Optimisation of the depth range of UIV on cohesive sediment (fluid mud) flows

Abstract

AbstractUltrasound Image Velocimetry (UIV) is a technique that enables the monitoring of flows in natural high-density cohesive sediments, also known as fluid mud. Using conventional medical ultrasound imaging equipment, the depth range of UIV in mud is however too limited. This manuscript discusses how this depth range can be improved based on the results of experimental UIV measurements in fluid mud. The results show that the depth range limitation is determined by the pixel brightness of the generated speckle pattern images. Since pixel brightness is a measure of the intensity of the backscattered signals, it degrades with increasing depth due to the attenuation of the ultrasound radiation by the mud. The rate at which ultrasound is attenuated by fluid mud is constant and is governed by the density of the mud and the applied ultrasound frequency. A greater depth range can thus be achieved by reducing attenuation and increasing the intensity of the emitted ultrasound. The former was confirmed by the results of experiments with variable mud density and ultrasound frequency. Flexibility of both parameters is however restricted, allowing only a limited increase in depth range. Another experiment confirmed an additional increase in depth range when a higher ultrasound intensity is applied. Patient safety measures of medical ultrasound imaging equipment, however restrict the intensity of the emitted ultrasound. To increase the depth range even further, potential solutions are therefore suggested, such as the use of high-power non-medical ultrasound imaging equipment or a novel procedure for processing the acquired acoustic data.