Capacitively Coupled Resistivity Research Articles

Application of Complex Geophysical Methods for the Detection of Unconsolidated Zones in Flood Dikes

The flood levees in the vicinity of Krakow city (Poland) are, in some places, over 100 years old. Thereupon, in the flood dike, and its subsoil, can appear unconsolidated zones, which, during the flood stage, can be a simple way of water flow and/or even can be the place where the levee body will be destroyed. This phenomenon took place in Wawrzeńczyce village, near Krakow city, during the flood in 2010. The geophysical research was carried out, in order to develop a methodology of recognition of potential areas where the levee body can be damaged. The geophysical surveys were conducted with the use of electrical and electromagnetic methods, as well as utilizing the seismic method. The general identification of examined media was realized by the electrical resistivity tomography (ERT) method. The ERT surveys were supplemented by capacitively-coupled resistivity (CCR) measurements, in order to analyze the usefulness of the CCR method for the examination of river dikes and reduction of interpretation ambiguity. The ground penetrating radar (GPR) method detected small anomalies in the body dike, due to the very high resolution of this method, which were not detected by the ERT and CCR techniques. During GPR surveys, non-standard measurement techniques were applied. Finally, the high-resolution seismic reflection (HRSR) method provided a clear and high-resolution image of the dike structure up to the water table and assisted with the identification of the hazard non-consolidated zones.

Application of Combined Geophysical Methods for the Examination of a Water Dam Subsoil

The paper presents the results of geophysical measurements that were carried out in the vicinity of the water dam/water reservoir supplying the city of Bielsko-Biala with drinking water. The measurements were performed in order to non-invasively detect faults, fractured zones and areas filled with breccia, which may be, at the same time, a preferential path of groundwater flow. The aforementioned factors influence the stability of the dam. The general identification of the examined media was realized by the electrical resistivity tomography method. The ERT surveys were supplemented by capacitively-coupled resistivity. The electrical methods allowed them to recognize geological settings, indicate possible fault locations, and point out the fault plane as a path of water flow. The ground penetrating radar method detected fractured and filled water areas and underground water paths in the dam’s forefield as a result of the method’s very high resolution. The high resolution seismic reflection method provided a clear and high resolution image of the relatively deep geological structure and verified a location and the run of the faults. In general, the complex geophysical-geological interpretation enabled classification of the unconsolidated/fractured zones associated with faults as a place where the erosion process is the most intense and can bring danger on the dam. Finally, it was confirmed that the area should be the subject of geophysical monitoring.

A new levee control system based on geotechnical and geophysical surveys including active thermal sensing: A case study from Poland

Specific features of levees, particularly their large lengths, cause significant problems in the early and precise detection of leaks as well as erosion zones in both the levee body and subsoil. These processes are the primary threats to levees safety. Fast and relatively cheap methods for the internal investigation of levees, i.e., non-invasive geophysical methods, are used only during non-flooding periods and do not have equivalent accuracy to sensors installed in the levee. Additionally, there is no optimal monitoring method for levee leak detection during a flood. Such a method, which use a fibre-optic thermal linear sensor is very effective in the case of earth dams, but it has significant limitations when to applied to existing levees.In consequence, the primary goals of our research were to improve the thermal method for levee investigation and to propose a coherent levee investigation methodology that considers the entirety of the currently used methods.We therefore developed a new type of sensor: the Multi Points Thermal Sensor (MPointS). In our solution, thermal sensors are mounted next to one another inside the levee or/and in its subsoil by hammering them from the surface. This procedure creates a quasi-continuous line of sensors along the levee. This technology's key feature is that each sensor has an integrated micro-heater that uses an active (heating) thermal method for leak detection and in situ measurements of the seepage velocity.A pilot section of the Vistula River levee in Poland was chosen to verify the MPointS solution in addition to a geophysical, geological, and geotechnical survey. Some leakage zones on this levee were previously identified during the flood in 2010. We next used a wide variety of methods to investigate the levee during a period of non-flooding. Dynamic penetration light (DPL) soundings and geological drillings were applied in addition to geophysical methods such as ground-penetrating radar (GPR), electrical resistivity tomography (ERT), electromagnetic profiling (EMP), capacitively-coupled resistivity (CCR), and high-resolution reflection seismic imaging and multichannel analysis of surface waves (MASW).Loose zones of higher porosity in the levee and its subsoil were identified using geophysical methods. These zones also included the local leakage zones detected during the 2010 flood. However, the leakage zones themselves were not clearly and unambiguously detected by these geophysical methods. The thermal monitoring system using MPointS was next installed in accordance with the locations of the loose zones indicated by the geophysical methods. During the flood in 2019, MPointS allowed us to detect precise leakage zones and monitor whether the seepage velocity exceeded the critical values that may lead to the development of internal erosion.The results showed the importance of using a comprehensive and optimised methodology for levee investigation. MPointS proved to be an effective solution for levee leakage monitoring; however, it was found to be primarily applicable to the most vulnerable levee sections. Overall, levee investigations must be performed with cheaper and faster geophysical methods and supplemented by geotechnical surveys, which are also able to choose MPointS system locations.

