Resistivity Tomography Measurements Research Articles

Pressurised water flow in fractured permafrost rocks revealed by borehole temperature, electrical resistivity tomography, and piezometric pressure

Abstract. Rock slope instabilities and failures from permafrost rocks are among the most significant alpine hazards in a changing climate and represent considerable threats to high-alpine infrastructure. While permafrost degradation is commonly attributed to rising air temperature and slow thermal heat propagation in rocks, the profound impact of water flow in bedrock permafrost on warming processes is increasingly recognised. However, quantifying the role of water flow remains challenging, primarily due to the complexities associated with direct observation and the transient nature of water dynamics in rock slope systems. To overcome the lack of a quantitative assessment, we combine datasets of rock temperature measured in two deep boreholes (2016–2023), with electrical resistivity tomography measurements repeated monthly in 2013 and 2023; the site-specific temperature–resistivity relation determined in the laboratory with samples from the study area; and borehole piezometer data. Field measurements were carried out at the permafrost-affected north flank of the Kitzsteinhorn (Hohe Tauern range, Austria), which is characterised by significant water outflow from open fractures during the melt season. Borehole temperature data demonstrate a seasonal maximum of the permafrost active layer of 4–5 m. They further show abrupt temperature changes (∼ 0.2–0.7 °C) at 2, 3, and 5 m depth during periods with enhanced water flow and temperature regime changes between 2016–2019 and 2020–2022 at 10 and 15 m depth, which cannot be explained solely by conductive heat transfer. Electrical resistivity measurements repeated monthly reveal a massive decrease in resistivity from June to July and the initiation of a low-resistivity (< 4 kΩ m) zone in the lower part of the rock slope in June, gradually expanding to higher rock slope sections until September. We hypothesise that the reduction in electrical resistivity of more than an order of magnitude, which coincides with abrupt changes in borehole temperature and periods of high water heads up to 11.8 m, provides certain evidence of snowmelt water infiltration into the rockwall becoming pressurised within a widespread fracture network during the thawing season. This study shows that, in addition to slow thermal heat conduction, permafrost rocks are subjected to sudden push-like warming events and long-lasting rock temperature regime changes, favouring accelerated bottom-up permafrost degradation and contributing to the build-up of hydrostatic pressure, potentially leading to slope instability.

Identification of Architectural Roman Remains in the Complex Archaeological Site of Buto ‘Tell El Fara'in’, Northern Egypt, Using Geophysical and Remote Sensing Data

ABSTRACTThe integrated use of remote sensing (RS) techniques, vertical magnetic gradient (VMG) and electrical resistivity tomography (ERT) measurements, and, in particular, combined analysis of 2D and 3D data, can provide a viable option for the identification of targets of interest at complicated archaeological sites. In this regard, a case study is Kom C at the archaeological site of Buto (Tell El Fara'in) in the northern Nile Delta (Egypt), where satellite data (Google Earth, Landsat 8 and OrbView‐3), VMG and ERT measurements were collected prior to site excavation. In this particular case, soil salinity in the buried structures, a lack of contrast in magnetic susceptibility and electrical resistivity, as well as the orientation, complex spatial distribution and overlapping of the architectural elements, all contributed to a number of anomalies that were difficult to interpret using only 2D results. Initially, the archaeological remains were identified as being made of mud‐brick based on land surface temperature (LST) estimated from thermal bands (Bands 10 and 11) in Landsat 8. Then, the high‐resolution satellite data, as well as the VMG and ERT (2D, quasi‐3D and full 3D resistivity models), were integrated to produce a comprehensive map of buried archaeological features. Excavations by Kafrelsheikh University in collaboration with the Ministry of Tourism and Antiquities recovered archaeological remains, including architectural elements that were perhaps used for official or administrative purposes or pottery‐making workshops during the Late Roman period (between the 4th and 7th century ce). The direct comparison of geophysical results to archaeological evidence from the excavation enabled a robust interpretation of geophysical anomalies visible in the horizontal resistivity depth slice and magnetic maps. As a whole, this case study highlights the value of combining satellite data with the analysis of 2D data and 3D views of geophysical surveys to better understand the real distribution of buried archaeological remains at similar complex sites.

