3D bathymetry and bottom backscattering maps applied to identify potential phosphate deposits in the pelotas Basin, Brazil

The Brazilian sector of the Pelotas Basin (PB) covers an area of 210,000 km2, limited northwards by the Florianópolis High southwards by the boundary with Uruguay. The PB is economically and strategically important due to the presence of sub-surface mineral resources such as oil, natural and hydrate gas, as well as surface deposits, crusts and concretions of phosphorite (P2O5), in addition the PB it is an important fishing ground. In this work bathymetric and Bottom Surface Backscattering Strength (BSBS) data were used from twenty hydroacoustic assessment surveys undertaken either to map pelagic fish or to evaluate the abundance of cetaceans in the PB. Bathymetry and BSBS were collected with a calibrated scientific echo-sounder SIMRAD EK500, operating at the frequency of 38 kHz. A total of 5,249,191 acoustic readings were processed from 15,367 nautical miles. Bathymetric data were integrated to satellite altimetry data in order to produce a 3D map aiming the identification of the main features in the region, such as the Rio Grande Valleys, the Rio Grande Cone, the Rio Grande Terrace and the terrace off Florianópolis. A BSBS map was also generated. The BSBS acoustic parameter is the ratio between incident and reflected energy by the seafloor and it is usually expressed on a logarithmic scale (dB). Bottom backscattered signal is influenced by physical, chemical and morphological characteristics of the seabed. Thus, BSBS can be used to identify different bottom types due to their different acoustic responses. In our analyses the PB showed intermediate BSBS values. Lower BSBS values, associated with finer sediment, predominated in the continental slope and oceanic areas, while high values, associated with coarse sediments, did not occupy large areas. The BSBS map was then superimposed on the 3D bathymetric map, allowing the identification of six areas associated with the presence of phosphorite in the PB, based on the high BSBS values and depth, suitable for the formation of phosphorite. Because of their genesis phosphorite deposits tend to exist in areas with high levels of organic matter, at depths of up to 1000 m, covering the edge of the continental shelf and upper slope, marginal plateaus and submarine banks. Potential areas were identified in the outer shelf on the Rio Grande Cone, on the outer shelf and slope off Mostardas-RS, on the Rio Grande Terrace and off Florianópolis-SC. All these areas have good correspondence with those identified by the Geological Survey of Brazil with the presence of calcareous deposits and phosphorite.