Acoustic scattering to measure dispersed oil droplet size and sediment particle size

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1 Acoustic scattering to measure dispersed oil droplet size and sediment particle size Paul D. Panetta 1,2, Leslie G. Bland 1,3, Grace Cartwright 2 and, Carl T. Friedrichs 2 1 Applied Research Associates, Inc. P.O. Box 1346, 1208 Greate Rd. Gloucester Pt. VA Virginia Institute of Marine Science P.O. Box 1346, 1208 Greate Rd. Gloucester Pt. VA University of Virginia P.O. Box Charlottesville, VA Abstract The use of sound waves in oceanographic environments is well established including active sonar applications for mapping seafloors, for submarine detection, and for passive listening. Acoustic waves have been used for many years to study the ocean, including detecting and identifying objects in the water column, and the measuring the seafloor and sub seafloor properties. One of the key parameters of the acoustic field is the amplitude of the wave which scatters from the seafloor or from objects in the water column. The amplitude, time of flight, and frequency response can be used to map the seafloor, measure current flow, or to detect and classify objects in the water column. In addition to these standard uses, acoustics can also be used to size particulates including sediment, oil droplets and gas bubbles in the water. Our particular application for this work is to detect, classify, and size sediment particles and separately, oil droplets suspended in the water column using knowledge of the acoustic backscattering and attenuation. Specifically, we have measured and separated the absorption, single scattering and multiple scattering contributions to attenuation measurements. Our results show that the absorption dominates the attenuation at low ka values << 1 and multiple scattering and particle-particle interactions dominate at higher ka values when ka >~1 with a transition between theses ranges depending on the concentration of the suspensions. The physics has been proven out on silica particles in water and work is ongoing on suspended sediment and suspensions of oil droplets. Index Terms acoustic, particle size, attenuation, scattering I. INTRODUCTION The most common method to measure particulate size in oceanographic environments is laser scattering and transmission with the popular Laser In-Situ Scattering and Transmissometery (LISST) instrument. The LISST works well in low concentration suspensions below ~ 250 mg/l (400 ppm) for suspended mud and ~700 ppm for oil droplets in water, but in field deployments the LISST becomes fouled by biological growth. Acoustic technologies in oceanographic environments are effective at very high concentrations in the water column, in consolidated sediments and solids. Because of the access to much higher concentrations and resistance to bio-fouling, acoustic instruments are commonly used to measure current speed using the Doppler shift from suspended particulates and suspended sediment concentration. While acoustic instruments work well in practical applications, measuring the particulate size is currently limited by the lack of theoretical understanding of the scattering of the acoustic field and the complicated interdependence of attenuating mechanisms to the experimentally measured parameters. Research work is ongoing by the authors [1] and others [2,3] to develop high frequency particle sizing methods. For this paper we focus on studying the contributions to the attenuation with applications to oil droplets and suspended sediment. II. ACOUSTIC INTERACTIONS WITH PARTICULATES Several properties of an acoustic field can be used to measure the characteristics of a particle suspension. These properties include attenuation, backscattering, and the speed of sound. Physically, the attenuation of transmitted acoustic field and speed of sound are determined from waves that traverse across the media and received by a second transducer Figure 1b). Backscattering is the portion of the field which is scattered directly back to the transmitting transducer. The backscattered field at various concentrations for 70 microns glass spheres ranging from 5 to 40 wt% is shown in Figure 1c. A fourth quantity that can be measured is the scattered sound which travels as a diffuse field,

2 Fig. 1 (a) Schematic of ultrasonic measurement apparatus, (b) through transmitted RF signal for attenuation and velocity determination, (c) directly backscattered signal, and (d) diffuse field signal. shown in Figure 1d. Here the coherent reflections from the opposite side of the container can be seen in the early time (<200 microseconds). The amplitude of the diffuse field builds up in the 100 to 400 microseconds range, and then decays in time. This decay is a direct measure of the absorption contribution to the attenuation. Notable in these acoustic signals is the very large time range they span. The coherent signal used for determining attenuation and velocity cover approximately 1 microseconds. The direct backscattered, which is in incoherent summation of individual scattering events spans approximately 130 microseconds, while the diffusely scattered