Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) are two nearsurface
geophysical methods that are well suited to mapping air-filled cavities (e.g. burrows) in
the shallow subsurface. They both have the advantage that they are non-intrusive, so can
image the subsurface targets without disturbing them, which is advantageous for detecting,
characterising, and monitoring animal burrows.
GPR is a very rapid technique that can survey large areas relatively quickly; however, flood
embankments often have a clay component that can limit the depth of investigation of GPR
surveys, with tunnels beyond the penetration depth of the instrument. ERT surveys are typically
slower but are not limited by clay-rich ground. Both techniques are affected by metal objects in
the ground, such as sheet piling and wire mesh, which are installed at the stie to prevent animal
burrowing.
A desk study of badger sett morphology found that:
• Badgers rarely dig beyond 10 m from an entrance
• The tunnels are 30 cm wide and 20 cm tall
• Tunnels rarely go deeper than 2 m below the surface.
At the badger sett north of Drax power station, entrances were found within an area extending
along 75 m of the flood embankment. The location of the sheet piling was confirmed using the
GPR, and all badger tunnel entrances were found to be within this reinforced section of the
embankment. However, the entrances at the site's eastern end were very close to the end of the
sheet piling, making this area the focus of the geophysical survey.
In summary, the results of the geophysical survey (Figure 5) are as follows:
• Two areas of extensive tunnelling were found in the GPR data, matching tunnel
entrances' locations.
• GPR could only detect tunnels in the first three lines, beyond which the tunnels likely
continued but were over 1.5 m deep, deeper than the penetration depth of the GPR
signal.
• Most of the tunnelling seems to be concentrated in the floodplain and embankment toe.
• ERT focused on the eastern area but was partially affected by sheet piles, especially
the lines perpendicular to the embankment.
• Tunnels interpreted from the ERT data head towards the crest of the embankment close
to where the sheet pilling ends.
• We are unsure if we imaged the full extent of the tunnels or if they continued in to the
embankment, but they were beyond the detectability of the instruments at these depths.
Furthermore, the proximity of sheet piling at these locations caused significant noise in
the data.
This report concludes that ERT and GPR could successfully image areas of tunnelling in
the embankment northwest of Drax power station. Useable data could be collected despite
the sheet piling, which makes interpretation of the data significantly more difficult. While
GPR could only find tunnels in the first three lines, it provided some confidence to the ERT
interpretation and was able to investigate a much larger area. This highlights the potential
benefits of combining ERT and GPR for future badger investigations