Shallow Off-Shore Archaeological Prospection with 3-D Electrical Resistivity Tomography: The Case of Olous (Modern Elounda), Greece

Abstract

Abstract:It is well known that nowadays as well as in the past the vast majority of human habitation and activities are mainly concentrated in littoral areas. Thus the increased attention to coastal zone management contributed to the development and implementation of shallow-water mapping approaches for capturing current environmental conditions. During the last decade, geophysical imaging techniques like electrical resistivity tomography (ERT) have been used in mapping onshore buried antiquities in a non-destructive manner, contributing to cultural heritage management. Despite its increased implementation in mapping on-shore buried archaeological remains, ERT has minimal to non-existent employment for the understanding of the past dynamics in littoral and shallow off-shore marine environments. This work presents the results of an extensive ERT survey in investigating part of the Hellenistic to Byzantine submerged archaeological site of Olous, located on the north-eastern coast of Crete, Greece. A marine area of 7100 m 2 was covered with 178 densely spaced ERT lines having a cumulative length of 8.3 km. A combination of submerged static and moving survey modes were used to document potential buried and submerged structures. The acquired data from the marine environment were processed with two-dimensional and three-dimensional inversion algorithms. A real time kinematic global navigation satellite system was used to map the visible submerged walls and compile the bathymetry model of the bay. The adaptation of ERT in reconstructing the underwater archaeological remains in a shallow marine environment presented specific methodological and processing challenges. The in situ experience from the archaeological site of Olous showed that ERT provided a robust method for mapping the submerged archaeological structures related to the ancient built environment (walls, buildings, roads), signifying at the same time the vertical stratigraphy of the submerged sediments. The inherent limitation of employing ERT in a conductive environment is counterbalanced by the incorporation of precise knowledge for the conductivity and bathymetry of the saline water in the modelling and inversion procedure. Although the methodology definitely needs further refinement, the overall outcomes of this work underline the potential of ERT imaging being integrated into wider shallow marine projects for the mapping of archaeological sites in similar environmental regimes.

The Rise of 3D in Maritime Archaeology

“…Photogrammetry even facilitates 3D survey of the spaces inside large vessels, as demonstrated by the early results of the Thistlegorm Project (2018)-a comprehensive survey in 3D of one of the most well-known and dived wrecks in the world. Other important 3D sensing techniques for the marine environment also emerged around the same time, including lidar bathymetry (Doneus et al 2013(Doneus et al , 2015, 3D sub-bottom profilers (Gutowski et al 2015;Missiaen et al 2018;Plets et al 2008;Vardy et al 2008) and Electrical Resistivity Tomography (ERT) (Simyrdanis et al 2016;Passaro et al 2009;Ranieri et al 2010;Simyrdanis et al 2015Simyrdanis et al , 2018. These are enor-mously important due to their ability to non-invasively recover 3D data from shallow water (lidar bathymetry) sites and from below the seabed (sub-bottom profilers and ERT), but due to cost and availability are not nearly as widely used as multibeam and photogrammetry.…”

Resolving Dimensions: A Comparison Between ERT Imaging and 3D Modelling of the Barge Crowie, South Australia

“…There has been an increasing trend towwards the use of ERT methods in marine and freshwater environments, particularly for geological mapping (Rucker et al 2011) and the location of archaeological material (Passaro et al 2009;Passaro 2010;Ranieri et al 2010;Simyrdanis et al 2015Simyrdanis et al , 2016Simyrdanis et al , 2018. Electrical resistivity can be deployed in aquatic environments with either floating or submerged sensors, as shown in Fig.…”

Imaging a medieval shipwreck with the new PingPong 3D marine reflection seismic system

“…Nevertheless, there are different approaches to shallow water marine geophysical prospection like, e.g. Arcone, Finnegan, and Boitnott () who were able to achieve a GPR penetration of a few metres with 135 MHz GPR in water with low conductivity, searching unexploded ordnance (UXO), Simyrdanis, Papadopoulos, and Cantoro () who performed a marine ERT survey to image submerged ancient wall constructions in Greece, or Passaro () who imaged a World War II wreck using marine ERT. Offshore magnetic gradiometry suffers from the decrease of resolution caused by the enlarged distance between sensors floating near the water surface and targets located below the seafloor, whereas near‐seabottom magnetics or EMI requires the construction of underwater sensor vessels and special efforts in positioning.…”