field persists for many milliseconds. The physics governing the propagation of these fields is different in each time domain and is dominated by specific properties of the suspension. These measurements have been applied for many decades to study solid-liquid suspensions and elastic solids with varying degrees of success. Performing the conventional measurements of attenuation, backscattering, and speed of sound are relatively simple. The more difficult task is to accurately model the mechanisms contributing to the measured attenuation and to understand the effects of the different attenuating mechanisms on the various measurements. The inversion of the theory for comparison with experimental results to provide the specific characteristics of the slurry can be computationally intensive and inaccurate. When describing acoustic wave propagation in slurries, there are several important factors, including, particle size and shape, concentration, heterogeneities, and the degree of multiple scattering and particle-particle interactions present. The following sections describe the physics governing the attenuation, backscattering and diffuse field. A. Attenuation As an acoustic field moves through a solid-liquid suspension, the fluid and the solid attenuate the field through several different mechanisms. Specifically, the acoustic field can be scattered at the interfaces between the particle and the fluid. In addition, energy can be lost through heat generated by friction as the particle is moved through the viscous fluid. Changing the direction of motion of the particle as it oscillates (accelerating and decelerating the particle) also removes energy from the acoustic field. If the particle is not rigid, then it can also change shape as the acoustic field moves through the media, causing additional energy losses. These mechanisms can be broadly categorized as scattering losses and damping or absorption losses. While there are many contributions to energy loss as the acoustic field interacts with the fluid and the solid phases of the slurry, the dominant contributions are (1) the heat loss due to the friction between the viscous fluid and the particles as they move through the slurry, (2) energy loss to accelerate and decelerate the particle as it oscillates, and (3) the scattering of sound out of the propagating field. These three dominant loss mechanisms will now be described including the regimes where each is dominant. After this description of attenuation, the appropriate contributions to the backscattering and diffuse field will be discussed. Acoustic fields propagating in suspensions suffer attenuation due to viscous and thermal damping, and scattering mechanisms. Attenuation of sound propagation in suspensions can be divided into the following four major regimes. 1) Viscous regime: When k c R 1 and Re R / R l /(2 ) 1, attenuation 2 2 has been shown to scale with R /. Where k c is the wave number = 2, R is the particle radius, is the boundary layer thickness, is the frequency, l and are the density and viscosity of the fluid phase, respectively. 2) Inertial regime: When k c R 1and Re >> 1, the Basset history term begins to influence the particle drag. Consequently, the drag becomes dominated by inertia through the added mass effect. However, the added mass force is conservative and does not contribute to losses, and the dominant loss term is the in-phase component of the Basset force. In this regime, the attenuation scales with / R. 3) Multiple scattering regime: When k c R 1, the field scatters geometrically. In an assembly of many randomly arranged particles, the field will scatter randomly and thus penetrate poorly (high attenuation). Here attenuation scales with 4. 4) Resonance Regime: Particles may exhibit resonance characteristics and the frequency of the first mode of resonance can be approximated as the inverse of the time the acoustic field takes to travel twice the diameter of the particle. For particles that have sufficiently higher acoustic velocity, transition to multiple scattering will set in first. However, most plastic particles will often exhibit resonance absorption at frequencies below k c R 1. Attenuation has been used widely and successfully to characterize dilute slurries. Allegra and Hawley [4] provided the groundbreaking theoretical treatment for solid-liquid suspensions. Their model accounts for the attenuation due to

