Mapping of quick clay by electrical resistivity tomography under structural constraint

Geotechnical projects usually rely on traditional sounding and drilling investigations. Drilling only provides point information and the geology needs to be interpolated between these points. Near surface geophysical methods can provide information to fill those gaps. Norwegian case studies are presented to illustrate how two-dimensional electrical resistivity tomography (ERT) can be used to accurately map the extent of quick clay deposits. Quick clay may be described as highly sensitive marine clay that changes from a relatively stiff condition to a liquid mass when disturbed. Quick clay slides present a geo-hazard and therefore layers of sensitive clay need to be mapped in detail. They are usually characterized by higher resistivity than non-sensitive clay and ERT is therefore a suitable approach to identify their occurrence. However, our experience shows that ERT cannot resolve this small resistivity contrast near large anomalies such as a bedrock interface. For this reason, a constrained inversion of ERT data was applied to delineate quick clay extent both vertically and laterally. As compared to the conventional unconstrained inversions, the constrained inversion models exhibit sharper resistivity contrasts and their resistivity values agree better with in situ measurements.     

River terrace sand and gravel deposit reserve estimation using three-dimensional electrical resistivity tomography for bedrock surface detection

We describe the application of 3D electrical resistivity tomography (ERT) to the characterisation and reserve estimation of an economic fluvial sand and gravel deposit. Due to the smoothness constraints used to regularise the inversion, it can be difficult to accurately determine the geometry of sharp interfaces. We have therefore considered two approaches to interface detection that we have applied to the 3D ERT results in an attempt to provide an accurate and objective assessment of the bedrock surface elevation. The first is a gradient-based approach, in which the steepest gradient of the vertical resistivity profile is assumed to correspond to the elevation of the mineral/bedrock interface. The second method uses an intrusive sample point to identify the interface resistivity at a location within the model, from which an iso-resistivity surface is identified that is assumed to define the interface. Validation of these methods has been achieved through direct comparison with observed bedrock surface elevations that were measured using real-time-kinematic GPS subsequent to the 3D ERT survey when quarrying exposed the bedrock surface. The gradient-based edge detector severely underestimated the depth to bedrock in this case, whereas the interface resistivity method produced bedrock surface elevations that were in close agreement with the GPS-derived surface. The failure of the gradient-based method is attributed to insufficient model sensitivity in the region of the bedrock surface, whereas the success of the interface resistivity method is a consequence of the homogeneity of the mineral and bedrock, resulting in a consistent interface resistivity. These results highlight the need for some intrusive data for model validation and for edge detection approaches to be chosen on the basis of local geological conditions.     

Combining Electrical Resistivity Tomography and Ground Penetrating Radar to study geological structuring of karst Unsaturated Zone

This paper highlights the efficiency and complementarity of a light package of geophysical techniques to study the structure of karst Unsaturated Zone (UZ) in typical Mediterranean environment where soil cover is thin or absent. Both selected techniques, 2D Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT), are widely used in environmental studies and their application is accessible for a lot of scientists/engineers. However, GPR or ERT alone is not able to provide an enhanced characterization of geological features in karst media. In the present study, GPR results supply a near surface high resolution imaging and thus can provide relevant geological information such as stratifications and fractures. Despite the quality of the results GPR's investigation depth remains limited to around 12 m. Apparent and inverted resistivity provided by ERT surveys shows strong lateral and vertical variations. These variations can inform about general geological structuring and feature orientation. ERT is able to prospect down to 40 m but it's a low resolution integrative technique. In the study area the investigated limestone is a commonly electrical resistive formation (more than 2000 Ω.m). However deeper than 5–7 m, the ERT profiles reveal several zones of moderate resistivity (around 900 Ω.m). In these zones a stratification change corresponding to slanted bedding is clearly identified by GPR results. The combination of both GPR and ERT results can allow a well-established geological interpretation. These moderate resistivity zones with slanted beddings can explain the presence of a perennial water flow point 35 m below the surface of the studied site within the underground gallery of the Low-Noise Underground Laboratory (LSBB).     

Application of electrical and electromagnetic methods to study sedimentary covers in high mountain areas

The results of geophysical studies conducted with selected electrical and electromagnetic methods in the Kondratowa Valley in the Tatra Mountains (the Carpathian Mountains, Poland) are presented in the article. The surveys were performed with the following methods: electrical resistivity tomography (ERT), georadar (GPR) and conductivity meter (CM). The objective of the noninvasive geophysical measurements was to determine the thickness of the Quaternary postglacial sediments that fill the bottom of the valley and to designate the accumulation of boulders deposited on Quaternary sediments. The results of ERT surveys conducted along the axis of the valley allowed to determine the changeability of the thickness of the postglacial sediments and allowed to designate a few areas of occurrence of boulders. The ERT, GPR and CM surveys conducted across the valley allowed to designate with high accuracy the thickness of the accumulation of boulders sliding down the valley bottom from the couloirs surrounding the valley.     

