The reliability of muography applied in the detection of the animal burrows within River Levees validated by means of geophysical techniques

Animal burrows in a river's earthen levee leads to water piping phenomena causing structural damage and eventual collapse during floods. Currently, the state of the art comprises case studies that deal with management and maintenance, while very few documents attempt at assessing possible animal-induced failure mechanisms. For the latter, detection and characterisation of the animal burrows is crucial and Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar are the most employed geophysical techniques. Between 2017 and 2018 a team of physicists, engineering geologists, and geophysicists has for the first time probed the possibility of exploiting the Muon Transmission Radiography (MTR) to verify the internal conservation status of levees that were visibly damaged by animal activities. The technique is a non-invasive method, currently under development, based on the detection of muons, a highly penetrating component of atmospheric cosmic rays. MTR is capable of providing angular maps of the average density of the material present in front of the detector. A test site measurement campaign was carried out with a prototype instrument placed at the side of the levee. This new survey methodology was compared to a more traditional ERT measurement, performed with a pole-dipole and dipole-dipole configuration. Moreover, the actual burrows' distribution was mapped during the demolition works using Terrestrial Laser Scanner measurements to validate and constrain results. The comparison between ERT and MTR maps shows that, in spite of some limitations, the latter is a suitable and promising technique that could successfully complement a program of geological risk assessment.