Laboratory measurements and simulations of reflections from a water/clay interface during the diffusion of salt

Estuarine, riverine, and certain continental shelf environments experience significant temperature and salinity variability near the ocean bottom that can produce significant changes in how sound interacts with fine-grained sediments, presenting challenges in applications including shallow water sonar and bottom surveys. To begin to understand the effects of this variability on acoustic reflection, reflection measurements in the laboratory near 1 MHz were obtained from a water-clay interface while varying the salinity of the bottom water. At certain angles of incidence, salinity variations caused changes in bottom loss up to 15 dB and 180-degree phase shifts in the reflected signal, and induced changes in the reflectivity of the clay through the diffusion process, thereby leading to complicated, coupled interactions at the water-clay interface. By modeling the reflectivity of clay during molecular diffusion of salt, the diffusion coefficients were experimentally inferred and simulations at lower frequencies and longer timescales were performed. Derived characteristic length scales associated with the molecular diffusion of salt are compared with acoustic wavelengths to identify frequency regimes that are sensitive to salinity fluctuations. Results indicate that the dynamic nature of the bottom water can cause measurable and significant effects in reflectivity at and below frequencies applicable to sonar.