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July 2003-July 2010InteractiveDig Sagalassos
Kris Vanneste adjusting the resistivity measurement equipment
The electrodes used for the electrical resistivity profiling

Photos courtesy Sagalassos Archaeological Research Project. Click on images to enlarge.
by Marc Waelkens

Seismological Studies: Introduction

This week Dominique Similox-Tohon (Structural Geology and Tectonics Group, KULeuven), Kris Vanneste (Royal Observatory of Belgium), and Philippe Muchez (Physico-Chemical Geology, KULeuven) continued neotectonic research at Sagalassos. Their goal is to identify active faults in the area that can generate major earthquakes such as the devastating mid-seventh-century A.D. one that extinguished whatever remained of Sagalassos. To visualize such active faults they used electrical resistivity profiling. In this, an image of the electric resistivity variations in the subsurface along a profile is obtained. Different geological layers (e.g. sand, clay, limestone, screes, ophiolite, etc.) may be distinguished, if they are characterized by different porosity or water content. In this method, 64 electrodes are planted in the ground along a straight line perpendicular to the structures one wishes to investigate. A set of measurements with different electrode spacing is repeated for all possible electrode combinations along the line. The result is a "pseudo-section," which has to be inverted to obtain a model of the true subsurface resistivity.

Five profiles were measured at the northern extremity of Sagalassos. Three profiles were oriented north-south and two east-west. The profiles range from 126 to 475 meters in length with electrode spacing varying between two and five meters. One purpose of these profiles was to study the lateral relationship between the faults observed in the resistivity profiles (2002) and in the Neon Library (in the east), and the fault observed in a trench at the northwest necropolis (see Field Notes 2003, Seismological Studies, July 27-August 2). A second purpose was to correlate the variation of resistivity layers in depth. The preliminary results indicate the possible continuation of faults in the western part of the site and indicate that the deeper resistivity layers show mainly low resistivity values interrupted by areas with higher values. The low values correspond to the ophiolitic mélange bedrock. The higher resistivity could be from heterogeneities in the ophiolitic mélange, the presence of limestone massifs, or both. The team also recorded an electrical resistivity profile in the Basköy area where a travertine escarpment is present. Last year (see Field Notes 2003, Seismological Studies, August 24-30) a profile was already shot along a steep irrigation channel but the profile appeared to be too short to verify if the travertine deposits are related to an active fault. This year, this profile was extended and the results are currently processed.

[image] [image]
Location of resistivity profiles measured during the 2002 (black dots) and 2004 (blue dots) campaign. Faults identified during the previous campaigns are shown in red. Hatchings indicate the side of the hanging wall block. Preliminary resistivity profile SA04P4, oriented west-east between the Neon Library and the Potters' Quarter indicating the presence of limestone and the ophiolitic mélange in the subsurface.
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