Geophysical survey at Petit Portelet

Over the last few days we have also been undertaking a detailed geophysical survey of the site at Gorey.  In order to determine where the artefacts in the collections may have come from we need to understand the buried geology at the site. The geophysics is a way of using remote electrical and electromagnetics to “see” into the earth.  Our survey has included both an electromagnetic survey (EM 38/EM 31) as well as an Electric Resistivity Tomography  Survey.  These investigations have begun to reveal the underlying structure of our site sitting between the ancient granite cliff line and the outcrop on which the castle at Gorey sits.  We can now see where gravels from the collapsing cliffs and finer grained sands and silts blown into the site are resting beneath the ground.  We hope that these results will provide a basis for locating boreholes and other test pits in the future that might reveal the presence of artefacts.


Fig 1. EM 31 ground conductivity surveying


Fig 2. Electric Resistivity Tomography Survey cables set out on Castle Green


Fig 3. Maps of electrical conductivity from the EM 31 and EM 38 surveys at the site (reds are conductive areas, blues more resistive area)


Fig 4. An example of the Electric Resistivity Tomography  Survey line data and an interpretation of the results


Fig 5. A ‘fence diagram’ of the results from the study placed relative to a Google Earth image of the site.




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