3 viscous damping, as a particle moves and changes shape, the thermal loss as heat is exchanged between the acoustic field and the particle, and the scattering loss as the propagating wave is scattered at the interfaces between the fluid and the solid particles. However, they obtained good agreement between experimental measurements of attenuation and theoretical predictions only at low concentrations. Furthermore, their theory lacks an explicit term for the backscattering amplitude and does not incorporate particle size distributions, hydrodynamic effects of relative motion between particle and fluid, and contributions from multiple scattering. In addition to the work of Allegra and Hawley, there have been a significant number of researchers who have studied wave propagation in various suspensions, including Commander and Prosperetti [5], Soong et al. [6], Challis et al. [7], Holmes et al. [8], Atkinson and Kytomaa [9, 10], and Kytomaa [11]. In recent studies of attenuation in sols and gels, Holmes and Challis [12,13] showed that the single scattering models were inadequate at high concentrations. Recently, Dukin and Goetz [14] studied two particle sizes at low concentrations. Notable work has been performed by Dave Scott in industrial applications where he showed that the attenuation can be used for particle size determination at low concentrations. [15,16,17]. Furthermore, he produced data which showed a deviation for the Allegra and Hawley model at high concentrations, indicating that multiple scattering is significant and needs to be accounted for in industrial slurries. In recent work, Spelt and coworkers have shown excellent theoretical progress in developing a model for slurries that accurately predicts attenuation at low concentrations for specific weak scatterers [18]. In addition, they extended their models using an effective medium technique that shows promise for accounting for the effects particle-particle interactions on attenuation in concentrated slurries. Their results show good agreement in slurries at concentrations up to only ~ 30 volume % solids, indicating that it is likely that the theory does not account for enough of the multiple scattering to provide accurate predictions at higher concentrations. They also indicated that their approach had limited applicability for strongly scattering particles with elastic properties that vary significantly from water. Furthermore, their techniques are applicable only for dilute suspensions, and the inversion process is sensitive to the complex resonance behavior of the particles in slurries and the frequency range for which the comparison is made. Even though, their theoretical treatment has limitations, it represents significant progress towards including multiple scattering effects in the theoretical description of attenuation in slurries. Researchers have studied multiple scattering for several decades. Notably, Davis [19] studied the effects of multiple scattering on attenuation in suspensions and emulsions. In his studies, he coupled the single scattering amplitude developed by Allegra and Hawley [4] and the multiple scattering approach described by Waterman and Truell [20]. Varadan et al. [21] modeled multiple scattering by bubbles in water using a T- Matrix approach and Lax s quasi-crystalline approximation for scattering for kr <1, where k is the wave number and R is the particle radius. Agreement between the model results and the experimental results were quite good, which indicates that multiple scattering terms are important. The main obstacles for implementing attenuation measurements to slurry characterization have proven to be the mathematical complexities of accounting for multiple scattering at high concentrations, and the accompanying complex nature of the inversion process. In addition, the acoustic attenuation measurements require careful alignment of transducers, further decreasing the accuracy and precision of the methodologies based on attenuation alone. B. Backscattering Backscattering measurements have several advantages over attenuation measurements, including insensitivity to alignment of the transducer and small propagation distances, thus making them ideal for characterizing highly attenuated slurries. In addition, since the direct backscattered field usually can be described by single-scattering processes, the mathematical inversion processes is often more simple and stable than those used for attenuation. Figure 1c shows typical backscattered signals in a solid-liquid suspension. Backscattering in suspensions has been less thoroughly studied, relative to attenuation, with the efforts largely focused on geologic and oceanographic applications. Notably, Hay and Mercer (1985) used the theory developed by Allegra and Hawley [4] to explicitly determine the backscattering amplitude as a function of the elastic properties of the scatterers and the viscous fluid. They further extended their work to include particle size distributions of sand particles in suspension. In addition, He and Hay [22] accounted for the irregular shapes of sand particles by performing an ensemble average of the scattered pressure. They obtained good agreement with experimental results for sand particles in suspension up to kr~1. Hay [23] further studied backscattering in flowing suspensions and observed evidence of concentration fluctuations in the turbulent flow. These theories were then applied to the inversion problem of determining the concentration and size of sand particles in suspension [24 Crawford and Hay 1993]. Reasonable agreement was obtained for low concentrations and particle size for a wide range of materials including fused quartz and granite. Their level of agreement was poor at higher concentrations. Other efforts to use backscattering to characterize sediment have been undertaken by Thorne and Hardcastle [25] where they performed in-situ sampling during measurements of turbulent sediments. Agreement between experimental results and theoretical predictions of particle size and concentration were quite good over a large range of concentrations. At high concentrations, the inversion technique was difficult to perform due to a high attenuation. This difficulty could be related to the fact that the theory relied on a single scattering approach and the irregularly shaped particles were assumed to be spherical. The irregular shape of sand particles was modeled by Thorne et al. [26], who used a boundary element method and a finite element method to numerically model the complicated particle shapes. An alternative approach has been employed by

