Large-scale ERT surveys for investigating shallow regolith properties and architecture

Abstract

Abstract. Within the Critical Zone, regolith plays a key role in the fundamental hydrological function of water collection, storage, mixing and release. Electrical Resistivity Tomography (ERT) is recognized as a remarkable tool for characterizing the geometry and properties of the regolith, overcoming limitations inherent to conventional borehole-based investigations. However, ERT measurements with a high vertical resolution remain restricted to shallow depths, essentially due to the requirement of small electrode spacing increments (ESI). Under these circumstances, the use of ERT measurements for large horizontal surveys remains cumbersome and time-consuming. Here we focus on the need to optimize the ESI parameter in order to adequately characterize the subsurface fabric. We use a set of synthetic three-layered soil–saprock/saprolite–bedrock models in combination with a field dataset. We demonstrate that oversized ESI can significantly affect our perception of shallow subsurface structures by missing important layers and increasing the ill-posed inverse problem effects. More precisely, we document how a thin surficial layer can influence inverted ERT results and cause a resistivity bias, both at the surface and at deeper horizons. To overcome this limitation, we propose adding interpolated levels of surficial apparent resistivity based on a limited number of ERT profiles with small ESI. We demonstrate that our protocol significantly improves the accuracy of ERT profiles based on large ESI. Our protocol is time and cost efficient – especially in the case of large-scale ERT surveys.

Saturated areas through the lens: 2. Spatio‐temporal variability of streamflow generation and its relationship with surface saturation

“…A more stable percentage of contribution seemed to be associated to the reaches located in the middle and west part of the stream (i.e., reaches M3, M2, M1, and L1), in contrast to the reaches located in the east (R2 and R2) and lower part (S2) of the stream. As previously mentioned, the investigations of Gourdol et al () employing soil drilling and electrical resistivity tomography revealed some heterogeneities in the subsurface structure of the Weierbach catchment. In particular, they have shown that the northern and western part of the catchment is characterized by overall thinner solum (i.e., “true soil,” where pedogenic processes are dominant; cf.…”

“…In particular, they have shown that the northern and western part of the catchment is characterized by overall thinner solum (i.e., “true soil,” where pedogenic processes are dominant; cf. Gourdol et al, ) and shallower hard bedrock compared with the eastern portion of the catchment. This may determine differences in the way different sides of the catchment deliver water to the stream.…”

“…Figure 4c). Spring location and the delivery of water from the hillslopes to the streams are likely substantially controlled by bedrock characteristics as schists/slate weathering degree, fractures' size and orientation within the catchment (Gourdol et al, 2018;Scaini et al, 2018), and/or presence of faults (Shaw, 2016;Whiting & Godsey, 2016). The aforementioned bedrock characteristics have been shown to be variable within the Weierbach catchment (Gourdol et al, 2018), representing a substantial source of variability that can be hardly disentangled.…”

“…The climate is semioceanic, with an annual average precipitation of 918 mm (2011)(2012)(2013)(2014)(2015)(2016)(2017). Detailed information about the soil and bedrock characteristics and seasonal hydrological response is reported in the companion paper by Antonelli et al in review, as well as in several other studies (Gourdol, Clément, Juilleret, Pfister, & Hissler, 2018;Martínez-Carreras et al, 2016;Wrede et al, 2014). Here, we briefly summarize the information from the first manuscript about the riparian zone and the seven distinct areas within the catchment where we monitored riparian surface saturation ( Figure 1).…”

Reply to Editor (Access review)

“…Those algorithms can lead to inverted ERT images whose resolution is superior or equal, respectively with the same or fewer number of measurements, to those using standard survey designs (for example, Wenner-Schlumberger or dipole-dipole (eg. Stummer et al, 2004;Furman et al, 2004Furman et al, , 2007Wilkinson et al, 2006Wilkinson et al, , 2012Loke et al, 2014;Abdullah et al, 2018;Uhleman et al, 2018)). Thus, in the scope of large-scale ERT C1 surveys, such optimized non-conventional electrode arrays could help reducing the operational measurement time without reducing the information content.…”