Using seismic traveltime tomography in geoarchaeological exploration: an application at the site of Chatby cemeteries in Alexandria, Egypt

Abstract

We apply seismic traveltime tomography in an investigation of the subsurface of a geoarchaeological site at Chatby cemeteries (Alexandria, Egypt). We are interested in obtaining information about the geological structure and the remains of man‐made structures. Energy sources are placed at the surface and recorders are distributed vertically in an underground location. Traveltime data are inverted with a three‐dimensional tomographic algorithm. Inversion results are evaluated and interpreted with the aid of independent geophysical and archaeological information, and various tests are carried out with different initial velocity models and synthetic traveltimes. The inversion results show good correlation with the estimated geological structure, in particular at deeper levels, as suggested by independent geophysical and archaeological information and later confirmed by excavation. The depth and variations of the bedrock surface are satisfactorily imaged. Indications of possible man‐made structures are presented and discussed. Shallow velocity features are evaluated and interpreted using tests with synthetic traveltimes. The main goal of imaging the deeper structure was successful. As inversion tests with different initial velocity models indicate, the subsurface structure and the complexity of data information in this particular case are better handled when using a single‐layer model than when using an average two‐layer model. The single‐layer model allows for a more versatile velocity variation and raypath distribution, while the two‐layer model leads to an inversion result that is considerably influenced by the initial velocity distribution. Enhancement of particular aspects of the single‐layer inversion result can be derived with the use of an initial velocity model, that permits a gradual increase of velocity with depth. The gradient model improves the raypath distribution and limits velocity artefacts. The resulting images can assist in the assessment of the archaeological potential of a site, particularly in excavation design, in a non‐destructive manner. They can be also of great value in other applications, for example in engineering works, when detailed information on the subsurface is sought in areas of restricted access.

Seismic velocity model and near‐surface geology at Mycenaean Tiryns, Argive Basin, Peloponnese, Greece

“…Figure 7e shows a representative example of the v p /v s ratios obtained from profiles 04P and 04SH, which are next to the Tiryns ridge. The v p /v s archaeological sites with complex near-surface geologic structure and spatial limitations of an urban setting (Polymenakos and Papamarinopoulos 2007;Schicht et al 2007). Our v p and v s tomograms modeled with SRT have been interpreted to depths of 30 m without showing evidence of faulting, and geologic engineering parameters are derived for the bedrock.…”

“…The ability of SRT to image dipping refractors in karstic terrains (Sheehan et al ) provides us with in situ seismic information for large portions of the subsurface. Regardless of topographic relief, the non‐destructive nature of SRT has successfully imaged the underground geological structure at other archaeological sites with complex near‐surface geologic structure and spatial limitations of an urban setting (Polymenakos and Papamarinopoulos ; Schicht et al ). Our v p and v s tomograms modeled with SRT have been interpreted to depths of 30 m without showing evidence of faulting, and geologic engineering parameters are derived for the bedrock.…”

Investigation of clay sediments and bedrock morphology in caves with seismic traveltime tomography: an application at Alepotrypa Cave (Diros, Greece)

“…Velocity decreases significantly in fractured and weathered rocks and unconsolidated sediments in contrast to consolidated sediments and intact rocks (Jongmans & Garambois, 2007). As a result, vertical and lateral changes of velocity can discriminate the overlying sediment cover from the hard rock substratum (Lanz et al, 1998;Polymenakos & Papamarinopoulos, 2007;Samyn et al, 2012) and highlight the alternation of steeply dipping fracture areas and rocks (Heincke et al, 2006).…”