Detection of soil pipe network by geophysical approach : electromagnetic induction (EMI) and electrical resistivity tomography (ERT)

Studying soil pipes is a methodological challenge that needs improvement in detection methods in order to better recognize the role of piping erosion in land degradation and hillslope hydrology. This study explores electromagnetic induction (EMI) and electrical resistivity tomography (ERT) in order to identify soil pipes. The study was conducted in a mountainous area (the Bieszczady Mts., SE Poland) under a temperate climate, where pipes develop in silty-clayey soils. In the plot area, eight profiles were measured by the conductivity meter at different depths, and then interpolated to present apparent electrical conductivity ($EC_{a}$). Also, six ERT profiles were carried out using the Wenner-Schlumberger electrode configuration. The $EC_{a}$ values measured by EMI are not very diversified, suggesting its lower sensitivity to changes in the $EC_{a}$, whereas the $EC_{a}$ values measured by ERT are characterized by greater fluctuation, i.e., better detection possibilities. ERT has revealed soil pipes as zones of higher electrical resistivity (ER >268 Ωm) than their surroundings (characterized below pipes by ER 427 $\Omega$m), whereas EMI has revealed its higher sensitivity to water content. The EMI results have shown the lowering of the water table in the lower part of the slope, perhaps because of the drainage by a complex pipe network. EMI allows quick measurements of $EC_{a}$ providing information on water content, and thus indirectly soil pipes, but, it cannot delineate individual pipes. Only the integration of geophysical methods supported by field recognition provides an effective method to detect soil pipes