Multi‐Method Structural Investigation of the Schneiderberg–Baalberge Burial Mound (Saxony‐Anhalt, Germany) Including Seismic Full‐Waveform Inversion (FWI)

ABSTRACTThe construction history and subsequent usage of burial mounds are an important testimony for socio‐economic transformation in prehistoric societies. The Baalberge–Schneiderberg burial mound, subject of the presented study, falls in this category as it is considered as an important monument that indicates the emergence of early social stratification during the Chalcolithic period in central Europe. This hypothesis relies on the chronological development of the burial mound, which is not fully understood until now. Therefore, a reconstruction of the complex stratigraphy of the burial mound including construction phases and later alterations is highly relevant for archaeological research, but the required excavations would be onerous and inconsistent with preservation efforts. In this paper, we demonstrate that non‐invasive geophysical prospection, especially seismic sounding with shear and Love waves, is suitable to obtain the required stratigraphic information, if seismic full waveform inversion (FWI) and reflection imaging are applied. Complementary information on the preservation state of the mound is obtained through Electrical Resistivity Tomography (ERT) and Electromagnetic Induction (EMI) measurements. To support the seismic and geoelectric results, we utilize Dynamic Testing (DynP), geoarchaeological corings, 14C‐Dating and archaeological records. Our investigations reveal two construction phases of the Baalberge–Schneiderberg mound. The 14C‐Dating yields dates for the older burial mound that are contemporary to the Chalcolithic Baalberge group (4000–3400 bc). During the Early Bronze Age (EBA), the mound was enlarged to its final size by people of the Aunjetitz/Únětice society (2300–1600 bc). However, both seismic and geoelectric depth sections show an extensive disturbance of the original stratigraphy due to former excavations. For this reason, the exact shape of the older burial mound cannot be determined exactly. Based on our data, we estimate that its height was below 2 m. In consequence, the original Baalberge burial mound was less monumental as until now assumed, which potentially prompting a revision of its significance as indicator for social differentiation.

Application Research of Integrated Geophysical Methods in Hydrogeological Survey in Extremely Arid Desert Areas

The research area is located in the Wuyi Mountain area of Dunhuang, Gansu Province, belonging to an extremely arid desert region. Previous hydrogeological surveys in the area had not conducted geophysical exploration work, and most of the hydrological boreholes previously drilled had either low water yield or almost no water yield. In this study, based on a comprehensive analysis of existing regional geological and hydrogeological data, the researchers conducted Controlled Source Audio-frequency Magnetotellurics (CSAMT) measurements and high-density electrical resistivity tomography (ERT) measurements. This work essentially identified the hydrogeological characteristics such as buried fracture water in the structural zones of the research area and water-rich properties. Hydrogeological boreholes were then deployed, and subsequent drilling confirmed a relatively high water yield.

Use of electrical resistivity tomography measurements for investigation of different grouting materials for very shallow geothermal applications within varying seasonal conditions; Applied on a geothermal earth-air heat exchanger system

To achieve the current climate targets, heat pump systems like very shallow geothermal applications are gaining ground. However, the dimensioning of these ground coupled systems is soil-dependent and thus subject to significant differences in efficiency. Furthermore, the thermal properties of the grouting materials are essential for heat transfer in the direct vicinity of the geothermal application. To provide a non-invasive assessment of relevant soil and grouting characteristics, electrical resistivity tomography (ERT) measurements were performed.In this case, the surrounding of an Earth Air-Heat Exchanger – system (EAHE) with different grouting materials (fine sand and fine sand with bentonite) was investigated. For investigation the electrical resistivity (ER) of initial and modified soil conditions were measured regarding soil texture and moisture content. Thereby, the different grouting materials are clearly identified. Thus, based on the ERT measurements, a characterisation of key soil and grouting material properties for applications focusing on heat transfer in soil has been carried out.Furthermore, the depth of soil disturbance due to the EAHE installation could have been traced.Due to a monitoring over different season (February and May) changes in raw ER data could have been ascribed to temperature changes.

