The fluvial architecture of buried floodplain sediments of the Weiße Elster River (Germany) revealed by a novel method combination of drill cores with two‐dimensional and spatially resolved geophysical measurements

The complex and non-linear fluvial river dynamics are characterized by repeated periods of fluvial erosion and re-deposition in different parts of the floodplain. Understanding the fluvial architecture (i.e. the three-dimensional arrangement and genetic interconnectedness of different sediment types) is therefore fundamental to obtain well-based information about controlling factors. However, investigating the fluvial architecture in buried floodplain deposits without natural exposures is challenging. We studied the fluvial architecture of the middle Weiße Elster floodplain in Central Germany, an extraordinary long-standing archive of Holocene flooding and landscape changes in sensitive loess-covered Central European landscapes. We applied a novel systematic approach by coupling two-dimensional transects of electrical resistivity tomography (ERT) measurements and closely spaced core drillings with spatially resolved measurements of electromagnetic induction (EMI) of larger floodplain areas at three study sites. This allowed for (i) time and cost-efficient core drillings based on preceding ERT measurements and (ii) spatially scaling up the main elements of the fluvial architecture, such as the distribution of thick silt-clay overbank deposits and paleochannel patterns from the floodplain transects to larger surrounding areas. We found that fine-grained sand and silt-clay overbank deposits overlying basal gravels were deposited during several periods of intensive flooding. Those were separated from each other by periods of reduced flooding, allowing soil formation. However, the overbank deposits were severely laterally eroded before and during each sedimentation period. This was probably linked with pronounced meandering or even braiding of the river. Our preliminary chronological classification suggests that first fine-grained sedimentation must have occurred during the Early to Middle Holocene, and the last phase of lateral erosion and sedimentation during the Little Ice Age. Our study demonstrates the high archive potential of the buried fluvial sediments of the middle Weiße Elster floodplain and provides a promising time and cost-effective approach for future studies of buried floodplain sediments

Geophysics conquering new territories: The rise of “agrogeophysics”

Agriculture is facing immense challenges. We have to produce enough food while safe-guarding the environment for future generations. This results in the need to use less water and fertilizer, and to harness soil quality. Key to achieving this goal is improving the understanding of processes and interactions governing the soil–plant–atmosphere continuum of agricultural ecosystems. Geophysical tools have great potential to better characterize and quantify these processes noninvasively from the plot to landscape scale. Nevertheless, a number of challenges remain for geophysical results to be better exploited by different scientific communities and by decision-makers. In this special section, we explore ongoing research in the relatively new field of agrogeophysics, and we provide an overview of potential applications and highlight future research needs

DC Electrical Resistivity Imaging at a High-Arctic Continuous Permafrost Site in Svalbard, Norway

Direct-Current (DC) electrical resistivity imaging has proven to be a suitable technique for a number of permafrost related questions. We present measurements from a high-arctic continuous, maritime permafrost site near Ny Alesund, Svalbard (Norway). The area under investigation features a great diversity of soil types and soil water contents. The surface is characterized by sparse vegetation alternating with rock fields and exposed soil.We investigated 25 different transects each of 47.5m length using a DC-Resistivity and Electrode Control System (RESECS) with 96 electrodes at a spacing of 0.5m in Wenner-alpha configuration. At three transects, fixed electrode arrays were installed and measured on a weekly basis in order to capture temporal changes. The study started in August, just before the active layer reached its maximum thickness, and extended until the beginning of freeze-up in mid-September.The specific resistivities at the surface ranged from less than 50$\Omega$m in areas with damp clay to more than 1000$\Omega$m in rock fields and on dry hill crests. In most cases, areas with such high resistivities only extended to depths of less than 1 m. From depths between 1m and 1.5m onwards, specific resistivities increased continuously, indicating the position of the freeze-thaw interface. This is in general agreement with thaw depths that were determined by point measurements along individual transects using a drill.The repeated measurements of the fixed electrode arrays displayed the most pronounced changes in the beginning of August, where up to a 50\% decrease in specific resistivity over a period of two weeks was measured at depths below 1 to 2m. This is interpreted to be the seasonal thawing of the active layer. A subsequent increase in specific resistivities at these depths until mid-September corresponding to the refreezing of the soil was only observed in some areas, which suggests spatial variations in the course of the refreezing process

