Abstract
Increasingly, electromagnetic induction methods (EMI) are being used within the area of archaeological prospecting for mapping soil structures or for studying paleo-landscapes. Recent hardware developments have made fast data acquisition, combined with precise positioning, possible, thus providing interesting possibilities for archaeological prospecting. However, it is commonly assumed that the instrument operates in what is referred to as Low Induction Number, or LIN. Here, we detail the problems of the approximations while discussing a best practice for EMI measurements, data processing, and inversion for understanding a paleo-landscape at an Iron Age human bone depositional site (Alken Enge) in Denmark. On synthetic as well as field data we show that soil mapping based on EMI instruments can be improved by applying data processing methodologies from adjacent scientific fields. Data from a 10 hectare study site was collected with a line spacing of 1–4 m, resulting in roughly 13,000 processed soundings, which were inverted with a full non-linear algorithm. The models had higher dynamic range in the retrieved resistivity values, as well as sharper contrasts between structural elements than we could obtain by looking at data alone. We show that the pre-excavation EMI mapping facilitated an archaeological prospecting where traditional trenching could be replaced by a few test pits at selected sites, hereby increasing the chance of finding human bones. In a general context we show that (1) dedicated processing of EMI data is necessary to remove coupling from anthropogenic structures (fences, phone cables, paved roads, etc.), and (2) that carrying out a dedicated full non-linear inversion with spatial coherency constraints improves the accuracy of resistivities and structures over using the data as they are or using the Low Induction Number (LIN) approximation.
Highlights
Geophysical methods are used in archaeological prospecting, i.e., to help to distinguish between archaeological constructions and non-anthropogenically disturbed soils, or to make large-scale investigations to study paleo-landscapes
Gaffney [1] stated that electromagnetic induction (EMI) techniques remained underestimated, as the Geonics EM38 system was the only one used at this time
The use of EMI methods has been increased use outside the archeological community, especially for geotechnical applications and agricultural uses [8,9,10,11,12,13,14]. It is a common assumption for most of the above-mentioned applications that the instrument operates in what is referred to as low induction number, or Low Induction Number (LIN)
Summary
Geophysical methods are used in archaeological prospecting, i.e., to help to distinguish between archaeological constructions and non-anthropogenically disturbed soils, or to make large-scale investigations to study paleo-landscapes. Several instruments and mountings have been tested with more advanced multi-receiver EMI instruments on archaeological sites This includes Bonsall et al [4] with an instrument from GF instruments (CMD mini-explorer), Bates [5] used EM31 and EM34 to map a paleo-landscape, while De Smedt et al [6] and De Smedt et al [7] have used DualEM-21s to map soil, buried objects, and archaeological features. The use of EMI methods has been increased use outside the archeological community, especially for geotechnical applications and agricultural uses [8,9,10,11,12,13,14] It is a common assumption for most of the above-mentioned applications that the instrument operates in what is referred to as low induction number, or LIN. All foslulrovweyin(gphmotaopfrsoamreaneoxtahcetrlysitteh).e same (b) Schematics of the DualEM-421s system employed in the survey (photo from another site)
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