VR-EX — An immersive virtual reality serious game for science communication about the electrical resistivity tomography measurements in the Mont Terri Rock Laboratory, Switzerland

This paper presents the design, implementation, and evaluation of VR-EX, a combination of a virtual field trip and a serious game in immersive virtual reality. The application’s purpose is the communication of research conducted in the Mont Terri underground research laboratory in Switzerland. VR-EX enables users to actively attend electrical resistivity tomography measurements within a geological experiment, from planning to execution to analysis of the results, and in this way implements an active and playful learning approach. The work conducted in underground research laboratories has a high relevance for society as it contributes to research on the final disposal of nuclear waste. Therefore, the active communication of research methodology and results is crucial to increase understanding of scientific processes and boost interest. VR-EX was evaluated in a user study with 35 participants to measure its overall quality and its effectiveness of the knowledge transfer. Taking the evaluation’s qualitative results into account, the application was improved in an iterative process. Overall, the results prove the good quality of the application and its high effectiveness in terms of knowledge transfer. The reported high engagement, joy, and immersion indicate the benefits of employing immersive virtual reality for vivid science communication.

Analysis of electrical resistance tomography measurements for fast force localization

Abstract Safe human-robot collaboration requires the robot to monitor the location and intensity of a potential contact force. This is necessary to avoid a possible risk of injury to humans. The goal of this work is to develop a distributed sensor system that enables spatially resolved force measurement. By covering the entire robot’s surface with an elastic coating with a sufficiently pressure-dependent conductivity, electrical resistance tomography can be used for this purpose. The measurement of the transimpedance makes it possible to localize a force applied between the electrodes and thus increase the spatial resolution of the measuring system. By analyzing the obtained measurements, a fast method, compared to classical electrical resistance tomography, for force localization is proposed. This method is compared to the classical method of electrical resistance tomography. A reduction in the processing time to less than one tenth has been attained with the presented method for the case of force localization only.

The two salinity peaks mode of marine salt supply to coastal underground brine during a single tidal cycle

Seawater salt is constantly supplied from the marine environment to coastal underground brine deposits, meaning that brine has the potential for continuous extraction. There is currently a lack of information about the processes that drive the fluxes of seawater salt to underground brine deposits in tidal-driven brine mining areas. We chose the Yangkou salt field on the southern coast of Laizhou Bay, a brine mining area, as our study site. We monitored the spatial and temporal distribution of the underground brine reserve and the changes in water level and salinity in the mining area and adjacent tidal flats using electrical resistivity tomography and hydrogeological measurements. We monitored cross-sections along two survey lines and observed that the underground brine reserve receives a stable supply of seawater salt, and calculated that the rate of influx into the brine body in the mining area near the boundary of the precipitation funnel was 0.226−0.232 t/h. We calculated that a total salt flux of approximately 5.50 t enters the underground brine body every day through a 150 m long shoreline and a 1322.3 m2 window, which is sufficient to sustain the daily extraction of one brine well. During tidal cycles, there are two peaks in the salinity of the water supplied to the underground brine reserve, which means that the brine supply is from at least two high-salinity salt sources in different tidal stages. The first salinity peak occurs during the initial stage of the rising tide after seawater inundates the tidal flat. At this time, seawater, which is a solution and carries a large amount of evaporated salt, is transported into the brine layer through highly permeable areas or biological channels and replenishes the brine in the mining area. The second salinity peak occurs during the early stage of the falling tide. Influenced by hysteresis-driven tidal pumping, high-salinity brine from the lower intertidal zone is rapidly transported into the mining area, thereby increasing the salinity of the underground brine.