4 Schaafsma et al. [27], who used an equivalent spherical scatter method Schaafsma and Hay [28] to model the irregular shape of sand particles and showed much better agreement at frequencies where strong multiple scattering can be significant (kr~2). Recently, an attempt at accounting for inhomogeneities in sediments was performed by Guillon and Lurton [29] where they modeled the backscattering from buried sediment layers and accounted for the losses at interfaces between layers. One of the major problems of this approach is the complexity of the inversion process. C. Diffuse Field Absorption Another aspect of the scattered field that is intimately related to the properties of the particulate suspension is the portion that undergoes multiple scattering and diffuses through the media. These signals arrive at the detector after several milliseconds as compared to the direct backscatter signals which typically arrive in a few 10 s of microseconds [30]. After multiple scattering events, the acoustic diffuse field develops [31], and a portion of the scattered wave eventually returns to the transducer. Buildup of this field is governed by a diffusivity term that is a function of the mean free path and, thus, is related to the size of the scatterers. The decay rate is related to the energy absorbed from the acoustic field (see Figure 1d) and is also a function of particle type, size and concentration. The loss mechanisms in the long time limit for the diffuse field are dominated only by viscous losses and particle motion, with little or no scattering losses. One of the appealing aspects of the diffuse field measurement of absorption is that it can also be performed with one or more transducers and sensor alignment is not important. In addition, the insonified volume can be larger than for the backscattering for the same frequency because the wave travels outward as a diffusely propagating wave. These measurements are especially appealing because they can be used to probe the absorptive properties of the particles and thus can be used to isolate specific mechanisms when used in combination with the attenuation and backscattering. While the majority of applications of the diffuse field absorption measurements have focused on characterizing solids, Weaver and Sachse [32] focused their work on slurries. In their study, it was significant that measurements were performed on slurry containing strongly scattering glass spheres at a concentration of 62 wt %. In addition, Page et al. [33, 34] reproduced the work of Weaver and Sachse and then applied the diffuse field technique to measure particle motion in a non-stationary suspension. Panetta has also utilized these measurements to characterize solid liquid suspensions and isolate specific attenuation mechanisms [38]. While the simple model employed by Weaver and Sachse [32] described the fluid motion in a matrix of immobile particles, recent work by Rosny and Roux [36, 37, 38] takes into account the effects of the particle motion on the diffuse field characteristics. They obtained excellent agreement between experiments and theory for dilute solutions of moving particles and fish, showing applicability to marine environments. They also showed that multiple scattering can be important and should not be neglected. An important outcome of their work was the study of the correlation of the backscattering with time. Page has also advanced his work on diffusing acoustic wave spectroscopy (DAWS). Specifically, they have accounted for particle motion and show good agreement between experiments and theory [39]. A particularly significant theoretical advancement is given by a series of papers by Turner and Weaver [40, 41, 42, 43] where they modeled the backscattering in polycrystalline materials using a radiative transfer approach for all regions of backscattering from early-occurring single scattering through late-occurring multiple scattering. While these results are promising, it is evident that the characteristics of the diffuse field are rich with information and that theoretical work and experimental applications to particle size distributions are not yet well developed. III. ANALYSIS METHODS A. Separation of Attenuation Mechanisms The acoustic measurements of attenuation, backscattering, and the diffuse field are sensitive to particle properties in suspensions. To create a method to characterize particulate suspension, one must separate these attenuating mechanisms so they can be studied and