Marine electrical resistivity tomography for shipwreck detection in very shallow water: a case study from Agropoli (Salerno, southern Italy)

Abstract A marine electric resistivity survey was carried out over a submerged beach along the Agropoli shore (Salerno, Italy) to detect buried objects of archaeological interest below the sandy seabed. We found a shipwreck, a military vessel that probably sunk during the Salerno landing operations of the allied forces in the Second World War. Resistivity data provide information on the vertical and horizontal extension of the shipwreck, which is characterized by very low calculated resistivity values (about 2–5 ohm m). Such values differ significantly from the sand and the bedrock values (5–40 ohm m). Although the presence of the shipwreck is clearly visible from geoelectric data, the joint application of electric, magnetic and multibeam bathymetric techniques reduces the ambiguities inherent in each method. As shown in the Electrical Resistivity Tomography (ERT) and confirmed by the Digital Elevation Model (obtained from the processing of bathymetric data), the shipwreck extends more than 30 m in NE–SW direction and it is about 13 m wide. The global extension of the relic is consistent with the magnetic data, that are characterized by a magnetic anomaly with an amplitude of about 1800 nT and similar dimension, as inferred from the estimation of source boundaries obtained from the computation of the analytic signal. The results of our survey encourage the use of marine geoelectrical methods for the detection of buried archaeological targets, particularly in locations where the use of seismic prospecting is not effective (e.g. very shallow water with sandy sea-bottoms). The integration of different geophysical methods allows to better define the extension, depth and thickness of buried objects, suggesting that such an approach is the most effective for underwater archaeological investigations.

[1]  Sergio Negri,et al.  Geophysical investigation of the Temple of Apollo (Hierapolis, Turkey) , 2006 .

[2]  E. Forte,et al.  2-D and 3-D processing and interpretation of multi-fold ground penetrating radar data:a case history from an archaeological site , 1999 .

[3]  Misac N. Nabighian,et al.  The analytic signal of two-dimensional magnetic bodies with polygonal cross-section; its properties and use for automated anomaly interpretation , 1972 .

[4]  Niels B. Christensen,et al.  Analysis and application of a non‐conventional underwater geoelectrical method in Lake Geneva, Switzerland , 1998 .

[5]  G. Leucci.  Contribution of Ground Penetrating Radar and Electrical Resistivity Tomography to identify the cavity and fractures under the main Church in Botrugno (Lecce, Italy) , 2006 .

[6]  Joseph I. Boyce,et al.  Ultra-high-resolution marine 2D–3D seismic investigation of the Liman Tepe/Karantina Island archaeological site (Urla/Turkey) , 2009 .

[7]  E. Marsella,et al.  Late Pliocene-Early Pleistocene compressional tectonics in offshore Campania (Eastern Tyrrhenian sea) , 1994 .

[8]  G. Leucci,et al.  Integrated geophysical surveys at the archaeological site of Tindari (Sicily, Italy) , 2006 .

[9]  M. Geen,et al.  The application of the Submetrix ISIS 100 Swath Bathymetry system to the management of underwater sites , 2000 .

[10]  V. Morra,et al.  The proximal marine record of Somma-Vesuvius volcanic activity in the Naples and Salerno bays, Eastern Tyrrhenian Sea, during the last 3 kyrs , 2008 .

[11]  Avner Raban,et al.  Marine Magnetic Survey of a Submerged Roman Harbour, Caesarea Maritima, Israel , 2004 .

[12]  R. Sheriff.  Limitations on Resolution of Seismic Reflections and Geologic Detail Derivable from Them: Section 1. Fundamentals of Stratigraphic Interpretation of Seismic Data , 1977 .

[13]  T. Dahlin,et al.  A comparison of the Gauss-Newton and quasi-Newton methods in resistivity imaging inversion , 2002 .

[14]  Rory Quinn,et al.  Integrated Geophysical Surveys of The French Frigate La Surveillante (1797), Bantry Bay, Co. Cork, Ireland , 2002 .