Exploration of Undercurrent Path for Hot Springs in Tibet by UAV-based Frequency Electromagnetic Induce Method

Hot springs are useful resources for a wide range of fields, including tourist industry and power generation. In this study, an UAV-FDEM instrument is proposed, which use the unmanned aerial vehicle (UAV) to carry a multi-coil Frequency Electromagnetic induce (FDEM) instrument for scanning survey in the field. We exploit the new method to investigate the electrical conductivity (EC) distribution around the hot springs in the Yangbajing, Tibet. The results clearly present the boundary of high and low EC zones, and indicate the undercurrent path of hot spring based on the measured EC values. These discoveries by UAV-FDEM are confirmed by the electric resistivity tomography (ERT) measurements. Our investigations also reveal the causes of hot spring formation i.e., the exposure of shallow groundwater or the eruption of deep conduit hot water.

Estimating Permafrost Distribution Using Co‐Located Temperature and Electrical Resistivity Measurements

AbstractAssessing the lateral and vertical extent of permafrost is critical to understanding the fate of Arctic ecosystems under climate change. Yet, direct measurements of permafrost distribution and temperature are often limited to a small number of borehole locations. Here, we assess the use of co‐located shallow temperature and electrical resistivity tomography (ERT) measurements to estimate at high‐resolution the distribution of permafrost in three watersheds underlain by discontinuous permafrost. Synthetic modeling shows that co‐located temperature and ERT measurements allow for supervised classification schemes that provide 60% higher accuracy compared to unsupervised methods. Linking resistivity and size of the identified permafrost bodies to surface observations, we show that tall vegetation (>0.5 m) and gentle slopes (<15°) are related to warmer and smaller permafrost bodies, and a more frequent occurrence of taliks.

The summer heatwave in 2022 and its role in changing permafrost and periglacial conditions at a historic mountain pass in the Eastern Alps (Hochtor, Hohe Tauern Range, Austria).

Air temperatures in Europe in 2022 had been the highest on record for the meteorological summer season [June, July and August (JJA)], with +1.3°C above the 1991-2020 average. We studied the effects of recent warming on permafrost and periglacial conditions at a historical mountain pass in the Eastern Alps (Hochtor, 2,576 m asl, 47.08°N, 12.84°E). We used ground temperature data (2010-2022), repeated electrical resistivity tomography measurements (2019, 2022) and auxiliary data dating back to Roman times. We quantified permafrost conditions, evaluated frost-related weathering and slope processes and assessed the impact of atmospheric warming on it. Results show that summer ground surface temperatures increased by 2.5°C between 1891-1920 and 1991-2020, whereas frost-related weathering and periglacial processes decreased. The summers of 2003, 2015, 2019 and 2022 were the four warmest ones in 1887-2022. Hochtor changed in 2010-2022 from an active permafrost site to an inactive one with supra-permafrost talik. A general three-layer structure was quantified for all three ERT profiles measured. The middle, 5-10m thick layer is ice-poor permafrost detected in 2019, whose existence, although smaller, was confirmed in 2022. Resistivity decreased at the three profiles by 3.9% to 5.2% per year, suggesting permafrost degradation. We interpret the resistivity changes between the summers of 2019 and 2022 as a long-term signal of permafrost degradation and not as the single effect of the summer heatwave in 2022. As our data show how rapidly permafrost degrades and as we face an even warmer climate for the remaining part of the 21st century, we expect that near-surface permafrost at the Hochtor site will soon be history.

Electrical resistivity tomography (ERT) measurements during water flow in a date palm stem segment

Commercial cultivation of date palms has high economic significance, particularly in arid and hyper-arid regions. Achieving commercially viable yield of dates amid water scarcity and enforced restrictions on water use requires optimising the amount and timing of irrigation. To that end, better understanding of the temporal dynamics of the water use of date palm is of great importance. Traditionally, this can be achieved through sap flow estimates obtained using heat dissipation probes. However, such measurements lack the ability to provide information on the spatial distribution of sap flow, which is important considering that date palms transport water in the entire stem cross-section. The aim of this study is to evaluate whether electrical resistivity tomography measurements have the potential to provide information on the spatial variability of flow in date palm stems. For this, flow was induced through a stem segment that was obtained from a felled date palm tree. ERT measurements were continuously obtained throughout a cycle of flow and no-flow periods. The results showed that the mean bulk electrical conductivity varied strongly due to changes in the flow conditions. In addition, it was found that the electrical conductivity of the outflow was much higher than that of the inflow, which indicates the release of stored salt from the stem segment. Analysis of the spatial distribution of the electrical conductivity suggested that flow mainly occurred in a limited part of the cross-sectional area of the stem. Overall, it was concluded that the ERT is a promising tool to investigate the spatial variability of water flow in date palm stems.

