A low-cost approach to improve sediment mapping in shallow marine environment: A case study on the inner continental shelf of Parana, Southern Brazil

摘要

Sediment mapping is a quest that demands a high and extensive sampling effort. The costs involved in surveying shallow marine environments are high, particularly for emerging countries. In this sense, we propose a low-cost approach that is useful for both cases where past data is available and in the case of further surveys to be planned. We integrated side-scan data, sediment grain-size parameters and bathymetric data to produce more reliable sediment distribution maps. The mapped area is located at the Paraná inner shelf and covers 3,000 km^{2between 5 and 30 m isobaths. We used a set of 875 georrefered sediment grain-size data regularly spaced, with spacing from 1 to 4 km, surveys of side scan sonar in four small areas, about 9km2, and bathymetric data from nautical charts. Mean grain size vary from fine silt to very coarse sand, 0.016 to 1.032 mm. Most of the samples, 80 %, have mean diameter between 0.063 and 0.250 mm, 53 % are of fine and 27 % of very fine sand with broad distribution along the inner shelf. Medium to coarse sands, 12 % of samples, and silt, 8 % of samples, present a patchy distribution along inner shelf. Backscatter intensity of side scan sonar is related with sediments mean grain size. Areas of fine and very fine sand have the lowest backscatter while medium to coarse sands have highest backscatter. Sonograms showed that medium to coarse sands are in direct contact with the broad distributed fine to very fine sands of inner shelf. The medium to coarse sand bodies have an elongated shore oblique distribution. Bathymetric data showed that most of medium to coarse sands and silts are associated with sand ridges. Both sand ridges and medium to coarse sand bodies are spaced from 0.5 to 4 km. Therefore, considering the grain size data and the sand bodies spacing, the use of any spatial interpolation will provide wrong information between sampled sediment points. Surveys with side scan sonar allowed us to delineate a spatial continuity between georrefered sediment data. Then the patterns observed in side scan surveys could be used to elaborate more reliable surficial sediment maps. Even when side scan surveys of large areas are unviable, small areas surveys allow us to have a general idea of surficial sediment spatial distribution.}