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

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 >similar to 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.