modeled individually. The authors developed methods to isolate these attenuating mechanisms in dilute and highly concentrated slurries [1]. A brief description of the scientific basis behind the methods used to separate and isolate attenuation mechanisms will be given in this section. It was previously mentioned that (i) the attenuation measured from a through transmitted signal is a function of absorption, single scattering, multiple scattering, and particleparticle interactions,(ii) the backscattering is a function of absorption and single scattering, and (iii) the diffuse field is a function of the absorption only. The second point about the backscattering is an assumption that may or may not be valid under certain circumstances. We will accept this assumption as correct for the purposes of this paper and subsequent analysis. An illustration showing these acoustic signals and equations for calculating the attenuation is displayed in Figure 2. The attenuation from the through transmitted signal is calculated by comparing the signal in the slurry to a reference signal in water. The attenuation for the backscattering and diffuse field is calculated by measuring the decay rate of the scattered signals at individual frequencies. Here is the attenuation, f is the acoustic frequency, z is the separation distance of the transducers, D is the diffraction correction for a circular piston transducer, is the transducer efficiency, (f) is the Fourier amplitude of the received signal, is the decay rate and v is the speed of sound. The attenuation as a function of frequency for 70 micron silica spheres in water at 10 wt% for each of these measurements is shown in Figure 3. The through transmitted attenuation is

5 RMS Backscattering Attenuation velocity 5 wt% High concentration 1.9 X c e lls /m L 40 wt% Time (microseconds) 5 MHz 5wt% 5 MHz 10wt% 5 MHz 15wt% 5 MHz 20wt% 5 MHz 30wt% 5 MHz 40wt% 0 Low c e concentration lls /m L Backscattering Diffuse Field Absorption and single and multiple scattering 1 D ln slurry ref f ref f f z D ref slurry f slurry f Absorption and single scattering Absorption BS f f BS 2 v DF f f DF 2 v Fig. 2 Ultrasonic measurements of attenuation suing the through transmitted signal, the backscattering, and the diffuse field. The associated equations are shown on the right. highest; the backscattering attenuation is next, followed by the diffuse field measurement of the attenuation. The frequency response of the attenuation is shown in Figure 3 and within the included table where the through transmitted attenuation has a power of 2.5, backscattering 1.7, and diffuse field 0.3. These powers indicate the through transmitted attenuation is dominated by multiple scattering, the diffuse field by absorption, with the backscattering containing both scattering and absorption contributions. The dependence of the attenuation on each mechanism is shown in the equations below. These equations can be rearranged to solve for the individual contribution as follows: Through _ transmission f Multiple _ scattering f Single _ scattering f Absorption f Backscattering _ decay f Single _ scattering f Absorption f Diffuse _ field _ decay f Absorption f Multiple _ scattering f Through _ transmissionf Backscattering _ decay f Single _ scattering f Backscattering _ decay f Diffuse _ field _ decay f f f Absorption 2.0 Diffuse _ field _ decay 70 um SiO 2 spheres in water (10 wt%) (1) (2) (3) (4) (5) (6) The results from this process are shown in Figure 4 where each contribution to attenuation is plotted as a function of frequency. As expected, the frequency dependence of the multiple scattering increased to 3.5, the single scattering increased to 2.1, and the absorption dependence on frequency remained constant. Attenuation (N/cm) um SiO2 spheres in water (10 wt%) Multiple scattering and particle particle interactions Single scattering Absorption Frequency (MHz) Fig. 4. The isolated attenuation mechanisms plotted as a function of frequency. The power of the frequency dependence increased as expected. IV. RESULTS A. Silica Suspensions The dependence of these mechanisms on concentration is shown in Figure 5 and reveals the true power and insight this method provides into understanding the acoustic properties of the suspension. For 70 microns glass spheres in water at 5 MHz, the single scattering contribution to attenuation dominates below 10 wt % while the multiple scattering contributions to attenuation dominate above ~15 wt%. These results show why the commercial acoustic instruments, which rely on attenuation measurements dominated by multiple scattering and particle-particle interactions, become inaccurate at high concentrations. These results summarize the fundamental science underlying acoustic particle characterization, provide the insight needed to better use commercial instruments, and indicate where the commercial instruments are likely to fail or become inaccurate. Attenuation (N/cm) Through transmitted attenuation Backscattering Diffuse field Frequency (MHz) Fig. 3 The attenuation mechanisms as a function of frequency. Attenuation at 5 MHz (N/cm) um Glass Spheres Multiple scattering and particle particle interactions Single scattering Absorption Concentration (Wt%) Fig. 5. The isolated attenuation mechanisms plotted as a function of concentration. Above ~ 15 wt% the multiple scattering gang particle-particle interactions dominate the attenuation

