High-frequency acoustic scattering from oceanic microstructure

Andone Lavery

High-frequency acoustic scattering techniques provide us with a powerful tool for studying small-scale physical processes that occur in the ocean interior over relevant spatial and temporal scales. Sound is an exceptional probe of the ocean interior since, compared to other forms of radiation, such as light, it suffers significantly lower attenuation and can thus travel longer ranges. High-frequency sound (above 20 kHz) is ideally suited to studying small-scale physical processes since the corresponding wavelengths (smaller than a few centimeters) are commensurate with the processes under investigation.

My research interests lie in the development of high-frequency acoustic scattering techniques to study the smallest physical scales, the microstructure scale, where molecular diffusion starts to play a significant role. These small-scale physical processes are important since they contribute to the larger-scale mixing of oceanic properties, such as temperature and salinity. Examples of different types of microstructure include oceanic turbulence, and double-diffusive phenomena, such as double-diffusive interfaces and salt-fingers. At these small scales, large gradients, or fluctuations, in temperature and salinity can occur, which in turn give rise to gradients in the acoustical (and also optical) index of refraction and oceanic density. It is the changes in the acoustical material properties of the ocean (density and index of refraction) that give rise to the scattering of sound. We expect that different types of microstructure scatter sound in different ways, each with a unique spectral signature. My goal is to determine these unique spectral signatures and then exploit the differences to gain a better understanding of the temporal and spatial distribution of different types of microstructure.

Figure 1: 120 kHz acoustic curtain plot (acoustic scattering as a function of depth and latitude and longitude) obtained during an acoustic survey of the deep basins of the Gulf of Maine in December 1999. High-frequency acoustic scattering techniques provide us with a powerful tool to synoptically image large areas of the ocean interior. (click for larger image)

Though there are other more direct techniques for measuring oceanic microstructure, many are inherently one-dimensional and invariably time-consuming, involving measurements of temperature and conductivity at one location as a function of depth. In contrast to these more traditional techniques for measuring microstructure, acoustics scattering techniques may ultimately provide us with a remote sensing technique to rapidly and synoptically (Figure 1) survey different types of microstructure, and extract associated physical parameters, such as the dissipation rate of heat variance.

The challenge in using high-frequency acoustic scattering techniques is in the interpretation of the acoustic data (such as those shown in Figure 1) in terms of a particular physical process [1]. This difficult problem is simplified by the use of theoretical models that are based on the scattering physics for each different type of microstructure, followed by rigorous testing of the models in controlled laboratory experiments. It is the (expected) differences in the spectral signatures of the acoustic scattering from different types of microstructure may allow us to acoustically distinguishing between them.

My colleagues (Ray Schmitt and Tim Stanton) and I are currently working on developing scattering models for double-diffusive interfaces and salt-fingers. Recently we have also completed the development of a new scattering model for turbulent oceanic microstructure. Unlike previous work done in this area, we have included in our theoretical development the small-scale gradients in oceanic density, as well as index of refraction, since it is both these parameters that determine the acoustic impedance of the medium, and hence also the amount of sound that is scattered (Figure 2).  We have found that density fluctuations greatly contribute to scattering from salinity microstructure, although the effect of density fluctuations on scattering from temperature microstructure is less significant.

Figure 2: Predicted acoustic scattering spectra for turbulent temperature and salinity microstructure. Since molecular diffusion of heat is almost one hundred times faster than the diffusion of salt, scattering from salinity microstructure is expected to be most significant at higher frequencies than temperature microstructure, corresponding to smaller acoustic and spatial wavenumbers. (click for larger image)

We are also currently performing laboratory experiments aimed at understanding how high-frequency sound scatters from double-diffusive interfaces (Figure 3). In order to verify the accuracy of our scattering models and measurements, we use optical techniques as well as high-resolution temperature and conductivity probes to obtain direct measurements of the tank-generated microstructure. An optical shadowgraph image (Figure 3) is obtained by propagating collimated light through water containing temperature and salinity inhomogeneities. The light is refracted by the changes in the optical index of refraction caused by the inhomogeneities. The refracted light is projected onto a screen producing a shadow of the inhomogeneties along the propagation path.

Figure 3: Shadowgraph image of a tank-generated double-diffusive interface. This interface was generated in a laboratory tank by placing cold fresh water over warm salty water. To maintain the interface the water was resistively heated from below and chilled from above.

As soon as we have completed these measurements for scattering from double-diffusive interfaces, we will generate laboratory turbulence in order to test the validity of our scattering model for turbulent oceanic microstructure. In the future, we will apply these models to the interpretation of acoustic field data collected during surveys of the open ocean.

[1] There are also other significant sources of scattering in the ocean interior in addition to microstructure (e.g. suspended sediments, bubbles, zooplankton, fish).

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