A helicopter-borne electromagnetic survey of a former open-cast mining area was conducted in Lusatia (Germany) and combined with groundwater level maps and borehole profiles to predict subsurface conditions in terms of confining and permeable characteristics with a Random Forest (RF) model. The lignite mining region, located around 100 km south of Berlin, faces multiple hydro(geo)logical challenges connected to the former extensive mining, ranging from acid mine drainage to changing groundwater flow fields. Due to the extensive exploration, a large amount of borehole records was accessible around the former mining pits making the location valuable for a data-driven investigation. After translating borehole records into confining and permeable classes, we extracted the target variable using a discretization and a non-discretization approach. To account for data bias in the target variable, we included several sampling methods. Furthermore, we evaluated the variations of predictor values derived from different source models on the RF-model performance. The predictions were visually compared to independent external geological data. The best results were obtained using the oversampling method with a discretization approach of the borehole data. The results show a good level of consistency with previous interpretations of the subsurface. Larger Quaternary lacustrine deposits were successfully detected. However, the reliability of the detection process is highly influenced by both the thickness of the deposits to be identified and their depth below the surface. Our trained machine-learning model showed a fast and scalable addition that sheds light on subsurface characterization by detecting large-scale lithological patterns.

The region Lusatia in northeastern Germany, which is located about 100 km south of Berlin, is strongly affected by over a century of both former and on-going opencast lignite mining. Although, there is an abundance of borehole data from former excavation surveys both varying data quality and heterogeneous coverage is a challenge for deriving spatially continuous subsurface properties. To overcome these obstacles we combined airborne geophysical investigations with borehole data. Different machine learning-algorithms (Random Forest and K-means) are used to determine spatially and depth-related insights into the variability of geological and hydrogeological characteristics. An aeroelectromagnetic (AEM) survey was carried out in summer 2021 using BGR’s (German Federal Institute for Geosciences and Natural Resources) helicopter, which covered flight lines of 1680 km in an area of about 200 km². First results show that the machine learning approach can predict fine-grained sediments (clay and silt) in untrained areas and can distinguish between clusters of mining-affected regions and undisturbed ones. The results of the study will be further used to improve the parameterization of existing regional groundwater flow models to address challenges of water allocation in the region of Lusatia.

Extensive pumping, storm floods or sea‐level rise can lead to regional and local‐scale processes of saltwater intrusion into coastal aquifers. Particularly when exacerbated by the impacts of climate change, these processes can threaten freshwater resources and thus drinking water supplies and need to be prevented by investigation and monitoring, for example, with resistivity methods. In this study, we present a multi‐scale resistivity‐based imaging and monitoring concept that integrates regional‐scale airborne electromagnetic data, local‐scale surface electrical resistivity tomography data and borehole‐scale data from permanently installed electrode chains for resolving spatio‐temporal processes of groundwater salinization. Based on two field cases in Northern Germany, it is argued that comprehensive multi‐scale geophysical datasets are required for understanding the complex spatio‐temporal dynamics of groundwater salinization. Particularly long‐term monitoring data are essential for resolving groundwater salinization processes and for differentiating long‐term changes from seasonal effects. The approach complements traditional groundwater monitoring and yields crucial data for the development of density‐driven groundwater flow models to support sustainable coastal aquifer management. If installed at an early stage, such monitoring systems can anticipate threats to sustainable groundwater use and generate valuable time for taking counteractions.

A large contiguous former opencast lignite-mining district is located in Lusatia, Germany, about 100 km south of Berlin. Since this region has been heavily polluted by acid mine drainage from opencast lignite mines, time- and cost-efficient investigation methods are required to obtain comprehensive information on aquifers. In summer 2021, BGR carried out a helicopter-borne electromagnetic (HEM) survey with its helicopter and a RESOLVE system covering an area of about 250 km2. While current surface water levels are derived from in-flight altitude measurements, the spatial resistivity distribution shows indications for the water table in the entire survey area. Apparent resistivities agree very well with water resistivities measured in larger lakes, where the distance to the lakeshores outreaches the system’s footprint. While groundwater sampling requires observation wells, we use HEM results to close the gaps between the sparsely distributed wells. The analysis of groundwater samples shows that groundwater EC correlates with dissolved iron (Fe2+) and sulfate (SO42−) content. HEM resistivities help to approximate groundwater EC, and further Fe2+ and SO42− contents at the observation wells, and to estimate these within the mining area where oxidative pyrite weathering processes dominate other contamination processes.

