# Detection of Preferential Water Flow by Electrical Resistivity Tomography and Self-Potential Method
## 1 Introduction
Without discrete fracture networks, as discussed before, highly permeable interlayers, subsurface layer topography, or animal burrows can serve as preferential flowpaths as well [10; 11]. The integrated use of geophysical and subsurface monitoring techniques increases the accuracy of geophysical surveys for application in different scales and geological settings. The combined use of ERT and active seismics allowed for monitoring of water flow in the vadose zone with very high spatial and temporal resolution [28]. The effect of groundwater flow in fractured media on electrical resistivity distribution and streaming potential signals has been modeled numerically [24; 26]. For this research, we have selected ERT and self-potential techniques that provide a physical link both with water content and water flow in the subsurface [24; 26; 28]. A self-potential geophysical method using non-polarizing electrodes has been employed for measuring the streaming potential associated with preferential flow paths [18; 29; 30; 31; 32; 33]. Contrary to ERT, when an external source of current is applied, the self-potential method measures electrical charges developing naturally in the subsurface [34; 35; 36]. There are various sources of self-potentials. Redox potentials cause the most pronounced self-potential anomalies up to 1500 mV [37]. Anomalies caused by streaming potentials have lower magnitude, and they are generated by electrolytic groundwater flowing in porous or fractured media [35; 36]. Water in the space between mineral grains is never neutral, as there is an excess of charge in pore water to balance the charge on the surface of silicate and aluminosilicate mineral particles [35]. The presence of the excess charges in pore water causes the phenomenon of streaming potential when the charges are dragged by flowing groundwater. As a consequence, in unconfined aquifers, positive self-potential anomalies can be observed in the direction of the preferential flow [18; 35]. The literature review shows that preferential flow has an importance in many fields because it can radically alter the velocity and rate of groundwater flow. The newesthydrogeological models are able to consider various kinds of preferential flow mechanisms in a groundwater simulation. However, advanced investigation and testing in the field site is a prerequisite for the adequate conceptualization and parametrization of the preferential flowpaths in numerical models. This study was conducted in the study site of Deisermillen (Luxembourg), Upper Mosel region (Figure 1a), where in the autumn of 1964, a dangerous landslide occurred [38,39]. The typical morphological features of the landslide such as hummocky topography can be seen in a high-resolution digital elevation model (Figure 1b). In 1965, several drillings were done under the supervision of the State Geological Survey [39] in order to investigate the possible causes of the Deisermillen landslide. In 1974, additional fieldworks were done to conduct remediation measures in the landslide's area [40]. From previous research campaigns, 11 borehole records in all are available in the Archive of Geological Survey of Luxembourg [39,40]. In 2018-2019, an extensive field research program was conducted including 8 new geological drillings, hydrogeological monitoring and testing, and geophysical exploration to investigate the lithological and hydrogeological setting of the landslide's area. All of the borehole records from previous and actual studies were used for the geological characterization of the study area and the building of a 3D geological model (Figure 1c,d). The uppermost lithological sequence in the study area is the dolomites of the Upper Muschelkalk (mo). This formation appears as a steeply inclined cliff in the western part of the site (Figure 1b-d). Figure 1: _Cont_. This formation is underlain by a 70 m thick Middle Muschelkalk (mm) formation composed of gypysiferous marks.