[15]  G. Leucci,et al.  GPR, ERT and magnetic investigations inside the Martyrium of St Philip, Hierapolis, Turkey , 2009 .

[16]  G. Leucci,et al.  Three-dimensional image of seismic refraction tomography and electrical resistivity tomography survey in the castle of Occhiola ` (Sicily, Italy) , 2007 .

[17]  Marilena Cozzolino,et al.  Three‐dimensional resistivity probability tomography at the prehistoric site of grotta reali (Molise, Italy) , 2009 .

[18]  E. Landa,et al.  Detection of shallow objects using refracted and diffracted seismic waves , 1998 .

[19]  R. Quinn,et al.  Marine geophysical investigation of the inshore coastal waters of Northern Ireland , 2000 .

[20]  J. Phillips,et al.  National geophysical data grids; gamma-ray, gravity, magnetic, and topographic data for the conterminous United States , 1993 .

[21]  Justin K. Dix,et al.  The use of a high-resolution 3D Chirp sub-bottom profiler for the reconstruction of the shallow water archaeological site of the Grace Dieu (1439), River Hamble, UK , 2009 .

[22]  R. Barker,et al.  Rapid least-squared inversion of apparent resisitivity pseudosections by a quasi-Newton method , 1996 .

[23]  Walter R. Roest,et al.  Magnetic interpretation using the 3-D analytic signal , 1992 .

[24]  Rune Solberg,et al.  Automatic detection of circular structures in high‐resolution satellite images of agricultural land , 2009 .

[25]  Salvatore Piro,et al.  Integrated geophysical and topographical investigation in the territory of Ancient Tarquinia (Viterbo, central Italy) , 2007 .

[26]  A. Vallega.  The coastal cultural heritage facing coastal management , 2003 .

[27]  F. Baumgartner.  A new method for geoelectrical investigations underwater , 1996 .

[28]  S. Fletcher,et al.  Coastal management in the Solent: An introduction , 2007 .

[29]  Xin Wang,et al.  American Association of Petroleum Geologists Memoir , 2011 .

[30]  Sergio Negri,et al.  Application of 3D visualization techniques in the analysis of GPR data for archaeology , 2002 .

[31]  T. Missiaen,et al.  Very-high-resolution seismic and magnetic investigations of a chemical munition dumpsite in the Baltic Sea , 2008 .

[32]  C. Faccenna,et al.  Late Pleistocene N-S shear zones along Latium Tyrrhenian margin: structural characters and volcanological implications , 1994 .

[33]  Rory Quinn,et al.  Backscatter responses and resolution considerations in archaeological side-scan sonar surveys: a control experiment , 2005 .

[34]  Dean Goodman,et al.  Ground-Penetrating Radar: An Introduction for Archaeologists , 1997 .

[35]  Lanbo Liu,et al.  Marine electrical resistivity imaging of submarine groundwater discharge: sensitivity analysis and application in Waquoit Bay, Massachusetts, USA , 2010 .

[36]  M. Loke.  Tutorial : 2-D and 3-D electrical imaging surveys , 2001 .

[37]  Rory Quinn,et al.  The role of scour in shipwreck site formation processes and the preservation of wreck-associated scour signatures in the sedimentary record – evidence from seabed and sub-surface data , 2006 .

[38]  Rory Quinn,et al.  A side-scan sonar and high-resolution Chirp sub-bottom profile study of the natural and anthropogenic sedimentary record of Lower Lough Erne, northwestern Ireland , 2006 .

[39]  F. Budillon,et al.  Event beds in the recent prodelta stratigraphic record of the small flood-prone Bonea Stream (Amalfi Coast, Southern Italy) , 2005 .

[40]  D. Chianese,et al.  Electric and magnetic tomographic approach for the geophysical investigation of an unexplored area in the archaeological site of Pompeii (Southern Italy) , 2008 .