6 Initial results applying these methods to oil suspensions by measuring the attenuation of the backscattered signal are in the next section. Early indications in consolidated sediment cores shows the same methods may be used to determine the grain size of sediment in the seafloor. Detailed results will be in future publications. B. Oil Dispersions Sonar was used during the recent blowout and associated oil leak from the Deepwater Horizon incident to image the plume and calculate the flow rate. In the case of the Deepwater Horizon blowout they did not have the monitoring tools to determine the efficacy of the 1.1 million gallons of Corexit 9500 dispersant injected directly into a flowing plume of oil and natural gas. Dispersants main effect is to decrease the surface tension at the oil-water interface causing the oil to form droplets smaller than ~70 microns so they can remain in the water column long enough to be consumed by naturally occurring bacteria. There is a need to develop a deeper understanding of the scattering of acoustic waves from particulates, sediment, and oil droplets to effectively utilize the vast array of sonar and marine acoustic instruments. Specifically, to effectively size suspended particles in the water column and in consolidated sediments in the seafloor, it is important to understand the contributions from single scattering and multiple scattering as well as the absorption contributions to attenuation that control the wave propagation, especially in high concentration systems experienced in the seafloor sediment and subsea blowouts experienced during the Deepwater Horizon incident. Measurements were performed at both the Department of Interior Ohmsett outdoor wave tank in New Jersey, and at the Applied Research Associates, Inc. labs on the Virginia Institute of Marine Science Campus. The acoustic test chamber is shown in Figure 6. To ensure good mixing of the oil-water-dispersant, we suctioned the mixture into a 60 cc syringe multiple times prior to performing our measurements. A water mixture was mixed thoroughly within the acoustic test chamber while it was within the LISST testing area in order to calibrate the oil droplet size. Immediately after mixing with the syringe, the acoustic transducer was inserted into the chamber pointed down and data collection was started immediately. The acoustic data from multiple pings every 0.1 seconds for 300 seconds are shown in Figure 7a for oil and oil-dispersant mixtures respectively. The mixture of oil droplets caused the acoustic wave to scatter, creating a bright orange/red pattern between zero and ~30 seconds. After ~ 30 seconds, scattering from the droplets caused distinguishable orange/red streaks. As more droplets floated up towards the surface, the streaks caused by the droplets generally pointed up with some streaks pointing left to right or down presumably from random motion in the chamber. The droplets continued to move through the field of view of the transducer up to ~ 250 seconds. The raw acoustic signal approximately 10 seconds after the transducer was inserted is shown at the right of the image. Fig. 6. The acoustic test chamber for oil-dispersant mixtures. The syringe was used to uniformly mix the suspensions. The acoustic transducer is shown in the right figure pointing vertically down. The oil-dispersant mixture was created in a separate chamber by inserting the dispersant into the chamber full of water with a pipet. The dispersant-water mixture was then mixed with by repetitively drawing and emptying it from the syringe. Then, oil was added by first drawing approximately 20 cc of the dispersant-water mixture into the syringe then drawing the oil into the partially full syringe. The syringe with the oildispersant-water mixture was then placed back into the chamber and repetitively filled and emptied to uniformly disperse the oil. Care was taken to ensure the total oil concentration was below the striation limit of the LISST which was approximately 700 ppm. The image of the acoustic backscattering for each ping for the chemically dispersed oil is shown in Figure 7b. The ability for the dispersant to keep the oil in the water column can be seen by the scattering from the suspended droplets indicated by the orange/red region extending to ~100 seconds. The dispersant and water were incorrectly mixed prior to adding the oil diminishing the efficiency of the dispersant to disperse the oil. Individual droplets can also be seen as streaks across the image. Depth (cm) Depth (cm) (a) (b) Crude oil in water Dispersant and Oil DOR 1: Time (seconds) Fig. 7. Acoustic images of the scattering from oil droplets in water over 300 seconds. An individual ping is shown on the right hand side of each figure. The red/orange regions are high scattering and the black is low scattering. The long tracks are individual droplets as they float through the field of view of the 5 MHz transducer.