Identifying mountain permafrost degradation by repeating historical electrical resistivity tomography (ERT) measurements

Abstract. Ongoing global warming intensifies the degradation of permafrost. Permafrost thawing impacts landform evolution, reduces freshwater resources, enhances the potential of natural hazards and thus has significant socio-economic impacts. Electrical resistivity tomography (ERT) has been widely used to map the ice-containing permafrost by its resistivity contrast compared to the surrounding unfrozen medium. This study aims to reveal the effects of ongoing climate warming on mountain permafrost by repeating historical ERT and analyzing the temporal changes in the resistivity distribution. In order to facilitate the measurements, we introduce and discuss the employment of textile electrodes. These newly developed electrodes significantly reduce working effort, are easy to deploy on blocky surfaces and yield sufficiently low contact resistances. We analyze permafrost evolution on three periglacial landforms (two rock glaciers and one talus slope) in the Swiss and Austrian Alps by repeating historical surveys after 10, 12 and 16 years, respectively. The resistivity values have been significantly reduced in ice-poor permafrost landforms at all study sites. Interestingly, resistivity values related to ice-rich permafrost in the studied active rock glacier partly increased during the studied time period. To explain this apparently counterintuitive (in view of increased resistivity) observation, geomorphological circumstances, such as the relief and increased creep velocity of the active rock glacier, are discussed by using additional remote sensing data. The present study highlights ice-poor permafrost degradation in the Alps resulting from ever-accelerating global warming.

Coastal sedimentation and topographic changes in the Mataram Plain, Lombok (Indonesia) following the 1257 CE eruption of Samalas volcano

AbstractThe 1257 CE eruption of Samalas volcano, Indonesia, buried the entire island of Lombok beneath various thicknesses of pyroclastic material during this Volcanic Explosivity Index (VEI) 7 event. This study aims to reconstruct the paleo‐topography of western Lombok before the Samalas eruption in 1257 CE and analyse the sedimentation processes that led to its landscape evolution over the last 700 years. Stratigraphic data were collected from various surveys, such as outcrops, coring, drilling, hand auger and wells observation. Electrical resistivity tomography (ERT) measurements were performed to complement the stratigraphic data. A combination of stratigraphy, ERT and topographic data from various sources (topographic map, Digital Elevation Model Nasional [DEMNAS] and Real Time Kinematic [RTK] measurement) is employed to determine the depth of the paleo‐surface. Topographic modelling was performed by subtracting the delta‐DEM (DDEM) from the current‐DEM (CDEM). The result of this operation is a paleo‐DEM, which is used to reconstruct the paleo‐hydrographic features such as shoreline positions and river channels. Modelling results demonstrate that the relief has not been significantly modified, except in lowland areas, which is the sediment accumulation zone. River channels have experienced minor changes, except for the location of the river mouth and the degree of meandering. Significant changes occurred at the shoreline that has prograded by approximately 1.6 km during the last 700 years. A schematic model is built to illustrate the evolution processes of the study area, consisting of the pre‐eruption condition, the immediate post‐eruption condition and development of the current condition. This model further develops the previously proposed model with a higher resolution and simultaneously revises the estimated boundary of the pyroclastic density current (PDC) deposit from previous work.