The Case for Considering Polarization in the Interpretation of Electrical and Electromagnetic Measurements in the 3 kHz to 3 MHz Frequency Range

Usually, in situ electrical polarization measurements (in geophysical prospection referred to as induced polarization or spectral induced polarization (SIP)) are carried out at frequencies below 1 kHz. These techniques have mainly been used for mining exploration, followed by a larger panel of environmental applications. However, in this ultra low and extremely low-frequency domain, the duration of each individual measurement is long, i.e., typically several tens of minutes for a single full SIP spectrum down to the mHz range. This restriction makes it unrealistic to implement high-density measurement mapping campaigns over large areas, which would otherwise be possible at higher frequencies. In the intermediate frequency range [3 kHz–3 MHz], laboratory studies of soil and rock samples have shown that they can be strongly polarized notably in the presence of clays, and this property has been confirmed by several in situ mapping experiments using electromagnetic induction in the time and frequency domains (FDEM and TDEM), as well as by an electrostatic method (often named capacitive-coupled resistivity or CCR). The present paper recalls these results in an effort to promote polarization measurements at intermediate frequencies and to emphasize the importance of measuring this phenomenon.

Spectral Capacitively Coupled Electrical Resistivity Tomography for Breast Cancer Detection

Early detection of breast cancer is key for a better survival rate, so there are huge demands for a safe and reliable breast cancer screening method, which currently is not available. Earlier studies are showing that the impedance spectroscopy can be an effective discriminator for breast tumor against healthy tissues. Traditional electrical impedance tomography (EIT) has progressed well into clinical stages for such an application with some promising results. But EIT generally operates at low frequencies, ranging typically lower than 1 MHz. Previous research work has suggested that the important information on the tumor impedance spectra exists in the higher frequency domain. Thus, this work focuses on the introduction and the evaluation of a spectral capacitively-coupled electrical resistivity tomography (CCERT) for breast cancer diagnosis. Compared to the traditional EIT, CCERT can reach a much higher frequency domain producing the spectroscopic images in a broader range. This in turns offers improved discrimination of tumors against normal tissues via frequency difference imaging or spectral imaging as it is shown in this paper. Additionally, direct contact between the EIT electrodes and the surface of the body, which causes significant challenges and uncertainties in measured data, is avoided in CCERT inherently a contactless method. In this paper, a 3D in-silico model of the CCERT system is created and solved using the finite element method (FEM) to simulate the forward model for each frequency. The feasibility simulation analysis is conducted based on the existing clinical breast cancer data and a CCERT reconstruction. For the experimental verification, an 8-electrode laboratory phantom was tested with the excitation frequency ranging from 200 kHz to 13 MHz. The simulation and experimental results show the promising potential of this method for breast cancer imaging. The wide-range spectroscopic images provide better discrimination of tumors against healthy tissues through their spectral conductivity profile as well as the rate of changes in spectral conductivity images.