7 These images visually confirm the ability of the dispersant to keep oil in the water column over an extended period of time. To effectively size the droplets, we analyzed the raw acoustic waveforms shown on the right hand side of Figure 7 by determining the rate of decay of the acoustic signal. The signal was analyzed when oil remained within the water column of the testing chamber, often within the first 50 seconds. After this time, most of the oil droplets would have reached the top of the chamber. A ping range of approximately 15 to 20 seconds was taken within the data and a linear regression was fit over the data. The attenuation for isolated drops in water is given by the following equation [44]: (7) Fig. 8. The comparison between our acoustic determination of the droplet size and the LISST determination. The inset shows the droplet size distribution from the LISST measurements. V. CONCLUSIONS Where e = extinction cross section = sum of the scattering cross section and the absorption cross section a = droplet radius f = frequency f r = resonance frequency = damping constant at f r c 0 = speed of sound = ratio of specific heat at constant pressure and volume P = hydrostatic pressure = density of water k r = wave vector at the resonant frequency The resonant frequency for 100 micron oil droplets is ~ MHz. At our operating acoustic frequency of 5 MHz, which is much greater than the resonant frequency, the attenuation is proportional to the droplet size, a. Thus, we can directly relate the attenuation to the droplet size. Using this linear relationship, we then calculated the expected droplet size based on our acoustic measurements. The comparison between our acoustic predictions of droplet sizes with the droplet size measured from the LISST is shown in of Figure 8. The inset shows the droplet size distribution determined from the LISST. While there is some spread, the relationship is quite good considering some of the data is from Ohmsett and some from lab measurements using two different configurations. (8) We have shown the contributions to acoustic attenuation can be separated and isolated through multiple measurements for silica particles in water. The multiple scattering and particleparticle interactions dominate the attenuation at high kr and concentration. The absorption dominated the attenuation at low kr with sing scattering dominating the attenuation at intermediate kr values. These methods have been applied to oil dispersions to study the efficacy of dispersants to keep oil in the water column. Applications to sediment will be published later. ACKNOWLEDGMENT The authors would like to thank Wayne Reisner for fabricating the sample chambers, Randy Belore from S.L.Ross, Al Guarino from Mar, Inc. for assistance at Ohmsett and Kyle Winfield for assistance with the experimental measurements. REFERENCES [1] Paul D. Panetta Ultrasonic Characterization of Solid-Liquid Suspension US 7,739,911, 2010 [2] Thorne, P. D., and D. M. Hanes (2002), A review of acoustic measurement of small-scale sediment processes, Continental Shelf Research, 22(4), [3] Thorne, P. D., and R. Meral (2008), Formulations for the scattering properties of suspended sandy sediments for use in the application of acoustics to sediment transport processes, Continental Shelf Research, 28(2), [4] Allegra, J.R. and S.A. Hawley Attenuation of Sound in Suspensions and Emulsions: Theory and Experiments. The Journal of the Acoustical Society of America, 51(5): [5] Commander, K.W. and A. Prosperetti Linear Pressure Waves in Bubbly Liquids: Comparison between Theory and Experiments. Journal of the Acoustical Society of America, 85(2): 732.

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