# Detection of Preferential Water Flow by Electrical Resistivity Tomography and Self-Potential Method
## 2 Materials and Methods  
In this study, geological and hydrogeological survey techniques were applied in conjunction with geophysical methods to investigate the hydrogeological conditions of the study site. The geophysical techniques were applied with particular focus on preferential flow, with a scope to identify and parametrize the flow pathways in the subsurface. Subsequently a hydrogeological model considering preferential pathways was compiled and validated through observed groundwater heads in piezometer wells. In addition to 11 boreholes conducted during previous research campaigns [39, 40] in the study site, new boreholes 1-8 were drilled (Figure 2a) with a core driller, and samples were stored in boxes. The depth of the drillings was set two meters below the landslide mass-marl interface. The new boreholes Nos 1-8 and the 11 previous borehole records were used for the building of a 3D geological model applying GeoModeller software (Figure 1c,d). Further piezometers were installed in the new boreholes 1-7 (Figure 2a) equipped with barometric sounds measuring groundwater heads at a frequency of 15 min. The measurements of the groundwater heads are available for the time period from 7 March 2018 to 1 April 2019. The measurements were extended over a period of one year to consider seasonal fluctuation of the groundwater table. Injection tests were conducted in boreholes No 3 and 5 (Figure 2). These boreholes were selected because preferential flow was expected there due to a loud noise of flowing water and almost constant observed groundwater heads. Water was pumped into the boreholes at an average rate of 0.8 L/sec; however, no increase of the groundwater table was attained. Two geophysical techniques, ERT and self-potential mapping, were used for exploration of the hydrogeological settings of the study site. The ERT was used for the location of groundwater bodies, while self-potential mapping was selected for identification of groundwater flow in the subsurface. These two geoelecterical methods are complimentary and have been successfully applied for hydrogeophysical exploration [35]. The basic principle of ERT can be explained through four electrode measurements; namely, an electrical current is injected into the ground through two electrodes and voltage is measured between the second two [34]. An apparent resistivity of the ground can be calculated from applied current and measured voltage between the electrodes. When changing the arrangement of the electrodes, characteristics such as penetration depth, resolution, accuracy etc. can be varied [37]. An inversion procedure is applied to convert the pseudosection into a true resistivity section for a heterogeneous subsurface [34, 37]. As the charge carrier in most soils is electrolytic groundwater and mineral-grain interfaces, ERT can be effectively applied for the identification of groundwater in the subsurface [24, 34, 35]. In the study site of Desermillen, an ERT survey with 13 profiles was conducted using a LIPMANN 4-point light 10 W device. Using a maximum number of 50 electrodes with 2 m electrode spacing and a maximum profile length of 100 m allowed the attainment of approximately 15 m penetration depth. However, the length and number of electrodes varied because blocks of rock or intense vegetation crossed the way. The position of the first profile's start and end points was georeferenced using a GNSS receiver (HiPer HR Topcon Deutschland Positioning GmbH, Hamburg, Germany). Horizontal accuracy of the GNSS measurement did not exceed 1 m during the fieldwork. A further 12 additional profiles were aligned parallel to the first profile using measuring tape, wooden sticks, and wires. The ERT profiles were orientated perpendicularly to the expected direction of groundwater flow, and 4 m spacing was set between the profiles. The surveys were conducted using Wenner electrode allocation due to a high signal-to-noise ratio [37]. The ZondRes3D software was used for creating a joint 3D ERT model from 13 profiles.