3D Quantitative Interpretation of Archaeomagnetic Surveys: Application of Mathematical Modelling to Determine Depths and Physical Characteristics of Buried Materials

High-resolution total-field magnetic data can be collected rapidly and relatively cheaply over large archaeological sites due to recent advances in data collection. However, interpretation of these datasets still generally comprises a sequence of data correction and filtering operations prior to a 2D visual interpretation based on pattern recognition. In contrast, current developments in aero-magnetic interpretation have led to several tools for identifying location, shape and depth information of anomalous sources. Information regarding the horizontal location of sources can be obtained from derivative based methods such as the horizontal gradient magnitude, tilt-angle and theta-map, however these methods typically fail for archaeological purposes due to the high noise content of these datasets. Here it is shown that using pseudogravity data with these techniques overcomes the problem of noise amplification that has previously hampered archaeological implementation. Ideally, what it desired from the survey is information regarding the physical properties of causative features, rather than their magnetic responses. To achieve this, 3D inversion of the dataset can be used to produce a magnetic susceptibility model of the subsurface. It is demonstrated here that significant changes to the routine procedure employed during inversion of aero-magnetic data are required to avoid large discrepancies between the true and modelled depths of archaeological features. Here, a workflow is proposed that demonstrates how to process and model magnetic data for archaeological interpretation. A number of different stages are identified, dependant on the level of interpretation required, from enhancing the data image to full 3D quantitative results. Greater interpretation of the magnetic data will aid the planning of higher resolution surveys such as GPR or resistivity, or to position targeted excavations. In some situations the detailed 3D geophysical model may supply sufficient archaeological information to avoid excavation, thus reducing cost and risk to the archaeological material.