Combined Seismic Tomographic and Ultra-Shallow Seismic Reflection Study of an Early Dynastic Mastaba, Saqqara, Egypt

Abstract

Mastabas were large rectangular structures built for the funerals and burials of the earliest Pharaohs. One such mastaba was the basic building block that led to the first known stone pyramid, the >4600-year old Step Pyramid within the Saqqara necropolis of Egypt. We have tested a number of shallow geophysical techniques for investigating in a non-invasive manner the subsurface beneath a large Early Dynastic mastaba located close to the Step Pyramid. After discovering that near-surface sedimentary rocks with unusually high electrical conductivities precluded the use of the ground-penetrating radar method, a very high-resolution seismic data set was collected along a profile that extended the 42.5 m length of the mastaba. A sledgehammer source was used every 0.2 m and the data were recorded using a 48-channel array of single geophones spaced at 0.2 m intervals. Inversions of the direct- and refracted-wave traveltimes provided P-wave velocity tomograms of the shallow subsurface, whereas relatively standard processing techniques yielded a high-fold (50–80) ultra-shallow seismic reflection section. The tomographic and reflection images were jointly interpreted in terms of loose sand and friable limestone layers with low P-wave velocities of 150–650 m/s overlying consolidated limestone and shale with velocities >1500 m/s. The sharp contact between the low and high velocity regimes was approximately horizontal at a depth of ~2 m. This contact was the source of a strong seismic reflection. Above this contact, the velocity tomogram revealed moderately high velocities at the surface location of a friable limestone outcrop and two low velocity blocks that probably outlined sand-filled shafts. Below the contact, three regularly spaced low velocity blocks likely represented tunnels and/or subsurface chambers.