Detection of cavities in levees caused by the muskrat and other mammals by use of geophysical methods

A wide range of wildlife has their habitat around and within levees or other types of embankment structures. In the Netherlands the burrowing of the muskrat causes internal and external erosion, altering the geometry of the earthen structure. As often century-old embankments protect the densely populated land against the water, the burrowing can form a severe thread to the public safety. At present, the problem is managed by placing lethal and non-lethal capturing techniques in and around burrow founds by visual inspection. Although the population of the muskrat has been under control since 2004, a growing opposition of the media and public to the current control policy gives potential to a different way of monitoring embankments for animal cavities. This research focuses on a geophysical approach to detect muskrat burrows and to visualize the spatial extent of the tunnel network. Various methods have been judged on their potential to image the shallow cavities and evaluated on their advantages and disadvantages for field application. As outcome, fieldwork has been conducted with ground penetrating radar using multiple frequencies (250, 500, and 1000 MHz) and electrical resistivity tomography using different array setups (Dipole-Dipole and Wenner). Sites of investigation have been chosen throughout the Netherlands with different sized and shaped embankments and soil constituents. To ground truth the observations, holes have been drilled by hand to confirm several anomalies. This resulted in identification of various animal tunnels, rocks and plastic debris. However many drilled holes did not yield the desired outcome and many anomalous reflections remained unidentified. Moreover some entrances indicated by the muskrat controllers have not been detected by the methods. The 500 MHz antenna showed the highest resolution in the shallow subsurface and has been the best in detecting and mapping the muskrat burrows. The resistivity arrays failed to clearly distinguish any cavity from the surrounding soil. Organizations worldwide have reported similar nuisance activities of wildlife in dams and levee systems. An efficient instrument that reduces the labour-intensive management is highly desired. Optimizing geophysical methods, in particular the ground penetrating radar, for shallow small-sized cavity detection is a future interest.Applied GeophysicsSection Applied Geophysics and PetrophysicsCivil Engineering and Geoscience