Two-dimensional inversion of wideband spectral data from the capacitively coupled resistivity method – first applications in periglacial environments

Abstract. The DC resistivity method is a common tool in periglacial research because it can delineate zones of large resistivities, which are often associated with frozen water. The interpretation can be ambiguous, however, because large resistivities may also have other causes, like solid dry rock. One possibility to reduce the ambiguity is to measure the frequency-dependent resistivity. At low frequencies (< 100 Hz) the corresponding method is called induced polarization, which has also been used in periglacial environments. For the detection and possibly quantification of water ice, a higher frequency range, between 100 Hz and 100 kHz, may be particularly interesting because in that range, the electrical properties of water ice exhibit a characteristic behaviour. In addition, the large frequencies allow a capacitive coupling of the electrodes, which may have logistical advantages. The capacitively coupled resistivity (CCR) method tries to combine these logistical advantages with the potential scientific benefit of reduced ambiguity. In this paper, we discuss CCR data obtained at two field sites with cryospheric influence: the Schilthorn massif in the Swiss Alps and the frozen Lake Prestvannet in the northern part of Norway. One objective is to add examples to the literature where the method is assessed in different conditions. Our results agree reasonably well with known subsurface structure: at the Prestvannet site, the transition from a frozen lake to the land is clearly visible in the inversion results, whereas at the Schilthorn site, the boundary between a snow cover and the bedrock below can be nicely delineated. In both cases, the electrical parameters are consistent with those expected from literature. The second objective is to discuss useful methodological advancements: first, we investigate the effect of capacitive sensor height above the surface and corroborate the assumption that it is negligible for highly resistive conditions. For the inversion of the data, we modified an existing 2-D inversion code originally developed for low-frequency induced polarization data by including a parametrization of electrical permittivity. The new inversion code allows the extraction of electrical parameters that may be directly compared with literature values, which was previously not possible.

Mapping Subsurface Conditions and Detecting Seepage Channels for an Embankment Dam Using Geophysical Methods: A Case Study of the Kinion Lake Dam

Capacitively-Coupled Resistivity (CCR), Electrical Resistivity Tomography (ERT), and seismic surface wave testing using Love and Rayleigh waves were performed on Kinion Lake Dam, an embankment dam that has historically experienced significant seepage and internal erosion issues. The goal of this study is to detect seepage locations in the embankment dam for remediation purposes and map bedrock location across the area using nondestructive geophysical measurements. Surveys were completed along the crest and downstream toe of the dam to map subsurface conditions with a focus on seepage detection. The seismic surface wave data were analyzed using two different methods: Multichannel Analysis of Surface Waves (MASW) and Full Waveform Inversion (FWI). Both MASW and FWI methods were shown to be capable of resolving the bedrock layer below the dam, but the FWI method, provided a higher resolution image of the subsurface conditions compared to MASW method. However, the MASW and FWI provided little guidance regarding the seepage path through the dam. The CCR and ERT surveys, on the other hand, were shown to be very valuable for seepage detection. The seepage paths of the embankment dam were detected by comparing the resistivity measurements acquired in a wet winter and a dry summer. These seepage regions correspond well with previous sinkholes observed in the pool area following a drought. Additionally, new drilling in the two regions which showed differences in resistivity indicated the areas contained highly fractured limestone with evidence of solution cavities. It seems that the highly fractured limestone becomes fully saturated during the wet season causing very low resistivity values for these layers in wet seasons, but it quickly loses the water during the dry season leading to a highly resistive material in dry seasons. Therefore, seasonal resistivity measurements can be considered as a useful method for locating potential weak zones of earthen hydraulic structures in a cost-effective and timely manner.

The combined use of MASW and resistivity surveys for levee assessment: A case study of the Melvin Price Reach of the Wood River Levee

