Improved Geoarchaeological Mapping with Electromagnetic Induction Instruments from Dedicated Processing and Inversion

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.

Geophysical characterisation of the groundwater–surface water interface

“…Butler et al, 2004;Binley et al, 2013;Kinnear et al, 2013). In addition, as noted recently by Christiansen et al (2016), the majority of studies present apparent electrical conductivity, e.g. without appropriate data processing or inverse modelling.…”

“…without appropriate data processing or inverse modelling. Advances in data filtering and inversion schemes, such as EM4Soil (EMTOMO, 2013), Aarhus Workbench (Christiansen et al, 2016) or FEMIC (Elwaseif et al, 2017), permit more accurate modelling of subsurface conductivity structure and may lead to more reliable subsurface characterisation using FD-EMI Furthermore, temporally distributed FD-EMI surveys similar Robinson et al (2012), Shanahan et al, (2015) Huang et al, (2017) could prove useful in GW-SW interface characterisation. For instance, FD-EMI instruments could be used to investigate diurnal dynamics of salt water wedges in coastal environments or seasonal changes in GW upwelling, provided there are substantial contrasts in the electrical conductivity of GW and SW.…”

Geomorphological setting of a sacred landscape: Iron age post battle deposition of human remains at Alken Enge, Denmark

“…(a) Elevation map of Alken Enge and depth curves in Lake Mossø. (b) Enlarged map of main excavations with estimated extent of subaqueous spit platforms (partly based on data from Christensen et al ()) at the time of deposition of bones in first century A.D., location of human bones and position of a late Iron Age bridge construction. [Color figure can be viewed at http://wileyonlinelibrary.com]…”

Assessment of Managed Aquifer Recharge Sites Using a New Geophysical Imaging Method

“…§Modern FDEM instruments and ground conductivity meters can be efficient and capable of fast coverage for shallow (<10‐m) investigations (Christiansen et al, 2016). …”