Sediment facies determination using acoustic techniques in a shallow-water carbonate environment, Dry Tortugas, Florida

Abstract

Two high-frequency acoustic seafloor classification systems (12- and 15-kHz) were used in conjunction with sediment core analysis to characterize sediment facies at a study site near Garden Key in Dry Tortugas, Florida. The acoustic system uses echo return amplitude to compute acoustic impedance that is then correlated with sampled sediment impedance values calculated from wet bulk density and compressional wave velocity. Several bottom provinces were identified using the 15-kHz data to construct a real-time map of ship tracks in colors that represent the surficial sediment facies type. Sediment facies over the entire study site (36 km 2) range from sandy silts to exposed limestone rock and coral reef structures. Color contour maps using the 12-kHz data, created after correlating acoustic impedance predictions with core measured sediment properties, validates the initial facies pattern predictions made in real-time. The sediment facies patterns indicate a long-term pattern of deposition of fine-grained, silt-sized, surficial sediments in an area adjacent to the emergent carbonate embankment. Two-dimensional acoustic profiles along survey tracklines also provide cross-sectional views of seafloor and subbottom stratigraphy that confirm the buildup of these fine sediments in the northwest corner of the study site. A generous supply of sediment resulting from an abundance of benthic green algae ( Halimeda sp.) on adjacent shallow platforms form a thick sequence of fine sandy silt at the base of the southeastern edge of the embankment and fringing reef. Sediment cover over the limestone bedrock thins and becomes coarse southeast of Garden and Bush Keys, suggesting the likely existence of a dominant flow around the shallow carbonate embankment that restricts export of fine sediments out of the area.