Capacitively-Coupled Resistivity (CCR) and Multichannel Analysis of Surface Waves (MASW) were performed on the Melvin Price reach of the Wood River Levee that has experienced considerable piping through the foundation soil causing numerous sand boils along the landside toe of the levee. Tests were performed along the centerline crest, the landside toe, and the riverside berm of the levee. This study combined the strengths of MASW and CCR testing to resolve the distribution and stiffness of cohesive and non-cohesive materials in the body and foundation of the levee, where invasive testing was not ideal. MASW and CCR were shown to complement each other in the evaluation. CCR successfully classified the near surface clay and sand deposits along the levee and was particularly effective for soil deposits with a high degree of saturation. MASW revealed that a deeper clay layer, appears to be discontinuous at locations where old river bars cross under the levee. These bars may have eroded portions of the deeper clay layer, locally enhancing seepage rates through the foundation materials and influencing the formation of sand boils during large flood events. The use of MASW and CCR also successfully detected the locations of the major utilities crossing the levee. While CCR was the most effective method for detecting conductive utilities, MASW was more effective at detecting utilities which increased the overall stiffness of the subsurface either due to the utility itself or the backfill material around the utility. A site-specific SPT-Vs correlation was developed from the co-located boreholes and shear wave velocity profiles. Comparison of the correlation with previous studies indicates that soil stiffness and geologic age are important factors affecting SPT-Vs correlations. For levee soil deposits, which are typically young, a lower bound SPT-Vs correlation developed using similar soil deposits to the study area should be considered.

Quaternary sediment architecture in the Orkhon Valley (central Mongolia) inferred from capacitive coupled resistivity and Georadar measurements

Detailed information on shallow sediment distribution in basins is required to achieve solutions for problems in Quaternary geology, geomorphology, neotectonics, (geo)archaeology, and climatology. Usually, detailed information is obtained by studying outcrops and shallow drillings. Unfortunately, such data are often sparsely distributed and thus cannot characterise entire basins in detail. Therefore, they are frequently combined with remote sensing methods to overcome this limitation. Remote sensing can cover entire basins but provides information of the land surface only. Geophysical methods can close the gap between detailed sequences of the shallow sediment inventory from drillings at a few spots and continuous surface information from remote sensing. However, their interpretation in terms of sediment types is often challenging, especially if permafrost conditions complicate their interpretation. Here we present an approach for the joint interpretation of the geophysical methods ground penetrating radar (GPR) and capacitive coupled resistivity (CCR), drill core, and remote sensing data. The methods GPR and CCR were chosen because they allow relatively fast surveying and provide complementary information. We apply the approach to the middle Orkhon Valley in central Mongolia where fluvial, alluvial, and aeolian processes led to complex sediment architecture.The GPR and CCR data, measured on profiles with a total length of about 60km, indicate the presence of two distinct layers over the complete surveying area: (i) a thawed layer at the surface, and (ii) a frozen layer below. In a first interpretation step, we establish a geophysical classification by considering the geophysical signatures of both layers. We use sedimentological information from core logs to relate the geophysical classes to sediment types. This analysis reveals internal structures of Orkhon River sediments, such as channels and floodplain sediments. We also distinguish alluvial fan deposits and aeolian sediments by their distinct geophysical signature. With this procedure we map aeolian sediments, debris flow sediments, floodplains, and channel sediments along the measured profiles in the entire basin.We show that the joint interpretation of drillings and geophysical profile measurements matches the information from remote sensing data, i.e., the sediment architecture of vast areas can be characterised by combining these techniques. The method presented here proves powerful for characterising large areas with minimal effort and can be applied to similar settings.

Multidisciplinary geophysical approach to map a disposal site: The Ponza island case study

