Monitoring the hydrologic behaviour of a mountain slope via time‐lapse electrical resistivity tomography

Abstract

Catchment and hillslope hydrology is a major research area in geoscience and the understanding of its underlying processes is still poor. Direct investigation of steep hillslopes via drilling is often infeasible. In this paper, we present the results of non-invasive time-lapse monitoring of a controlled infiltration test at a site in the Italian Central Alps. The hillslope considered is steep (30-35 degrees), covered with grass and a soil layer 1-1.5 m thick above a variably fractured metamorphic bedrock. The key hydrologic question is whether rainfall infiltrates mainly into the underlying fractured bedrock, thus recharging a deeper hydraulic system, or flows in the soil layer as interflow towards the stream channel a few hundred metres downhill. In order to respond to this question, we applied 2200 mm of artificial rain on a 2 m x 2 m slope box over about 18 hours. We estimated the effective infiltration by subtracting the measured runoff (7% of total). Due to the limited irrigation time and the climate conditions, the evapotranspiration was considered as negligible. The soil moisture variation and the underlying bedrock were monitored via a combination of electrical resistivity tomography (ERT), TDR probes and tensiometers. A small-scale 3D cross-hole ERT experiment was performed using 2 m deep boreholes purposely drilled and completed with electrodes in the irrigated plot. A larger scale (35 m long) 2D surface ERT survey was also continuously acquired across the irrigated area. Monitoring continued up to 10 days after the experiment. As a result, we observed a very fast vertical infiltration through the soil cover, also favoured by preferential flow patterns, immediately followed by infiltration into the fractured bedrock. The surface layer showed a fast recovery of initial moisture condition nearly completed in the first 12 hours after the end of irrigation. The lateral transmission of infiltrating water and runoff were negligible as compared to the vertical infiltration. These experiment results confirm that the fractured bedrock has a key role in controlling the fast hydrological dynamics of the small catchment system under study. We concluded that deep water circulation is the key pathway to hillslope processes at this site

Monitoring hillslope moisture dynamics with surface ERT for enhancing spatial significance of hydrometric point measurements

“…A combination improves the spatial understanding of the ongoing hydrological processes and is more suitable for the identification of heterogeneities. Cassiani et al (2009) pointed out that a combination of geophysical and hydrometric data may be used for quantitative estimation of hillslope moisture conditions. Our study has shown that this may also be applied to mid-latitude hillslopes covered by periglacial slope deposits.…”

“…Besides hydrogeophysical methods such as electromagnetics (EM) (Popp et al, 2013;Robinson et al, 2012;Trompvan Meerveld and McDonnell, 2009), time-lapse ERT have been frequently applied to hillslope investigation in the runoff and interflow (Uhlenbrook et al, 2008;Cassiani et al, 2009) or preferential flow context (Leslie and Heinse, 2013).…”

Time-lapse monitoring of fluid-induced geophysical property variations within an unstable earthwork using P-wave refraction

“…Electrical resistivity tomography (ERT) (Loke and Barker, 1996), for example, has, over recent years, been used for near-surface time-lapse imaging of natural slopes and landslides (e.g., Miller et al, 2008;Uhlenbrook et al, 2008;Cassiani et al, 2009) or, more relevant to the topic of the present work, monitoring the internal condition and temporal dynamics of geotechnical assets (e.g., Sjodahl et al, 2008, Wilkinson et al, 2011Chambers et al, 2014;Gunn et al, 2015a). Favored by the sensitivity of resistivity data to changing hydrogeological conditions, ERT, however, requires an additional stage of calibration, often site dependent, to invert for parameters more relevant to the mechanical description of the investigated target (e.g., moisture content, as in Cassiani et al, 2009;Chambers et al, 2014). Seismic geophysical methods, which enable seismic velocity models of the subsurface to be reconstructed, are more directly related to the mechanical properties of the subsurface.…”

Geophysical-Geotechnical Sensor Networks for Landslide Monitoring

“…Cassiani et al, 2009). The aim of this testing is to establish resistivity-moisture content, and moisture content-pore pressure relationships for WMF and SSF.…”