USING GROUND PENETRATING RADAR (GPR) TO DETERMINE THE GEOPHYSICAL PROPERTIES OF SUBMERGED SEDIMENTS: AN EXAMPLE FROM A CARIBBEAN COASTAL LAKE

Lacustrine basins contain sediment types with unique physical and chemical properties that are unknown until sediment cores can be extracted and examined. Variations in sediment properties over time and space within lake basins provide researchers with important clues as to how climatic and/or tectonic conditions have changed over time. It is commonly assumed that the best lake sediment package for paleoenvironmental study is found in the center of most lakes, where greater amounts of finer silt- to clay-sized particles accumulate. However, this assumption about sedimentation may be incorrect, depending on the geologic history of the basin. Some researchers have extracted multiple sediment cores from single lakes to examine the spatial variability of submerged sediments, although this approach is costly in time and other resources. Here we report an alternative approach to determine the physical properties (particle size) of lake sediments using ground-penetrating radar (GPR). We used a 400 MHz antenna along 26 transects across Laguna Saladilla (19.61 N, 71.7 W; ~7 m asl), a large lake (220 ha) on the north coast of the Dominican Republic. Our objective was to identify subaqueous depositional facies and deltas deposited by the Rio Masacre. A change in the reflectivity of the radar signal (a function of electrical conductivity and magnetic permeability of the underlying sediments) was indicated by a change in the velocity of propagating radar energy at the submerged sediment-water interface. The relative dialetric permittivity (RDP) of the overlying (water) and underlying (sediment) material was estimated for 13 surface sediment samples collected with a dredge from Laguna Saladilla, and compared to results of our GPR survey. The coefficient of reflectivity of each of the 13 samples was found to be lowest in samples with high percentages of sand and highest in samples with larger proportions of clay- and silt- sized material. This may be due to greater saturated pore spaces in sand that will increase the RDP of the underlying material. These results show how GPR can be used in lacustrine research to determine changes in the physical properties of subaqueous sediments by non-invasively measuring changes in signal velocity across lake basins.