Frequency domain electromagnetic methods for coastal archaeology – a new(ish) approach for the detection of ancient settlements

As sea levels rose since the end of Last Glacial Maximum (LGM) ancient coastal communities were often forced to abandon their settlements and move inland. Today, many of these abandoned sites are covered by sand or lay in shallow water. Examining these can shed light on past coastal communities as well as settlement patterns in ancient times. Archaeological excavation along the coast is particularly tricky and often sporadic in nature. Thus, high‐resolution shallow geophysical methods, which have become a standard in archaeological studies since they provide a noninvasive way of imaging the subsurface before an excavation, would seem like a perfect solution. However, most methods are limited in their ability to work near the shoreline – the transitional zone between classical land-based methods and standard marine ones. Ground penetrating radar (GPR), for example, is greatly affected by moisture and salinity and is therefore limited in its ability to work in areas saturated with seawater. Seismic reflection is time consuming to overcome issues of poor vertical and spatial resolution and sensitive to urban noise, while magnetics would provide poor results for sand covered sandstone. Other techniques, such as electrical resistivity tomography (ERT) have been shown to work in coastal areas and in shallow water. However, this method can be slow, as it involves setting up complex arrays for each cross section measured. This study will present the frequency domain electromagnetic (FDEM) method, which has the potential to overcome these problems and can bridge the gap in knowledge by measuring in the nearshore environment. The ease of use and quick scanning capability means that large areas can be covered in a relatively short time. There are no electrodes or loops to set up. Since it measures swaths, results are obtained in map-view and not cross-section, with little interpolation. Different frequencies penetrate to different depths (lower frequencies corresponding to deeper penetration). Therefore, the result is a series of frequency maps corresponding to the integration of all subsurface data in a specific sampled volume (i.e. down to the frequency-related depths), providing important information on shallow subsurface properties. The use of multiple frequencies allows for the resolving of internal structures within the depth range. Overall, the FDEM method has proven to be a valuable tool for studying coastal archaeology, and it is likely to continue to play an important role in the field in the coming years. Its ability to detect buried objects and structures and to study the geomorphology of submerged landscapes makes it an essential tool for researchers working in this field.