1The site of Bodøsjøen (Bodø Municipality in northern Norway) is close to a medieval church and associated manor farm, and historical aerial photos reveal the presence of at least several circular and long mounds of significant size, making it an elite site from the later Iron Age (550-1030 CE) (Holberg & Hutchinson, 2009). Pending major urban development in the area, a large-scale ground-penetrating radar survey was conducted in November 2019, covering an area of almost 10 hectares. The investigation was performed by researchers from the research group TEMAR–Terrestrial, Marine and Aerial Remote Sensing at the Department of Archaeology and Cultural History, NTNU University Museum in Trondheim, on behalf of Nordland County Council for Bodø Municipality.
2The objective of the survey was to contribute additional data on the cultural heritage of the area and to provide the municipality with better background knowledge for cultural heritage management decisions in Bodøsjøen in the future (Stamnes & Kiersnowski, 2020). In this presentation, however, the goal is to showcase how the geophysical survey results can be integrated into a larger cultural historical context from a research perspective.
3The project involved combining aerial photos and archival sources with the newly acquired geophysical survey data. For this particular site, this involved investigating the entire field of 9.67 hectares (96 700 m²) with the Ground Penetrating Radar (GPR) method using a 3D-radar Mark IV step-frequency GPR (see Fig. 1), ensuring total coverage of the area. This particular GPR system provides an ideal combination of depth versus frequency response and resolution at each specific depth and location. The antenna used was the ground-coupled DXG1820 system, giving with a crossline and inline spacing of 7.5 cm and of 1.5 m for each pass with the system, and data in the frequency range of 200–3000 Mhz. The driving speed achieved was up to 10 km/h. Positioning was ensured with a GNSS system with RTK correction over the cell-phone network. The data was collected in the course of three long days of field-work over a frozen field with approximately 5-10 cm of snow. The dataset was processed and interpreted in the 3D-radar Examiner software version 2.97. The data processing involved interference suppression, Inverse Selective Discrete Fourier Transform (ISDFT), background filtering by High Pass Filtering, and a sliding window mean, Range Gain, Time Domain Migration and Thick Slicing to ≈ 3 cm thick slices. This software is native to this particular system, and makes it possible to combine all processed data in one large, full-coverage data block called a “stitch”. This makes it possible to instantly double-click on any interesting reflection and investigate it both in X, Y and Z-direction as well as at any particular depth, and easily switch between migrated and unmigrated data in both plan and section view by the touch of a button in the software. This was a huge asset in the interpretive work, translating the geophysical observations to cultural historical data to be integrated into the analysis.
Figure 1. Data collection at Bodøsjøen with the 3D-radar Ground Coupled DXG antenna and RTK GNSS system behind a 4x4 UTV.
4The following are the results of the analysis of this dataset (see Fig. 2):
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Ring ditches from at least eight Iron Age burial mounds, as well as seven boat/house-shaped long mounds. One of these is around what might be a central cairn. Several are very large, with the largest being 32 m in diameter, having a 7.5 m ring ditch preserved down to about 1 m depth (see Fig. 3).
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At least 32 oval ditches spread over a larger part of the field. Most of these are interpreted as ditches surrounding possible house foundations. Two of them are probably prehistoric burial mounds.
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At least 1257 pits of varying sizes, where 293 of these are concentrated in a relatively small quarter at the centre of the investigated area.
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A larger dug feature interpreted as the possible remains of the first hospital in the county, built in the early 19th century, which is known to be located within this field.
Figure 2. Plan, inline and crossline sections showing ring ditches surrounding the monumental burial mound at the site. Two of the oval ditches are visible as strong semi-oval reflections at the bottom right of the image. Data as presented in the Examiner software.
Figure 3. Map of the site at Bodøsjøen presenting archaeologically interpreted data.
5The geophysical data revealed a plethora of different geophysical responses. The burial site itself showed up well, and revealed additional circular and boat-shaped burial mounds not observed as cropmarks in extant aerial imagery. Before this investigation and the aerial photo only three boat-shaped mounds were known at Bodøsjøen. That number has now increased to seven. Also, the investigation provided exact dimensions of the mounds and details of their state of preservation as well as possible burial pits within the features. The huge concentration of what is interpreted as pits has a similar geophysical expression as postholes and cooking pits observed at other sites investigated with the same equipment elsewhere in Norway.
6The most surprising observation at this particular site are the 32 oval ditches, marked in blue in Fig. 3. They are all of similar size and orientation, with the short end angled south-west and towards the sea and the dominating wind direction –measuring 5-11 m in length (most typically 7-9 m) and 3.4-5.5 m in width. The shape and form is unknown from other archaeological sources, and can be interpreted in several ways: temporary settlements, burial mounds or remnants of platforms for drying hay. The settlement-interpretation is considered as the most probable one and can be an indication of a cultural–historical function as either an assembly site and/or a site for seasonal trading and exchange. While additional archaeological investigations have not been conducted by this date following the geophysical survey, the clarity and contrast of the geophysical responses interpreted as archaeological are convincing as such. Questions regarding definitive age, usage and purpose of the oval ditches remain open, as does the exact makeup of the possible settlement observed as a concentration of pit-like geophysical responses.
7Whichever interpretation is assumed as the most probable, the presence of oval ditches, the whole burial ground, in combination with the presence of the known medieval church and larger manor, indicates a seat of power and wealth over a long period of time. This makes Bodøsjøen as one of the major farms in the region, and one of the most important Viking-age localities in the region.