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

Caruthers, Jerald W.; Stanic, Steve; Elmore, Paul; Goodman, Ralph R.

doi:10.1121/1.427030

Cited by 20 publications

(11 citation statements)

References 7 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…When bubble clouds reached the delta frame, they were readily apparent through their attenuation of acoustical signals measured on the multiple frequency transmission paths [Caruthers et al, 1999]. Over the 10 day duration of the experiment, approximately one in three rip current events brought significant bubble concentrations down to the sensor depth.…”

Section: Observationsmentioning

confidence: 99%

Bubble transport in rip currents

Vagle¹,

Farmer²,

Deane³

2001

J. Geophys. Res.

View full text Add to dashboard Cite

Abstract.A number of remote and in situ acoustical sensors were mounted in the surf zone off Scripps Pier, La Jolla, California, during March 1997 and used to characterize rip currents and their ability to transport bubbles offshore from the surf zone. The bubble size distributions and air fractions at the injection point were measured using an acoustical resonator and a conductivity sensor right in the surf zone. The offshore moving bubble plume was tracked with a multibeam 100 kHz Doppler sidescan sonar, giving the spatial extent and offshore variability in the rip events as well as information about the surface wave field. Furthermore, three acoustical resonators were mounted at 2.5 m depth 55-60 m farther offshore to probe the bubble field 300-400 s after injection. These results are interpreted using models that include bubble buoyancy and dissolution, rip current advection, and the turbulent boundary layer, which is represented by the Christoffersen and Jonsson [1985] formulation. IntroductionCoastlines are of great importance for recreation, commerce, and the military. However, man-made interference, sea level rise, and large storms threaten the sandy shorelines. Understanding the nearshore processes is therefore increasingly important. Sea and swell waves incident on a beach can drive strong quasi-steady currents in the surf zone. Gradients in wave radiation stress caused by breaking waves drive setup. The response of bubble clouds to advection and turbulence in the nearshore zone depends in a subtle way on buoyancy and gas dissolution. Indeed, a primary benefit of the interpretation of bubble measurements is that it leads to deeper insights on the flow dynamics and other aspects that may be only peripherally related to the bubbles themselves. Here we make use of acoustical techniques to measure the bubble field both in the surf zone and within rip currents and use the results to test our Bubble clouds generated in the surf zone move alongshore in the background flow. Intermittently, fluid is expelled seaward from the surf zone, carrying the bubbles with it. Smith and Largier [1995] used these bubbles as acoustic targets in studies of nearshore circulation. As the bubbles move seaward, the size distribution evolves. Larger bubbles are more buoyant and rise toward the surface. Smaller bubbles, especially at greater depth, gradually dissolve [Thorpe, 1982]. Added to these effects, the turbulent boundary layer, which is maintained both by the relatively steady rip current and the harmonic forcing by the waves, enhances vertical diffusion within the water column and serves to offset vertical gradients in bubble concentration, which are an inevitable consequence of bubble buoyancy. The bubble size distribution then becomes a 11,677

show abstract

Section: Observationsmentioning

confidence: 99%

Bubble transport in rip currents

Vagle¹,

Farmer²,

Deane³

2001

J. Geophys. Res.

View full text Add to dashboard Cite

show abstract

“…for S v = -25 dB, a equals about 0.5 dB/m, which is comparable to direct measurements of attenuation made at 244 kHz by Caruthers et al 5 It is useful to evaluate how backscattering measurements made at 240 kHz respond to this bubble size distribution. For this we define the probability density function (PDF) p(a s )…”

Section: =1mentioning

confidence: 60%

“…More information concerning the delta frame is given by Caruthers et al, 5 who discuss the results of the multi-frequency sound propagation and attenuation measurements made by NRL-SSC during the Scripps Pier Experiment. Also, Vagle et al, 6 Farmer et al, 7 and…”

Section: Experimental Measurement Systemmentioning

confidence: 99%

See 1 more Smart Citation

Bubble clouds and their transport within the surf zone as measured with a distributed array of upward-looking sonars

Dahl

2001

The Journal of the Acoustical Society of America

View full text Add to dashboard Cite

A collaborative, multi-institute experiment called the Scripps Pier Experiment was conducted in the vicinity of the Scripps pier in La Jolla, California, in March 1997 to study the fate of bubbles in the surf zone and the effects of these bubbles on acoustic propagation. This paper discusses data gathered by the Applied Physics Laboratory, University of Washington, using a set of four upward-looking sonars (frequency 240 kHz), which simultaneously measured vertical profiles of acoustic volume scattering from bubbles at four locations.The transport of bubbles via rip currents emerged as an important, though episodic and localized, feature of the acoustic environment in the surf zone.

show abstract

“…A sector-scan sonar used during the Scripps Pier Bubble Experiment also observed the acoustic backscattering [3]. A preponderance of evidence collected by the participants established the fact that backscattering is due to microbubbles [1][2][3][4][5]9,10].…”

Section: Distribution Statement a *«S»3sr 20000313 066mentioning

confidence: 99%

Tomographic reconstruction of shallow water bubble fields

Rouseff

Henyey

Caruthers

et al. 2001

IEEE J. Oceanic Eng.

View full text Add to dashboard Cite

In the winter of 1997, a shallow water experiment was conducted near the Scripps Pier in La Jolla, California, USA. The goal was to determine the dynamics, distribution and acoustic effects of bubbles just offshore from active surf. A major component of the experiment was the "Delta Frame," an apparatus that supported two acoustic sources and eight receivers. Acoustic intensity was measured at frequencies between 39 and 244 kHz over the resulting 16 horizontal ray paths. Paths ranged in length from 2.5 to 8.6 m. In the present paper, a tomography algorithm is developed and implemented using Delta Frame data. Measurements are combined to produce quantitative cross-sectional images of the attenuation associated with the bubble cloud. Numerical simulations predict that the frame can resolve details of the field down to about 2 m. Images constructed at different acoustic frequencies are scaled and compared. A five-minute sequence of images is studied in detail. Swell waves are shown to cause rapid fluctuations in the images due to the vertical structure of the bubble cloud. At a wave trough, the bubble cloud dips into the horizontal plane probed by the Delta Frame and the attenuation increases. IntroductionBubbles, even in small concentrations, have strong effects on underwater acoustic propagation. They produce strong attenuation, and they change the sound speed. Unlike most acoustic environments, bubble clouds cause the sound to be strongly dispersed. As a crude approximation useful to guide one's thinking, the complex index of refraction at any frequency depends on the bubbles that resonate at that frequency, and the resonant frequency is inversely proportional to the radius of the bubble. The processes that remove bubbles from the water determine the index of refraction as a function of frequency. Large bubbles rapidly rise to the surface, and small bubbles dissolve, leaving intermediate size bubbles to dominate the acoustic effects.

show abstract

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

Cited by 20 publications

References 7 publications

Bubble transport in rip currents

Bubble transport in rip currents

Bubble clouds and their transport within the surf zone as measured with a distributed array of upward-looking sonars

Tomographic reconstruction of shallow water bubble fields

Contact Info

Product

Resources

About