Electrical resistivity tomography (ERT) and magnetometric surveys are extensively used for environmental investigations of uncontrolled landfills, where the presence of waste with potential contaminants and metal objects requires a detailed characterization and monitoring of their subsurface location. Capacitively-coupled resistivity (CCR) measurements applied for similar environmental studies are far less used. In this study, we show the results and discuss the advantages of a combined application of ERT, CCR and magnetometric surveys applied to the characterization of the disposal site of Mt. Pagliaro, in the island of Ponza (central Italy). The survey area is located on volcanic deposits, which characterize the low resistive geological bedrock. We acquired four CCR profiles and five ERT profiles in addition to a magnetic survey covering a total area of about 7000m2. The recovered smooth resistivity models suggest the presence of a shallow resistive layer (ρ>75Ωm) of variable thickness (2.0–6.0m), overlying a relatively low-resistive layer, which we interpret as the electrical response of the volcanic bedrock. This interpretation is supported by few shallow boreholes and field observation in the northern part of the landfill area. The magnetic maps show three suspicious dipolar magnetic anomalies, probably ascribed to the presence of a high concentration of buried ferrous waste. Several small-scale dipolar anomalies have been interpreted as due to the presence of sparse and shallow metal objects within the waste material. Due to the resistivity models' smoothness, to improve the characterization of the interface between the bedrock and the waste material we performed a statistical analysis of the resistivity data. Following the philosophy of the steepest gradient method, we found a significative change in resistivity computed on an averaged depth resistivity function of the ERT data. Accordingly, we classify two distinct units: a) an upper unit, with resistivity >75Ωm, which we assigned to the coarse waste and dry soil materials; and b) a lower unit, with resistivity <75Ωm, which we assigned to the volcanic deposits. We chose the 75Ωm value as representative of the top-bedrock\\waste interface and we interpolated this boundary from both the ERT and the CCR profiles to reconstruct a smooth 3D representation of the top bedrock within the landfill area. This allowed us to evaluate the total volume of the waste material to be about 35,000m3.

Evaluation of the capacitively coupled resistivity (line antenna) method for the characterization of vadose zone dynamics

The electrical resistivity survey, traditionally realized by the direct current (DC) resistivity method, has shown great value for characterizing vadose zone dynamics. Compared with the DC resistivity method, the capacitively coupled (CC) resistivity method has a higher ratio of measurement speed to data density, and thus is economically preferred for resistivity surveys that require high data density, e.g., hydrological characterizations. To test the applicability of the CC resistivity method to the study of vadose zone dynamics, we conducted time-lapse resistivity surveys using a commercial CC resistivity (line antenna) system, the OhmMapper, to monitor the water content change in an unsaturated zone due to artificial rainfall infiltration . Special considerations were paid to the inversion of CC resistivity (line antenna) measurements in order to increase the accuracy of inversion results. The derived resistivity of the subsurface clearly captures the water movement in the vadose zone and shows the applicability of the CC resistivity method. The experiment also showed a limitation of the equipment: when the ground surface became extremely conductive, the OhmMapper falsely interpreted the current level. If the wrong current level is identified in the measurement, the measured resistance should be corrected accordingly. The overestimation of the ground resistivity of the CC resistivity method, arising from the decrease in the ground resistivity, was also examined and discussed. Although the measurement bias was found to be negligible in our study, one should still be cautious about it when using the CC resistivity method for similar applications, especially when the measurement is made with a short dipole cable. • Water infiltration was monitored using the capacitively coupled resistivity method. • It proved the applicability of the method for vadose zone dynamics studies. • It revealed that the system sometimes falsely interpreted the current level. • The potential experimental bias of this study is examined and discussed.

Comment on “The application of CCR and GPR to characterize ice conditions at Parsons Lake, Northwest Territories” by Angelopoulos et al.

As part of a permafrost characterization study using capacitively-coupled resistivity data, Angelopoulos et al. (2013) suggest a methodology for selecting the inversion damping factor based on the depth of investigation. I argue that the suggested methodology conflicts with the discrepancy principle which states that the damping factor should be chosen based on a target misfit. Furthermore, a misfit-based selection of the damping factor is required for the depth of investigation to provide a reasonable measure of model resolution in the context of noisy data.

Capacitive resistivity inversion using effective dipole lengths for line antennas

Non-contacting capacitively-coupled resistivity (CCR) surveys find application in permafrost investigations and investigations over engineered surfaces. However, we observe discrepancies between line-antenna CCR data and galvanic resistivity (GR) data. Inverse models recovered from the different data types exhibit differences in both resistivity magnitude and structure. We apply and test the concept of effective dipole length for line-antenna CCR data collected over permafrost terrain in Iqaluit, Nunavut. We compare inversions of corrected CCR data to the GR counterpart. Results show that correcting CCR data with an effective dipole length of 80% of the physical antenna length results in a resistivity model most in accordance with the GR model. After correction, the CCR model does not precisely emulate the GR model; potential sources of remaining discrepancy are incomplete representation of the line-antenna nature of the CCR data and the realities of field data acquisition including significantly different noise levels and the potential violation of CCR operating conditions.

