at mid depths in about 10 m of water. During the time of the experiment, there were two stormy days with breaking waves and nearby high surf. It was observed that pulse-to-pulse variations (over seconds) in travel times, over a range of frequencies, increased dramatically from those observed on quiet days. Average travel times also increased by about 3%. Dispersion was also observed. The distance between the source and the receiver towers was approximately 60 m. By assuming that bubbles were either generated by breaking waves and advected down ward and/or generated by surf and advected outward, these results are explained. Estimates of the average bubble density and bubble-density variations are made.

Higher order corrections to an iterative approach for approximating bubble distributions from attenuation measurements

A formal theory exists for determining sound attenuation from a known distribution of bubble sizes in the ocean; however, an integral equation must be inverted if attenuation is given and the distribution of bubbles is not. An approximate distribution can be determined based on the resonant bubble approximation (RBA). An iterative approach, for which the RBA represents the zeroth iteration, was proposed and carried out to the first iteration in a previous paper . It was suggested that additional iterations would improve the bubble-distribution results. Here we formulate the procedure to carry the results to a higher order and demonstrate, based on a theoretical distribution of a multiple power law form, the improvements in successive approximations of the bubble distribution to the fourth iteration level. A recursion relation is developed that allows one to carry the iteration out to an arbitrary order. It is shown that regions of the distribution that change in the power-law exponent are places where the higher order corrections improve the results the most.

Effects of naturally occurring bubbles on multibeam sonar operations

The acoustic effects of bubbles can have a major impact on hydrographic operations using multibeam sonars and on the quality of the data they produce. It is well-known that bubble sweepdown, bubbles entrained beneath the hull due to high ship speeds or heavy weather, is a significant problem, and means to mitigate the effects are sought. The bubble problem discussed in this paper has a different source - one which hydrographers have not yet taken into account. The bubbles that cause this problem are those naturally occurring in the sea. Natural bubbles exist in the ocean due to many causes such as breaking waves, rip currents carrying bubbles produced in surf zones out into deeper waters, impact of rain drops, swim bladders of sea life, and gas hydrates bubbling up from the seafloor. This paper discusses the characteristics of bubbles caused by breaking wind waves and rip currents and their effects on multibeam-sonar operations.

Vertical profiles and horizontal scales of bubble clouds in the surf zone as measured with a distributed array of upward‐looking sonars

A collaborative, multi‐institute experiment was recently performed in the vicinity of the Scripps Pier, whose goal was the study of bubble‐field generation, transport, and distribution, as influenced by surf zone conditions. The collaboration produced an ensemble of instrumentation at the site for measuring bubbles, ambient noise, temperature and salinity, currents, and surface waves. This paper emphasizes data gathered by APL‐UW using a set of four upward‐looking transducers (frequency 240 kHz), which simultaneously measured vertical profiles of acoustic volume scattering from bubbles from four locations. The primary locus of measurement activity was defined by NRL‐SSC’s triangular frame (Delta Frame) and the four transducer locations for the APL‐UW system bracketed this region. The transport of bubbles via rip currents emerged as a key feature surf zone bubble environment. Images of volumetric backscattering strength versus time and depth reveal episodic events (by way of increased scattering level) whi...