Last modified: 2011-12-21
Abstract
The application of geophysical methods to investigate the near-surface soil layers containing anthropic manufactures has been recognized as an important element of archaeological research by the international community. Geophysics can be used to rapidly delineate the presence of archaeological buried structures without invasive and expensive stratigraphic excavation. In particular, resistivity surveying can be used to understand the geometry and the depth of the anthropic element buried in the subsoil, due to the different resistivity properties between the potential archaeological targets and the surrounding environment; by GPR methods it is possible to easily produce high definition maps of buried remains, thanks to the transmission of high frequency radar pulses from a surface antenna into the ground.
Geoelectrical data are traditionally acquired with a galvanically-coupled resistivity system. The most practical difficulty is to emplace electrodes in the soil; this operation is time consuming and prevents the fast realization of the investigation, especially in case of a three-dimensional survey. This problem can be avoided using the new OhmMapper (Geometrics Inc.) capacitively-coupled resistivity system designed to be pulled along the ground as a streamer that realizes an almost continuous profile.
The main problem of the OhmMapper resistivity-meter is the length of the instrument and the consequent necessity of a lot of space to use it. To avoid this problem in small spaces, we use the multi-frequency GPR RIS MF HiMod (IDS S.p.A.), top level product dedicated to underground mappings: the 200 and 600 MHz dual frequency antenna is the ideal solution to quickly realize an accurate mapping of the subsoil in archaeological context.
In order to verify the presence of buried structures and bedrock in the subsoil of the ancient town of Lixus (Larache - Morocco), we carried out some geophysical surveys in different areas using GPR and OhmMapper resistivity-meter. The site of Lixus is located on the northern Atlantic coast of Morocco, on the bank of the Loukkos River, approximately 80 kilometres south of Tangier and adjacent to the city of Larache.
Thanks to the archaeological excavation carried out on the hill of Lixus it appears that the archaeological site had different building stages from the Phoenicians (8th century BC) until the Marinid dynasty (14th century).
We decided to use GPR in restricted areas to detect buried structures under the foundation of the visible archaeological structures; we set a grid characterized by parallel and perpendicular lines close to each other to intercept the walls: we created several resistivity maps at different depths from a few centimetres from the ground surface to two meter deph. Instead we used the OhmMapper resistivity-meters along the dirt roads of the hill in order to locate the buried section of the city walls and the depth of the sandstone bedrock, which is untouched by anthropic activities.
The resistivity and GPR data were also compared with magnetics data acquired in the ancient town of Lixus. This multi-method approach permits us to check the data from several independent measurements and increase the geometrical and physical information useful for interpretation.