Characterizing Subsurface Archaeological Structures with Full Resolution 3D GPR at the Early Dynastic Foundations of Saqqara Necropolis, Egypt

Abstract

Currently, Ground Penetrating Radar (GPR) used in archaeological prospection is based on 2-D parallel line methodologies characterized by line spacing from 0.25 to 1 m (common line separation is 0.5 m) with different GPR antennas and extensive interpolation used to fill data gaps. High resolution 3-D GPR images of the subsurface can be obtained by recording data with a quarter wavelength grid spacing in all directions. Recently, we used a new GPR system which is a combination of commercial GPR with a rotary laser positioning system developed at Tohoku University for full-resolution subsurface imaging. In this paper we will show how the high density 3-D GPR data acquired over an area of about 14 m × 28 m can improve the image quality and reveal the subsurface archaeological structure of early dynastic foundations in the Saqqara area. The GPR vertical cross-sections and the horizontal depth slices extracted from the full-resolution 3-D GPR reveal great information about ancient human activities, most likely burial mounds. GPR data at depth greater than 1.3 m were overwhelmed by “ringing features” (repeated horizontal harmonic-like features) most probably caused by the presence of underlying shallow layers of low resistivity shale and claystone. A 2-D electric resistivity tomography (ERT) profile was acquired using a multi-electrode system with 1 m electrode spacing. The ERT section shows high resistivity for the near surface desert sand and gravel deposits. The second geoelectric layer detected by ERT shows a low resistivity value consistent with the presence of a highly conductive layer at a depth of about 1.3 m. Integration of such different geophysical tools (e.g. GPR with ERT) helps to interpret the repeated horizontal features in the 3-D GPR data.