Use of echo-sounders in marine archaeology, survey of Porto Grande in Syracuse

Over the last ten years, the high-resolution marine geophysical survey field has witnessed significant advances in survey investigation equipment. New equipments are addressed towards the use of quantitative acoustic methods for the high-resolution mapping of the seafloor for archaeological heritage management. In particular, side-scan sonar and sub bottom profiler are been tested in Syracuse areas, especially in Porto Grande, about a project led under the supervision of the Sea Superintendence of Palermo and in collaboration with Aurora Trust, Malta and Pharos s.a.s., Milan. Generally, this high-resolution equipments are becoming affordable for use on archeological projects thanks to the high accuracy of the measures, the high spatial resolution, the repeatability of the results and the possibility to integrate the data with other scientific ones. In addition of this, through the use of geographic information system (GIS), quantitative results are stored in digital format for post-survey processing. Side-Scan Sonar system is applied to survey the morphology of the seafloor. This system identifies the different type of seafloor (such as sand, rocks, mud, posidonia, and so on) and objects eventually dispersed on the seafloor (such as wreck). The system is based on the different backscattering given by various material and lithotype and it records the backscattering data through a digital technology that gives back high-resolution images. Finally, the individual side-scan strips can be joined together to obtain a side-scan mosaic of the surveyed region through SonarWizMap software. Sub bottom profiler allows to identifying the litho-stratigrafic sequence of the seafloor and it is based on the different sound speed through the sediments. The penetration and the reflection of the backscattering depend from the physics characteristics of the material surveyed and from signal power and frequency. The backscattering impulses are recorded by the same echosounder that transmits the signal. The system uses the digital Chirp technology to create high-resolution images. On the surveyed area it's usually realized a mesh of strips, in relation to the accuracy required. The two systems (sub-bottom profiler and side-scan sonar) can be used together to have an exact correlation of the areal and depth information. Keywords: side-scan sonar, sub-bottom profiler, marine archaeological surveys.