The Aceh River delta, northern Sumatra, is a subject of interest since the coastline was struck, more than any other, by the 2004 Indian Ocean Tsunami, the largest recorded in human history. Thereafter, significant scientific efforts focused on short-term dynamics to address the environmental effects of the tsunami, but the long-term evolution of the delta in this specific context of volcanic eruptions, megathrust earthquakes and tsunami landing, however, remains to be understood. This study investigates the subaerial delta, based on shallow sediment borehole stratigraphies and C-14 ages, in order to provide a partial reconstruction of the western and eastern fluvial and coastal evolution over the past 7 ky. It also benefits from comprehensive resistivity maps produced during recent helicopter-borne electromagnetic (HEM) surveys, and from earlier geomorphological mapping. By prograding, the delta expanded seaward, exposure to swell increased, and a large strandplain accreted along the eastern delta front from 4 to 1 ky BCE. The delta thus evolved asymmetrically, with higher, tightly-stacked beach ridges in the east, which accreted at similar to 6 km(2)/ky. Meanwhile, the Aceh River remained stable, along the western side of the delta, burying the western strandplain under its floodplain. After 0.5 ky BCE, delta progradation increased to similar to 14 km(2)/ky, generating low-lying and wider spaced beach ridges to the east. A series of river avulsions between 0.2 ky BCE and 1.6 ky CE shifted the river course from the west to the center of the delta. An asymmetric cuspate promontory grew at 23 km(2)/ky after 0.5 ky in front of the current mouth of the river, projecting 1 km offshore of the current coastline, before undergoing erosion in the past few centuries. Here, we discuss which combination of global and local factors, including sea level change, sediment supply, wave climate, tectonics, land use and tsunamis may explain the most salient processes during the growth of the Aceh River delta. Beyond its local interest, this study provides clues for a wider understanding of the complexity of subaerial delta development.

Fluvial and coastal landforms are indicative of landscape river delta evolution over time and provide clues for understanding coastal adjustments to sea-level and fluvial dynamics fluctuations, tectonic displacements, and extreme waves. We have mapped the surface and sub-surface footprints of fluvial and coastal geomorphological features in the Aceh River delta, northern Sumatra, using imagery dataset, vertical facies logging and helicopter electromagnetic surveys. The result is a geomorphological map at the scale of 1:75.000 which outlines the main features of the deltaic plain, including rivers, tidal and buried channels, fluvial levees, beach ridges, swales, tidal flats and lagoons. We compare their spatial distribution to the geometry of buried sediment bodies, revealed by boreholes and resistivity maps. Buried channel belts and floodplain deposits document the former locations of the distributary channels of the Aceh River. Coastal-parallel beach ridges evidence 7–8 km of asymmetric delta progradation since the mid-Holocene sea-level high stand.

Magnetic data can be acquired from a number of different platforms (e.g., ground, drone, helicopter) using a variety of sensors (e.g., caesium vapour-type optically pumped magnetometers, fluxgate, superconducting quantum interference devices) with different flight line configurations. To detect a magnetic anomaly associated with a mineral commodity that is not exposed but is thought to be associated with the anomalous magnetic mineral content, it is necessary to optimize the survey parameters through a complete data integration process. Prior petrophysical measurements provide insight into the physical contrast that might be expected between adjacent lithologic units and between the ore zone and the encompassing lithology. Oriented rock samples provide access to magnetic remanence data through palaeomagnetic laboratory measurements. Knowing the typical morphology of the ore zone one can compute a forward model of the expected anomalous response and determine which combination of survey parameters provides the highest probability of detecting the commodity being sought. In this study, we analyse magnetic patterns associated with thin dipping skarn bodies from the Geyer mining district in Erzgebirge, Germany. Petrophysical measurements indicate that the skarns are more magnetic than the surrounding host rock. Partially oriented samples from a bore core record a Variscan age metamorphic remanence. Forward modelling indicates that clusters of skarn bodies are required to produce a reliably detectable magnetic signal. Ground, or low elevation drone surveys are needed to detect these anomalies with standard scalar-type optically pumped magnetometer or fluxgate magnetic surveys. The enhanced spatial resolution and long-wavelength rejection of a superconducting quantum interference device based full-tensor magnetic gradiometer provide an improvement over optically pumped magnetometers for an aircraft-based survey platform.