Theoretical and experimental examinations of the capacitively coupled resistivity (line antenna) method

The capacitively coupled (CC) resistivity method is an emerging geophysical technique for near-surface investigations. However, there are two major problems associated with using the CC resistivity (line antenna) system in ground investigations. These two problems are (1) the geometrical factor and (2) measurement biases. We first derive the geometric factor for the line-antenna array in a dipole-dipole configuration. The new expression of the geometric factor considers the separation between the two current (and the two potential) line antennas, thereby improving the accuracy of the measured apparent resistivity. Second, from the electroquasistatic point of view, a quadruline model that can describe the characteristics of the CC resistivity (line antenna) system is derived based on the previously published quadrupole model. The validity of the quadruline model is verified experimentally. Based on the quadruline model, it is found that the dielectric properties of the line-to-ground capacitor and the ground permittivity produce measurement bias at low and high current frequencies, respectively. If the operating current frequency is around the kHz range and the ground resistivity is relatively high, such as in permafrost areas, the ground permittivity can also create measurement bias to cause the actual value to be underestimated. The experimental results and the predictions by the quadruline model suggest that the measurement bias induced by the gap between the line antenna and the ground surface becomes significant and cannot be ignored as the gap height is large and the associated ground resistivity is low. In general, the CC resistivity (line antenna) measurements are not biased by the gap effect when the gap height is less than 0.01 m, which can easily be achieved for tests on flat ground.

Characterization and Delineation of Potential Evaporite Geohazards Using Electrical Resistivity Methods along FM 2185, Culberson County, Texas

Extensive karst development within the Delaware Basin of West Texas and southeastern New Mexico poses a significant threat to infrastructure. Dissolution of regional evaporite strata have led to karst geohazards including sinkholes, subsidence features, and caves. The study area is located within the Gypsum Plain in Culberson County, Texas, and includes outcrops of Permian Castile and Rustler strata that host gypsum karst. Land reconnaissance surveys conducted during summer of 2019 documented numerous surface karst features proximal to Farm to Market Road 2185 (FM 2185). In combination with traditional survey techniques, electrical resistivity methods were used to delineate karst features along a 48 km segment of FM 2185. Capacitively-coupled resistivity (CCR) and direct-current resistivity (DCR) methods were used to characterize evaporite karst features that do not manifest surficially but pose potential geohazard concerns. CCR data were acquired using the Geometrics OhmMapper G–858 resistivity system, which uses a dipole-dipole configuration composed of five receivers connected by 2.5 m coaxial cables and a transmitter offset of 2.5 m. In combination with the medium analyzed, this geometric configuration enabled resistivity soundings up to 2.5 m deep. DCR data was collected with a SuperSting (R8/IP) multi-electrode earth resistivity meter using 112 electrodes with 2 m spacing and a dipole-dipole array configuration. This enabled a depth of investigation of up to 25 m. Data were processed using Advanced Geosciences Inc.’s (AGI’s) EarthImager 2D software and used to delineate and characterize karst-related geohazards in the shallow subsurface within the study area. Five sites are presented to demonstrate karst variability and electrical resistivity method effectiveness in geohazard detection in gypsum strata.

The application of CCR and GPR to characterize ground ice conditions at Parsons Lake, Northwest Territories

Abstract Capacitively-coupled resistivity and ground-penetrating radar surveys were performed in an area of ice-rich permafrost at Parsons Lake 55 km southwest of Tuktoyaktuk in the Northwest Territories. The Parsons Lake area has been identified as a possible staging site for future hydrocarbon development. Since ground ice is an important factor affecting the level of potential thermokarst, improving the use of non-invasive geophysical tools to map its nature and extent is important. This paper focuses on the application of capacitively coupled resistivity to detect and classify different ground ice conditions by relating the tool's outputs to permafrost properties available from borehole logs and thermal modeling results. By employing the depth of investigation index and varying inversion parameters, optimal degrees of data smoothing were calculated, and thus the resistivity system's capacity to identify various forms of ground ice was maximized. Ground penetrating radar cross-sections were used to provide more detailed information on cryostructures and contacts between massive ice and gravelly sand deposits.

