Investigation of magnetic anomalies in archaeological prospection

Hahn, Sandra (2023): Investigation of magnetic anomalies in archaeological prospection. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

Magnetometer prospecting refers to mapping the magnetic field and is an essential method in archaeological prospecting and archaeogeophysics for investigating large surface areas. Permanent and induced magnetic fields of potential features, as well as the Earth’s magnetic field, are registered, but the identification of the archaeological features relies fundamentally on the magnetisation contrast to the surroundings. The devices used are categorised as scalar and vector magnetometers, which provide different elements of the superimposed fields, resulting after data processing in different visual outputs. A new approach is presented to merge vector gradiometer with high-pass filtered total field magnetometer data to obtain visually uniform magnetograms for better interpretation. The registered anomaly of a feature depends on several source-specific parameters, among them the direction of its magnetisation. Inspired by the fieldwork at Artanish, synthetic magnetograms were generated to investigate the effect of the total magnetisation variation on a spherical object’s anomaly pattern. The results are discussed in terms of the impact of the remanence and their significance for the survey interpretation. The anomaly pattern also depends on the direction of the Earth’s magnetic field. From synthetic magnetograms and profiles, it was determined that the effect of the declination is avoidable for prospecting. The effect of the inclination is not compensable as anomaly patterns and peak ratios vary significantly with the inclination of the Earth’s magnetic field. For the case of an inclination of 15°, modelling results are compared to data collected at an isocline of 15° and the effect of the inclination discussed for spherical and linear features. Three case studies show the applicability of magnetometer prospecting under different conditions and for different research questions. Prospecting at the Sumerian site of Suruppak, the modern Fara, brought new insights into the excavation of 1902/1903 and answered opened questions about the existence of a city wall and harbour, canal routing and settlement structure of the ancient site. The case study of Yeha and Melazo portrays how complex magnetograms appear at sites still inhabited today. Archaeological features can only be reliably identified if geological and surface anomalies of recent buildings or agricultural use are recognised. The original idea of magnetometer prospecting in Gumbati was to re-locate the mid-1990s excavation or to find related features to the Achaemenid complex discovered back then. The resulting magnetograms show prominent rectangular features that were targeted in a subsequent excavation, but the source of the magnetic anomalies was not uncovered. Chemical analyses and susceptibility measurements indicate that fertiliser residues could be the reasons for these anomalies.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 3. Mai 2023
• 1. Bericht­erstatter:in: Fassbinder, Jörg