River deltas are strongly affected by demographic growth and by the intensification of land use. The migration of deltaic coastlines is often rapid, threatening urban settlements, coastal farming, and coastal biotopes. Some deltas benefit from centuries of monitoring, such that the evolution of their coastline is well documented. For most deltas, however, such long records do not exist. The study of their geomorphological evolution can benefit from overlapping maps drafted over time, combined with aerial photographs and satellite images, to track the evolution of fluvial and coastal landforms. Both fluvial and coastal landforms are sensitive to variations in water and sediment supply, such that covariations in the evolution of these landforms, or the lack thereof, provide clues on the contribution of water and sediment supply to delta evolution. We document the evolution of river channels and coastlines in the delta of the Aceh River in northwest Sumatra, by overlying maps, ortho-rectified aerial photographs, and satellite images covering the past 130 years. We assess the accuracy of the overlays, and then use multivariate statistics to analyze the co-evolution of fluvial and coastal landforms. We propose that a progressive decrease in sediment supply spurred river channel lengthening and narrowing, landward migration of the shoreline, and narrowing of beach ridges. The 2004 Indian Ocean tsunami generated an instantaneous retreat of the coastline that amounts to similar to 53% of the coastal retreat from 1884 to 2019 ce. Post-tsunami evolution is marked by an irreversible acceleration of previous trends. Beach ridges located up-drift of rivers and tidal channel mouths are more sensitive to long-term landward retreat and tsunamigenic erosion.

Aeromagnetic data are routinely acquired by mineral exploration programmes. The objective is to obtain a raster image of the spatial variations of magnetic field intensity; these variations are associated with mineralogical variations in the subsurface. When the survey is conducted in a populated area, much of the signal, however, may be associated with anthropogenic sources such as buildings and roads. Identification and minimization of the anthropogenic-related signal then are essential to derive a useful product for geological mapping. In this work, we examine a scalar magnetic dataset from Geyer, Saxony, and we apply five approaches for locating regions of anomalous anthropogenic signal: signal amplitude, absolute fourth difference, signal standard deviation, enhanced horizontal gradient and curvedness. All are shown to produce similar responses, and the summation of the five results compares favourably with the standard Keating kimberlite (circular anomaly) approach for detecting anthropogenic signals. Complications arise when geological features produce signals of similar amplitude to anthropogenic sources. Differentiating the probable origin of any specific pattern can be assessed by using a 2D shape index and increased flight height. Verification of an anthropogenic anomaly is achieved by comparison of anomalous solution grids with Geographic Information System-based reference data.

Airborne measurements are very useful to cover very large areas. Nine month after the Aceh Tsunami and Earthquake in 2004, BGR (Federal Institute for Geoscience and Natural Resources) conducted a fresh water supply exploration survey within a project called Helicopter Project Aceh (HELP ACEH). The helicopter-borne electromagnetic (HEM) device operates at five frequencies. The HEM can estimate the 1D resistivity models down to a depth of 150 m for the high electrical resistivity areas and 50 m for low electrical resistivity areas. In this paper, the airborne data of 2005 are compared with resistivity data acquired in Banda Aceh basin in 2018. The HEM output consists of 1D resistivity models derived by inversion of the processed data. These 1D resistivity models are compared with the 2D resistivity models derived from ground-based resistivity measurements. However, the 2D models on the ground are transformed into 1D resistivity models so it can be used for comparison. The transformation is conducted by averaging the resistivity values in the each layer, so every layer only has one resistivity value. Both methods are influenced by many factors. For example, resistivity on the ground is affected by local conditions. The airborne measurements are also influenced by objects that are at the surface of the ground. In some cases, the airborne resistivity models have some differences in absolute resistivity values, but they often have the same structural pattern compared with the ground-based resistivity models.

Subterranean estuaries the, subsurface mixing zones of terrestrial groundwater and seawater, substantially influence solute fluxes to the oceans. Solutes brought by groundwater from land and solutes brought from the sea can undergo biogeochemical reactions. These are often mediated by microbes and controlled by reactions with coastal sediments, and determine the composition of fluids discharging from STEs (i.e., submarine groundwater discharge), which may have consequences showing in coastal ecosystems. While at the local scale (meters), processes have been intensively studied, the impact of subterranean estuary processes on solute fluxes to the coastal ocean remains poorly constrained at the regional scale (kilometers). In the present communication, we review the processes that occur in STEs, focusing mainly on fluid flow and biogeochemical transformations of nitrogen, phosphorus, carbon, sulfur and trace metals. We highlight the spatio-temporal dynamics and measurable manifestations of those processes. The objective of this contribution is to provide a perspective on how tracer studies, geophysical methods, remote sensing and hydrogeological modeling could exploit such manifestations to estimate the regional-scale impact of processes in STEs on solute fluxes to the coastal ocean.

