The Effect of Seasonal Temperature Fluctuations in the Water Column on Sediment Compressional Wave Speed Profiles in Shallow Water

In shallow water areas, the temperature of the water column undergoes large fluctuations during the course of a year. In the Gulf of Mexico, for example, temperature fluctuations of as much as twelve degrees Centigrade have been observed. These seasonal variations in water column temperature affect the pore water temperature of the bottom sediments which, in turn, affects the compressional wave speed profile. Using Biot theory, it is shown that the sediment compressional wave speed varies approximately linearly with pore water temperature and this effect is, to first order, independent of the porosity and sediment type. It is also shown that the velocity ratio (ratio of sound speeds in the water and the sediment at the sediment/water interface) is independent of the temperature but dependent on sediment type. These effects are demonstrated using two experimental measurements made at the same site in the Gulf of Mexico but during different seasons. In both cases, perturbative inversion techniques were used to infer the sound velocity profiles in the bottom. The differences between the two profiles fall within the error bounds predicted by linear inverse theory everywhere except the top 10 m of sediment, where the differences are attributed to the seasonal temperature fluctuation phenomenon. The experimental results suggest that the influence of the water column extends to greater depths than those predicted by theory.