A computational package to compute the electrical resistivity tomography response for regular bodies immersed in a homogeneous half-space

A 3D forward algorithm is presented to model electrical resistivity tomography (ERT) measurements with the capability to use different electrode array configurations. We have implemented the algorithm as an open-source package with the assumption that the medium can be modeled as a homogeneous half-space with immersed bodies of different resistivities with regular geometry. This algorithm incorporates a novel numerical approximation by using the volume integral method, with the hypothesis of weak scattering, where the secondary currents generated in the bodies due to the main field are not considered. The algorithm also includes a new method called cubature to calculate volume integrals and the ERT response. In addition, the code has a module to display the results as color resistivity profiles. This method avoids the limitation of the complex geometry of the medium or even of the immersed bodies. The method is useful for planning a field survey, exploring the use of a specific electrode array, or implementing new arrays. The package, called the volume integral model of the electric response, is computationally efficient and can be easily ported to any operating system that supports MATLAB.

Study of the groundwater regime in unsaturated slopes prone to landslides by multidisciplinary investigations: Experimental study and numerical modelling

The triggering of shallow landslides in granular deposits is highly controlled by the groundwater regime, namely, the pore water pressure distribution, which directly affects soil shear strength. The slope hydraulic state is usually variable over time and space due to the high variability of atmospheric loads at the ground surface and local geologic conditions, such as stratigraphic irregularities and preferential groundwater flow paths. The unfavourable combination among critical geomorphological and topographical settings with stratigraphic and hydrogeological features are commonly recognized as predisposing factors of flowslides and debris flow occurrences.This paper proposes a multidisciplinary approach that combines geological, geophysical and geotechnical investigations to identify the role of local geological and geotechnical factors on the groundwater regime in slopes prone to flow-like landslides. The study is based on seasonally repeated electrical resistivity tomography measurements integrated with geotechnical numerical modelling of hydraulic phenomena affecting the soil cover. The latter is used to analyse the effects of the stratigraphic variability in terms of the geometry, continuity, and thickness of the soil horizons on the groundwater regime over time. The proposed approach has been applied to a test site located on the northern slope of Faito Mt. in the Lattari/Sorrento Peninsula mountain chain (southern Italy), an area historically affected by many rapid instability phenomena, such as flow-like landslides and flash floods. Both geophysical and geotechnical models obtained for the test site were cross-checked and validated, providing significant insights into the hydraulic response of the soil cover to rainfall and its hydraulic interaction with the underlying bedrock. Specifically, the integrated approach proved that i) the buried paleo-morphology of the bedrock severely affects the pore water distribution in the soil cover and ii) ashy soil fills the upper karst portion of the bedrock, providing a hydraulic connection of the water flow infiltrating from the topsoil downwards.

Brief communication: Combining borehole temperature, borehole piezometer and cross-borehole electrical resistivity tomography measurements to investigate seasonal changes in ice-rich mountain permafrost

Abstract. A novel combination of borehole temperature, borehole piezometer and cross-borehole electrical resistivity tomography (ERT) data is used to investigate changing ice and/or water contents in the creeping ice-rich Schafberg rock glacier in the Eastern Swiss Alps. Instrumentation techniques and first results are presented. The rock glacier ice is close to its melting point, and the landform has locally heterogeneous stratigraphies, ice and/or water contents and temperature regimes. The measurement techniques presented continuously monitor temporal and spatial phase changes to a depth of 12 m and should provide the basis for a better understanding of accelerating rock glacier kinematics and future water availability.

Comparison of Measured and Derived Thermal Conductivities in the Unsaturated Soil Zone of a Large-Scale Geothermal Collector System (LSC)

The design, energetic performance, and thermal impact of large-scale geothermal collector systems (LSCs) are dependent on the thermal conductivity of unsaturated soils (λ). The aim of this study was to investigate the benefits of two different λ measurement methods using single-needle sensor measuring devices on a laboratory scale. Since large-scale determinations are required in the context of LSCs, the potential for deriving λ from electrical resistivity tomography measurements (ERTs) was also examined. Using two approaches—the continuous evaporation method and the punctual method—thermal conductivities of soil samples from Bad Nauheim (Germany) were measured. The results were compared with averaged λ derived from three ERT sections. With the evaporation method, significant bulk density changes were observed during the experimental procedure, which were caused by the clay content and the use of repacked samples. The punctual method ensures a sufficiently constant bulk density during the measurements, but only provides a small number of measurement points. The thermal conductivities derived from ERTs show largely minor deviations from the laboratory measurements on average. If further research confirms the results of this study, ERTs could provide a non-invasive and unelaborate thermal exploration of the subsurface in the context of large-scale infrastructure projects such as LSCs.