A comparative integrated geophysical study of Horseshoe Chimney Cave, Colorado Bend State Park, Texas

An integrated geophysical study was performed over a known cave in Colorado Bend State Park (CBSP), Texas, where shallow karst features are common within the Ellenberger Limestone. Geophysical survey such as microgravity, ground penetrating radar (GPR), direct current (DC) resistivity, capacitively coupled (CC) resistivity, induced polarization (IP) and ground conductivity (GC) measurements were performed in an effort to distinguish which geophysical method worked most effectively and efficiently in detecting the presence of subsurface voids, caves and collapsed features. Horseshoe Chimney Cave (HCC), which is part of a larger network of cave systems, provides a good control environment for this research. A 50 x 50 meter grid, with 5 m spaced traverses was positioned around the entrance to HCC. Geophysical techniques listed above were used to collect geophysical data which were processed with the aid of commercial software packages. A traditional cave survey was conducted after geophysical data collection, to avoid any bias in initial data collection. The survey of the cave also provided ground truthing. Results indicate the microgravity followed by CC resistivity techniques worked most efficiently and were most cost effective, while the other methods showed varying levels of effectiveness.

Using Ground-Penetrating Radar and Capacitively Coupled Resistivity to Investigate 3-D Fluvial Architecture and Grain-Size Distribution of a Gravel Floodplain in Northeast British Columbia, Canada

Abstract This study combines ground-penetrating radar (GPR) and capacitively coupled resistivity (CCR) for geophysical architecture-analysis of a bar platform and channel bend on the floodplain of a poorly organized wandering gravel-bed river. An important objective of fluvial architectural analysis is linking fluvial style with preserved subsurface sedimentology. However, architectural analysis relies on opportunistic outcrops with locations or orientations that may not provide appropriate data. GPR is a well-established geophysical method that images reflections interpreted to represent bed geometry and bounding surfaces and is therefore ideal for imaging fluvial architecture. Unfortunately, grain-size information, which is integral to architectural classification, is more elusive using GPR. Resistivity data can be used as a proxy for sediment grain size. When GPR and CCR are combined they offer an effective tool for geophysical fluvial architectural analysis. Five trenches provided direct observation of the subsurface sedimentology and are used to calibrate the two geophysical methods. Eight radar facies and one radar element are classified from the GPR survey and grouped into four categories: horizontal and subhorizontal, laterally continuous reflections (Group 1), clinoform reflections (Group 2), discontinuous reflections (Group 3), and concave-up elements (Group 4). The 2-D resistivity data are combined in a GIS to establish a 3-D resistivity model for the upper 5 m of the floodplain. Resistivity values correlate well with grain size and are categorized into fine-grained ( 800 ohm-m) sediments. In general, resistivity values indicate coarse sediment in the bar platform and sand to fine-grained material in the main channel and subordinate depressions. Resistivity profiles were extracted from the 3-D model along the GPR lines so that resistivity values could be directly compared to radar facies. The shape and spread of the resistivity distributions provide dominant grain size as well as an indication of sediment sorting for individual radar facies. In some cases, the same radar facies is associated with markedly different grain sizes, indicating different architectural elements (i.e., horizontally bedded gravel verses horizontally bedded sand or fine-grained sediment). This demonstrates the utility of combining GPR and CCR, insomuch as radar architecture alone is not diagnostic of fluvial architectural elements. In this case study, the bar platform and channel have a planform that might suggest a meandering fluvial style. However, GPR-CCR results indicate that the dominant depositional process across the bar platform was vertical accretion of gravel sheets, an architecture more consistent with a wandering gravel-bed fluvial style. Lateral migration was limited to the outer downstream margin of the platform, a location dominated by a mix of sand and gravel. Coarse gravel likely occupies the base of the main channel, with fine-grained sediment contributing to the remainder of the channel fill. A four-phase history is presented where the initial phase of development involves vertical accretion and migration of stacked gravel-sheets during floods, forming the core of the bar platform. After flooding subsides and the interior bar emerges, the channel becomes established and lateral migration becomes the dominant depositional process. The evolution concludes with avulsion and eventual abandonment of the channel.

