SHALLOW STRATIGRAPHIC SEQUENCES AND THEIR ACOUSTIC VELOCITY IN THE BOHAI STRAIT AND SURROUNDING AREAS

Through the interpretation of high-resolution seismic profiles and three drilling holes collected from the Bohai Strait and its surrounding areas, the subsurface strata of the study area were divided into nine distinct acoustic units (SU₁¹、SU₁²、SU₁³、SU₂、SU₃、SU₄¹、SU₄²、SU₄³、SU₅) from the top downward, defined by nine bounding surfaces (T₀、T₁、T₂、T₃、T₄、T₅、T₆、T₇、T₈). The acoustic unit SU₁ is interpreted as a Holocene marine sediment upon a transgressing erosional surface. The unit SU₂ is a terrestrial sediment underlying the seismic unit SU₁. The acoustic unit SU₃ is a marine sediment formed during the late Wurm Interglacial Stage. The surface between SU₂ and SU₃ is a erosive discontinuity. The acoustic units SU₄¹, SU₄² and SU₄³ correspond to the continental sediment during the late stage of early Wurm Glaciation, marine sediment during the early Wurm Interglaciation and continental sediment during the early stage of early Wurm Glaciation respectively, separated by two distinct erosional surfaces. The unit SU₅ is a marine sediment deposited during the Wurm-Riss Interglacial Stage, bounded by a unconformity on the top. The seismic units are correlated well with the cores in the study area. Average acoustic velocity of the stratigraphic units beneath the sea bottom were calculated as basic data for further study of different stratigraphic sequences. With sea level changes in the study area, the acoustic velocity shows a pattern of high-low-high-low with depth in an ascending order.