Mapping of soil moisture by time domain reflectometry and electrical resistivity tomography at Velika Gorica well field, Zagreb aquifer

Knowing the soil moisture distribution in the unsaturated zone can improve understanding the water flow through the unsaturated zone and thereby enable the calculation of aquifer recharge, which occurs through precipitation. One part of the Zagreb aquifer recharge occurs through infiltration from precipitation. In order to observe and model infiltration from precipitation through the unsaturated zone, the research polygon was constructed at the Velika Gorica well field, located in the southern part of the Zagreb aquifer, Croatia, where hourly measurements of electric conductivity (EC) and soil moisture content were carried out. EC and soil moisture data are measured by Time Domain Reflectometry (TDR) probes which are placed at different depths in the unsaturated zone. Furthermore, electrical resistivity tomography (ERT) measurements were conducted. Geophysical data, along with moisture and EC data from TDR probes, were used as input data for MoisturEC software, in order to obtain soil moisture distribution along a 2D profile. MoisturEC program offers three options for translating EC data to moisture content data which are all tested in this research. We obtained eight moisture content distributions along the observed profile and concluded that MoisturEC provides reasonable results with input data from geophysical measurements and TDR probe measurements. Soil moisture distribution in the unsaturated zone represents the initial conditions for further unsaturated flow modeling. Understanding the flow in the unsaturated zone enables the quantification of effective infiltration and can improve groundwater management.

Combining UAV remote sensing and pedological analyses to better understand soil piping erosion

The study and mapping of evidences of soil piping using non-destructive techniques is a key issue in quantitative geomorphology. This paper attempts to combine Aerial mapping systems (Unmanned Aerial Vehicles (UAV)), soil physical and chemical attributes, and near-surface geophysical survey tools (ground penetrating radar and electrical resistivity tomography) to provide a comprehensive understanding of geometric features of soil piping in arid and semi-arid regions. The UAV Mapping System was applied to prepare ortho-photos, topography, analytical hill shading, drainage density, and land use maps of two different piping sites (site 1: rangelands, and site 2: agricultural lands) in Sarakhs plain, Razavi Khorasan Province, northeastern Iran. The physical and chemical soil attributes were analyzed in six soil profiles to check the hypothesis that these soil attributes control the occurrence of piping-related features (i.e., sinkhole, blind gully, gully), and to test if there are any differences in soil properties between the two land use types. The near surface geophysical tools were used to determine the approximate size of soil pipes, and to simulate their internal structure. The results of the derived UAV’s independent variables confirmed that typical soil erosion features related to piping (i.e., sinkhole, blind gully, gully) developed exactly in adjustment with subsurface processes (i.e., drainage density). The quantitative algorithms of pedology revealed significant differences of Na+ and SAR for soil profiles with and without piping erosion, but these soil properties themselves are not enough to explain piping development at the two study sites. The volume of GPR line surveys was 1701.5 m3 in site 1 and 1203 m3 in site 2. The potential distribution of subsurface pipes revealed by applying GPR were more extensive than those deduced from soil surface observations: three-dimensional pipe number density (number of soil pipes per unit soil volume; # m−3) of potential pipes and collapsed cavities simulated with a mean pipe length and pipe depth of 143.4 cm and 88 cm in rangelands and 175.75 cm and 79.14 cm in agricultural lands, respectively. The maximum density of pipes or pipe roof collapses occurred within soil depth boundaries of 0–50 cm at site 1 and 30–100 cm at site 2. The ratio of surface erosion features to subsurface potential piping in rangeland and agricultural lands was ca. 26.1 % and 10.4 %, respectively. The electrical resistivity tomography (ERT) measurements indicated higher resistivity values in piping-prone areas, where pipes were initiated at the bedrock interface. Integrating all data obtained by different techniques in this study allowed to better understand the extent of piping erosion in both surface and subsurface soil horizons.