Geophysical mapping of ground ice using a combination of capacitive coupled resistivity and ground‐penetrating radar, Northwest Territories, Canada

The nature and distribution of ground ice are two of the most unpredictable geological variables in near‐surface deposits characterized by continuous permafrost. Subsurface information about ground ice distribution and structure can be obtained either by invasive and environmentally destructive techniques like drilling and excavation or by noninvasive low‐impact geophysical methods. In this study, coordinated measurements by two complementary geophysical tools, capacitive‐coupled resistivity (CCR) and ground‐penetrating radar (GPR) were used to map ground ice in a variety of locations in the Mackenzie Delta region of the western Canadian Arctic. Both CCR and GPR systems are highly portable (especially on snow covered surfaces) and very effective in collecting data under winter conditions when cold ground temperatures ensure that nearly all liquid water is frozen and signal penetration is enhanced. CCR and GPR readily detect stratigraphic differences including the contacts between massive ice deposits and enclosing sediments. GPR is widely used in permafrost research, but CCR has been used in only a few studies. This is the first study to combine results from both systems by collecting complementary data sets along coincident transects. We demonstrate that when combined, these data increase the quality and interpretation of subsurface information beyond what could be determined by either of the instruments alone. The complementary nature of these two geophysical tools facilitated the detection and mapping of massive ground ice, ice‐rich sediments, ice wedges, thermokarst, and basic stratigraphic relationships. This study breaks new ground by documenting the benefits of using these techniques together in permafrost investigations.

COMPARISON OF ELECTROMAGNETIC INDUCTION, CAPACITIVELY-COUPLED RESISTIVITY, AND GALVANIC CONTACT RESISTIVITY METHODS FOR SOIL ELECTRICAL CONDUCTIVITY MEASUREMENT

In situ measurement of apparent soil electrical conductivity (ECa) is an important precision agriculture tool usefulfor determining spatial changes in soil properties. Three near-surface geophysical methods are available for rapid,continuous measurement of ECa in agricultural fields. These methods are electromagnetic induction (EMI), capacitivelycoupled resistivity (CCR), and galvanic contact resistivity (GCR). Acceptance for using geophysical methods to gauge spatialchanges in soil properties hinges to a significant degree on there being consistency of the measured ECa spatial patternbetween geophysical methods. Testing of all three methods was conducted on two adjacent test plots having fine-grained soilsand during two time periods with dissimilar shallow hydrologic conditions. Different operational modes for each of the threegeophysical methods were evaluated, including three primary electromagnetic field frequencies (8190, 14610, and 20010 Hz)used for the EMI method, four spacing distances (0.625, 1.25, 2.5, and 5.0 m) between the two dipoles employed with the CCRmethod, and two different Schlumberger electrode array lengths (0.7 and 2.1 m) utilized for the GCR method. Therefore, atotal of nine geophysical method - operational mode combinations were tested.<br><br>Based on spatial correlation analysis, the areal ECa patterns measured by the nine method - operational modecombinations showed substantial similarity to each other, with one exception. The exception was the short electrode arraymode of the GCR method that when paired with the various modes of the other two geophysical methods, exhibited an averagecorrelation coefficient, r, that ranged between only 0.30 and 0.45. All other average r values for pairs of different geophysicalmethod - operational mode combinations ranged between 0.62 and 0.97. Spatial correlation coefficients, for both test plots,between the same method - operational mode combination, but at two different times in which hydrologic conditions varied,ranged from 0.55 to 0.95 for eight of the nine method - operational mode combinations, with the GCR short electrode arrayhaving values of 0.32 and 0.58. Regarding the test plot EC average or median, there were substantial differences in valuesobtained by the three geophysical methods. Electromagnetic vertical sounding measurements along with results obtained byCCR, and GCR surveying, when combined, indicate that for both test plots, from the surface to a depth of a little over 2 m,soil electrical conductivity generally increased first and then decreased. Most importantly, although the measured ECamagnitudes vary between the three geophysical methods, results show that EMI, CCR, and GCR all provide useful andconsistent information on soil electrical conductivity spatial patterns.