The Münsterdorf sinkhole cluster: void origin and mechanical failure

Since 2004, collapse sinkholes occur on the sports field of Münsterdorf, a village north of Hamburg in Germany. The sinkholes, around 2–5 m in diameter and 3–5 m deep, develop in peri-glacial sand, which at around 20 m depth is underlain by Cretaceous chalk. The chalk has been pushed up close to the surface by a salt diapir. The sinkhole formation initiated suddenly and occurs with a frequency of one every 2 years. We use a variety of geophysical results (e.g. gravity, electrical resistivity imaging, ground-penetrating radar) from previous fieldwork campaigns together with a new data set from direct-push-based methods to infer mechanical and hydrological properties of the material beneath the sports field (peri-glacial sand, glacial marl, Cretaceous chalk). Based on the derived material properties, we develop a mechanical model for the sinkhole collapse, starting from simple analytical considerations and then moving towards a three-dimensional distinct-element model explaining the sudden onset of collapse sinkholes for the sports field. The mechanical model supports our hypothesis that the sudden onset of sinkholes is triggered by changes in groundwater level.</p

DC Electrical Resistivity Imaging at a High-Arctic Continuous Permafrost in Svalbard, Norway

Direct-Current (DC) electrical resistivity imaging has proven to be a suitable technique for a number of permafrost related questions. We present measurements from a high-arctic continuous, maritime permafrost site near Ny Alesund, Svalbard (Norway). The area under investigation features a great diversity of soil types and soil water contents. Sparse vegetation alternating with rock fields and exposed soil characterize the surface.A total of 25 different transects each of 47.5m length were investigated using a DC-Resistivity and Electrode Control System (RESECS) with 96 electrodes at a spacing of 0.5m in Wenner-alpha configuration. At three transects, fixed electrode arrays were installed and measured on a weekly basis in order to capture temporal changes. The study was conducted from August until mid-September, thus covering the period of maximum active layer thickness and the beginning of freeze-up.The specific resistivities at the surface ranged from less than 50 Ohm m in areas with damp clay to more than 1000 Ohm m in rock fields and on dry hill crests. In most cases, areas with such high resistivities only extended to depths of less than 1 m. From depths between 1m and 1.5m onwards, specific resistivities increased continuously, indicating the position of the freeze-thaw interface. This agrees well with thaw depths that were determined by point measurements along individual transects using a drill.The repeated measurements of the fixed electrode arrays displayed the most pronounced changes in the beginning of August, where decreases in specific resistivities of up to 40% over one week period were detected at depths between 1m and 2m. Afterwards, only insignificant changes were observed at these depths. This is interpreted to be the seasonal thawing of the active layer, which stagnates during in the second half of August. At depths less than 1m, both decreases and increases in specific resistivities were detected, most likely due to changes in the water content of the soil

High resolution monitoring above and below the groundwater table uncovers small-scale hydrochemical gradients.

Hydrochemical solute concentrations in the shallow subsurface can be spatially highly variable within small scales, particularly at interfaces. However, most monitoring systems fail to capture these small scale variations. Within this study, we developed a high resolution multilevel well (HR-MLW) with which we monitored water across the interface of the unsaturated and saturated zone with a vertical resolution of 0.05-0.5 m. We installed three of these 4 m deep HR-MLWs in the riparian zone of a third-order river and analyzed for hydrochemical parameters and stable water isotopes. The results showed three distinct vertical zones (unsaturated zone, upper saturated zone, lower saturated zone) within the alluvial aquifer. A 2 m thick layer influenced by river water (upper saturated zone) was not captured by existing monitoring wells with higher screen length. Hydrochemical data (isotopes, total ions) were consistent in all HR-MLWs and showed similar variation over time emphasizing the reliability of the installed monitoring system. Further, the depths zones were also reflected in the NO3-N concentrations; with high spatial variabilities between the three wells. The zonation was constant over time, with seasonal variability in the upper saturated zone due to the influence of river water. This study highlights the use of high resolution monitoring for identifying the spatial and temporal variability of hydrochemical parameters present in many aquifer systems. Possible applications range from riparian zones, agricultural field sites to contaminated site studies, wherever an improved understanding of biogeochemical turnover processes is necessary

Digital soil mapping: Approaches to integrate sensing techniques to the prediction of key soil properties

The guest editors summarize the special section, Digital Soil Mapping, beginning with a look at the challenges that are pushing advances in soil mapping, and then introducing the contributors’ efforts at improving current approache