The use of side scan sonar in Brazilian Underwater Archaeology

Abstract

Underwater Archaeology is a relatively young science in Brazil, if compared to its status in other countries such as Canada, the US, France or Spain. Scientific researches date back to the beginning of the 1990's, with some isolated previous references in the 70's and the 80's. Because of that, some essential geophysical methods and techniques are overlooked at the time the research parameters are drawn. The aim of this presentation is to point out the crucial role of one type of acoustic survey — side scan sonar — to provide data for future Underwater Archaeological scientific research. This presentation focuses on some case studies in Brazilian Underwater Archaeology that have successfully used side scan sonar to locate objects and structures of archaeological interest. I will focus on my Master's dissertation, in which was used an analog side scan sonar (Klein Hydroscan 530 model, 100 KHz), to find an 18th century British cast iron cannon in Cananeia bar, in the southern coast of the state of Sao Paulo. Some additional points will be made, by means of the analysis of side scan records of a 19th century Brazilian steamboat sunk at Cananeia's downtown ferry terminal in 1858 which has also been surveyed with the same analog sonar. In addition, we wish to highlight some situations in which even more sophisticated side scan sonars (digital models, with low and high frequencies transducers) were relatively useless to Archaeology. Because the goal of the surveys was the acquisition of geological data for engineering purposes, it was impossible for the archaeological team to determine whether some specific kinds of regular geometrical anomalies that were recorded were natural or human made. Only direct examinations — with scuba divers — revealed that those anomalies were related to lateritic rocks and rays. In those cases, although the side scan sonars were newer and more sophisticated than Klein 530, the survey's aims resulted in poor accuracy for archaeological purposes due to the fact that the operators of the sonar were not originally focused on the acquisition of archaeological data.