Geophysical characteristics and migration mechanism of active submarine sand waves off the coast of Dongfang, Hainan

A large numbers of sand waves are developed on the seafloor off the coast of Dongfang, Hainan. This paper is devoted to the study of the geophysical characteristics of active submarine sand waves by means of multi-beam echo-sounding, side-scan sonar, sub-bottom profile, and single-channel seismic, and to the discussion on the distribution pattern, migration mechanism, activity, and morphologic evolution of the sand waves. The results show that there are significant spatial differences in the distribution and scale of submarine sand waves, for examples, large and medium-sized sand waves are mainly developed on the top of sand ridges, small sand waves developed mainly on the two sides of sand ridges, and near-symmetrical sand waves developed in the areas of pits. Submarine sand waves are not developed in the southwest of the study area. Constrained by tidal current and Coriolis force, the migration direction of the sand wave on the west side of the submarine sand ridge is mainly northwards with a few towards east, while those on the east side is mainly southwards with a few slightly towards west. Restricted by topography, the migration of near-symmetric sand waves may stop or change direction. Strongly active submarine sand waves are characterized by the features as follows. (a) Asymmetric shape with "sharp ridge and gentle trough"; (b) Small sand waves and sand ripples superimposed on sand waves; (c)Sub-bottom profile shows transparent layers; (d) Blank reflection on the steep slope; (e) Internal oblique progradation configuration. The analysis shows that the movement of the sand waves is closely related to its morphology, and three stages of evolution from inactive to highly active to motionless are recognized.