Integration of Geophysical Techniques for Archaeological Prospection in Sudan.

Abstract

The geophysical data in the current study have been measured in four archaeological sites in Sudan. The studied sites include Meroe city, Domat Al-Hamadab, Musawwarat es Sufra and Jebel Barkal. The geophysical methods, which are suitable for each site, were selected on base of the physical contrast of the specific parameter of each method. This parameter represents magnetic susceptibility in magnetic method, electrical resistivity in resistivity method and dielectric permittivity in ground penetrating radar. The magnetic data in the studied archaeological sites in Sudan have been measured using three different magnetometers. The first one (caesium magnetometer with accuracy 1 0.01 nT) measures the total magnetic fields. The second one (Overhauser magnetometer :t: 0.1 nT) measures the vertical gradient of the total magnetic field and the third one (Fluxgate gradiometer :\: 0.1 nT) measures the vertical gradient of vertical component of the magnetic field using portable magnetometer/gradiometer. The magnetic surveys were complemented by resistivity mapping to investigate the archaeological features horizontally when these features have contrast in_ the magnetic susceptibility and resistivity to their surroundings. The complementary magnetic and resistivity mapping surveys have been integrated with electrical resistivity imaging (ERI) and ground penetrating radar (GPR) data, in profiles with locations selected on base of the magnetic and resistivity results. The ERI and GPR profiles provided complementary information about the variation of the anomalies related to the archaeological features with depth. The integrated interpretation of these methods leads to better understand the target features and accordingly helps the archaeologists witl1 some hints for correct conclusions. The integrated prospection results (chapter 5) convinced the archaeologist to replace some of the archaeological excavation by geophysical surveys to complete the missing parts of the site maps. Test excavations have been carried out for field check of the geophysical interpretation. This study shows how it is successful using geophysical processing techniques to improve signals of the geophysical data from archaeological sites so as to extract the maximum information from these data, and then gives better chances for successful interpretations. Basic theory and algorithms of the geophysical methods and processing techniques are used not only for explaining the successful techniques, but also for defining the problematic ones and introducing and recommending methods to overcome the encountered problems. It is demonstrated that geochemical analysis and magnetic susceptibility measurements can give some indications for explaining the success and failure of archaeological prospection using magnetic surveys. The magnetic gradient revealed the presence of some fairly clear archaeological building structures in the different studied sites. It is shown that such measurements are more effective to detect the shallow archaeological features than the total magnetic field measurements, which enhance the anomalies'of deeper sources. The geochemical analysis and magnetic susceptibility results of the collected samples, which represent the construction and the surrounding materials in the studied sites, indicate that sandstone as non-magnetic material can be detected successfully, when it is embedded in magnetic background. The fired-bricks may have higher magnetic susceptibilities. Therefore, they are shown in the magnetic images very clearly. Depending on the scale of the prospected features, comparison between coarse sampling pattern (e.g. 0.5 m) and fine sampling pattern (0.25 m) has been done. The first one was used in the reconnaissance‘ survey, but small-scale sampling pattern was required to resolve the small-scale anomalies more clearly. 3D modeling and inversion of the magnetic data of the studied area have been done using a priori infonnation, which were of great help to select a good starting model for the inversion process and then ending in a plausible model for the modeled anomalies with low rms error. ' 1D vertical electrical sounding (ID-VES) measurements have been used to give general estimation of the vertical extension of the prospected features. Furthermore, it helps to describe geological conditions of the archaeological features and their surroundings. 2D resistivity imaging profiles with small electrode- spacing (0.25 m, 0.5 or l m -depending on the scale of target anomalies) have been carried out along locations selected on base of magnetic, 1D-VES and resistivity mapping results. The resistivity inversion results of these profiles helped to estimate the vertical extensions of the archaeological targets more precisely. The applicability of combining 2-D electrical resistivity imaging (2D-ERI) profiles in 3-D data set for inversion using a 3-D resistivity inversion code is also demonstrated. High frequency (500 MHZ) ground penetrating radar (GPR) replaced 2D-ERI in arid and semi-arid area, where conditions favor the use GPR. The integrated interpretation of these methods leads to better understand the target features and accordingly helps the archaeologists with some hint for correct conclusions. This study can be used to pave the way for an increased use of geophysical techniques in the cultural heritage management of archaeological sites in Sudan.