A Pleistocene buried valley, the Cuxhaven Valley, located in Northern Germany, is extensively studied by three different electromagnetic methods. In order to enhance the resolution of the very shallow subsurface structures, the radiomagnetotelluric (RMT) measurements were conducted on the buried valley in addition to the helicopter-borne electromagnetic (HEM) and transient electromagnetic (TEM) data, which were available from the study area along the same profile. First, the RMT soundings are interpreted using the conventional 1D and 2D inversion algorithms. The RMT interpreted models inferred two-layer structure of the subsurface up to a depth of 30 m, which comprises a resistive layer at the top. Available lithological data is used for correlating the resistivity values derived from the 2D inversion of the RMT data with the subsurface formation. Subsequently, a 1D weighted joint inversion algorithm is utilized to interpret these three data sets jointly. The 2D interpreted models of RMT soundings are compared with the 1D weighted joint interpreted models of RMT, HEM, and TEM soundings.

Information on chloride (Cl) distribution in aquifers is essential for planning and management of coastal zone groundwater resources as well as for simulation and validation of density-driven groundwater models. We developed a method to derive chloride concentrations from borehole information and helicopter-borne electromagnetic (HEM) data for the coastal aquifer in the Elbe-Weser region where observed chloride and electrical conductivity data reveal that the horizontal distribution of salinity is not uniform and does not correlate with the coastline. The integrated approach uses HEM resistivity data, borehole petrography information, grain size analysis of borehole samples as well as observed chloride and electrical conductivity to estimate Cl distribution. The approch is not straightforward due to the complex nature of the geology where clay and silt are present. Possible errors and uncertainties involved at different steps of the method are discussed.

Zusammenfassung Die Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) führte in den Jahren 2000–2014 umfangreiche (etwa 5900 km 2 ) aerogeophysikalische Erkundungen an der niedersächsischen Nordseeküste durch. Die Aeroelektromagnetik liefert Informationen über die elektrisch leitfähigen Strukturen im Erduntergrund und ermöglicht die Unterscheidung von Süß- und Salzwasser wie auch von Tonen und Sanden. Im Landesamt für Bergbau, Energie und Geologie (LBEG) sowie am Leibniz Institut für Angewandte Geophysik (LIAG) wurden diese Daten zur Kartierung der Tiefenlage der Süß‑/Salzwassergrenze genutzt. Dadurch ist unter anderem eine detaillierte Karte der Grundwasserversalzung innerhalb der küstennahen Aquifere entlang der niedersächsischen Nordseeküste im Maßstab 1:50.000 entstanden. Diese zeigt den aktuellen Stand der Grundwasserversalzung, abgeleitet aus den gewonnenen Elektromagnetik-Modellen. Die Modelle waren auch Grundlage für eine hydraulische Modellierung der Süßwasserlinse der Nordseeinsel Borkum. Darauf aufbauende Simulationen zeigen die Entwicklung der Grundwassersituation für den Zeitraum bis 2100.

Knowledge on peat volumes is essential to estimate carbon stocks accurately and to facilitate appropriate peatland management. This study used airborne electromagnetic and radiometric data to estimate the volume of a bog. Airborne methods provide an alternative to ground-based methods, which are labor intensive and unfeasible to capture large-scale (>10 km2) spatial information. An airborne geophysical survey conducted in 2004 covered large parts of the Ahlen-Falkenberger Moor, an Atlantic peat bog (39 km2) close to the German North Sea coast. The lateral extent of the bog was derived from low radiometric and elevated surface data. The vertical extent resulted from smooth resistivity models derived from 1D inversion of airborne electromagnetic data, in combination with a steepest gradient approach, which indicated the base of the less resistive peat. Relative peat thicknesses were also derived from decreasing radiation over peatlands. The scaling factor (µa = 0.28 m−1) required to transform the exposure rates (negative log-values) to thicknesses was calculated using the electromagnetic results as reference. The mean difference of combined airborne results and peat thicknesses of about 100 boreholes is very small (0.0 ± 1.1 m). Although locally some (5%) deviations (>2 m) from the borehole results do occur, the approach presented here enables fast peat volume mapping of large areas without an imperative necessity of borehole data.

The knowledge of the subsurface down to about one hundred meters is fundamental for a variety of economic, ecological, and geoscientific tasks, particularly in coastal zones. Marine and terrestrial processes influence coastal zones and both seawater intrusion and submarine freshwater discharges may occur. The Federal Institute for Geosciences and Natural Resources (BGR) conducted airborne geophysical surveys in the coastal region of the German Bight between 2000 and 2014. The helicopter-borne system used simultaneously collected electromagnetic (HEM), magnetic (HMG), and radiometric (HRD) data. An area of about 5900 km2 was covered with parallel flight lines at 250 m line separation and additional tie-lines at larger separations. In total, about 25,000 km of data at sampling distances of 4 m (HEM, HMG) and 40 m (HRD) were acquired. The electrical resistivity (HEM), the anomalies of the magnetic field (HMG), and the exposure rate (HRD) are the resulting geophysical parameters derived from the data. The results are displayed as maps of the geophysical parameters as well as vertical resistivity sections (only HEM). Both data and products are publicly available via BGR’s product center. The airborne geophysical results helped to outline the fresh–saline groundwater interface, freshwater lenses on islands, submarine groundwater discharges, buried tunnel valleys, mires, and ancient landscapes.

In the framework of the Deep Electromagnetic Sounding for Mineral EXploration (DESMEX) project, we carried out multiple geophysical surveys from regional to local scales in a former mining area in the state of Thuringia, Germany. We prove the applicability of newly developed semi-airborne electromagnetic (EM) systems for mineral exploration by cross-validating inversion results with those of established airborne and ground-based investigation techniques. In addition, supporting petrophysical and geological information to our geophysical measurements allowed the synthesis of all datasets over multiple scales. An initial regional-scale reconnaissance survey was performed with BGR's standard helicopter-borne geophysical system deployed with frequency-domain electromagnetic (HEM), magnetic and radiometric sensors. In addition to geological considerations, the HEM results served as base-line information for the selection of an optimal location for the intermediate-scale semi-airborne EM experiments. The semi-airborne surveys utilized long grounded transmitters and two independent airborne receiver instruments: induction coil magnetometers and SQUID sensors. Due to the limited investigation depth of the HEM method, local-scale electrical resistivity tomography (ERT) and long-offset transient electromagnetic (LOTEM) measurements were carried out on a reference profile, enabling the validation of inversion results at greater depths. The comparison of all inversion results provided a consistent overall resistivity distribution. It further confirmed that both semi-airborne receiver instruments achieve the bandwidth and sensitivity required for the investigation of the resistivity structure down to 1 km depth and therewith the detection of deeply seated earth resources. A 3D geological model, lithological and geophysical borehole logs as well as petrophysical investigations were integrated to interpret of the geophysical results. Distinct highly-conductive anomalies with resistivities of less than 10 Omega rn were identified as alum shales over all scales. Apart from that, the petrophysical investigations exhibited that correlating geophysical and geological information using only one single parameter, such as the electrical resistivity, is hardly possible. Therefore, we developed a first approach based on clustering methods and self-organizing maps (SOMs) that allowed us to assign geological units at the surface to a given combination of geophysical and petrophysical parameters, obtained on different scales. (C) 2020 The Author(s). Published by Elsevier B.V.

Knowledge on peat volumes of peatlands is essential to estimate carbon stocks accurately and to facilitate appropriate peatland management. This case study uses helicopter-borne electromagnetic and radiometric data to investigate a bog in Germany. Airborne methods provide an alternative to ground-based methods, which are labour intensive and unfeasible to capture large-scale spatial information. One of the airborne surveys BGR conducted at the North Sea coast over the past two decades covers the Ahlen-Falkenberger Moor, an Atlantic peat bog (39 km²) investigated by the Geological Survey of Lower Saxony. This enables comparison of airborne and borehole results. The lateral extent of the bog is derived from low radiometric and elevated surface data. The vertical extent results from smooth resistivity models in combination with a steepest gradient approach as well as from radiometric data. The latter requirs scaling of the relative depth values. Depths derived from electromagnetic data are usable for this scaling due to their similarity to borehole peat depths. The mean difference of the combination of electromagnetic and radiometric depths and the peat depths of about 100 boreholes is very small (-0.08 ± 1.09 m), but may differ significantly (>±2 m) at some points.

Summary Expected results of climate change in the North Sea region are rising groundwater table and sea level rise, both affecting the dynamics of the groundwater systems. In the EU INTERREG North Sea Programme project TOPSOIL consequences of climate change on groundwater and soil are investigated in 16 areas around the North Sea. In two German areas on both sides of the river Elbe groundwater flooding and saltwater intrusions are in the focus of interest. To quantify the enhanced demand for drainage and changes in groundwater salinity, groundwater flow models based on geological models are established. The contribution of geophysical measurements to the geological models is shown. Area GE-1 in Schleswig-Holstein consists of flat marshland with marine sediments covering the aquifer. Resistivity methods give thickness of the covering layer and the aquifer, borehole NMR gives a first assessment of the hydraulic properties of both layers. Area GE-2 in Niedersachsen consists of moraine areas and marshland. Resistivity formation factor was determined from borehole results, formation factor combined with resistivity logs and HEM data gives the electrical resistivity of the groundwater, which was converted to chloride content. Interpolation of these values gives a map of groundwater salinization in different depths.