The use of non-conventional CPTe data in determination of 3-D electrical resistivity distribution

The spatial distribution of the electrical resistivity data provides useful information for investigating and modeling the fluid transport processes. 3D electrical resistivity distribution provides information about water flow and changes in electrical resistivity of the pore fluid.Therefore, to assist in understanding and modeling of the fluid transport process, 3D spatial distribution of the electrical resistivity data with the corresponded 3D geological section were mapped and interpreted in the test site located in western Germany. A process of deriving electrical resistivity values from the mechanical and radioactive parameters of cone penetration tests (CPT) and geological information of boreholes was presented. A reliable method which gives accurate resistivity values in cases of near surface sediments was introduced. Then a field test was executed where the calculated resistivity values were compared with the measured CPTe resistivity data. The CPTe (cone penetration test with electrical extension) data were also used in correlating to the ERT (electrical resistivity tomography) data. Consequently, obtained dense CPT surveys give us the possibility to determine a high resolution resistivity distribution of the investigated area.     

Landslide-dammed lake sediment volume calculation using waterborne ERT and SONAR profiling

Lake sediment volume calculation is a challenging task, namely in cases when detailed drilling is complicated, expensive, or impossible, information on the pre-sedimentation surface unavailable, and record of siltation rate non-existent or too short. This study shows how waterborne, non-invasive geophysical survey, such as electrical resistivity tomography (ERT) can be very effective in acquiring the missing data, namely when combined with sound navigation ranging (SONAR) water depth measurements and supported by information from auxiliary sources. However, ERT surveying in water environment requires specific approaches, as we illustrate on the case of the Mladotice lake study. The lake was created after a landslide in May 1872, and since its formation, the depth has gradually decreased due to sedimentation. We have reconstructed the original surface, calculated the sediment volume, and compiled information on sedimentation to estimate its remaining life span. To achieve this, we measured nine waterborne ERT profiles across the lake. To reach the necessary depth, all ERT profiles were extended on land and crossed the lake using custom-built flotation pads. ERT profiling was combined with SONAR depth measurements, historical bathymetric surveys, borehole core analysis, sediment flux measurements, volumetric calculations, and water conductivity probing. The study has achieved three main results. First, practical applicability and advantages of stationary waterborne ERT profiling in combination with bathymetric sounding were demonstrated. Second, the original lake volume and accumulated sediment was calculated. We estimate that the volume of lake sediment is 187 000 m³, two-thirds of the original lake volume (over 275 000 m³). Finally, based on three volumetric data sets from 1972, 2003, and 2017, and recent monitoring of the sediment inflow, we propose scenarios of lake filling and its future development. Most interestingly, the sedimentation rate has decreased significantly in the last 20 years, suggesting that the lake may survive much longer than hitherto expected. © 2020 John Wiley & Sons, Ltd.     

Evaluation of trenching,ground penetrating radar (GPR) and electrical resistivity tomography (ERT) for sinkhole characterization

This paper explores the suitability and advantages of combining the trenching technique with geophysical surveys [ground penetrating radar (GPR) and electrical resistivity tomography (ERT)] for sinkhole characterization in a mantled karst area. The approach is applied to two active sinkholes concealed by anthropogenic deposits and formed by contrasting subsidence mechanisms; collapse and sagging. The ERT section acquired across the collapse sinkhole images the clayey fill of the depression as an obvious low resistivity area, showing the approximate location of the sinkhole edges. Spatially dense GPR surveys provide information on the position of the boundaries of the concealed subsidence structures and their three‐dimensional (3D) internal geometry, revealing the dominant subsidence mechanism. We illustrate the impact of several factors on the quality of the GPR data such as sinkhole size, nominal frequency of the antennas, antenna shielding, and the presence of backfilled excavations and above‐surface objects. Trenches provided detailed information on the subsurface structure of the sinkhole, subsidence magnitude, partitioning of the strain, and the position of the sinkhole edges, especially when they are deep enough and excavated across the central sector and perpendicular to the boundaries. The stratigraphic and structural relationships observed in the trench were then used to infer the spatial evolution of the sinkholes (e.g. enlargement), their kinematic behavior (episodic versus progressive), and to differentiate discrete subsidence events and their associated magnitude. Numerical dates were used to estimate average subsidence rates and the recurrence of subsidence events. Such integrated data sets may be used as an objective basis to forecast the future behavior of potentially damaging sinkholes and to assess the associated hazard and risk. Copyright © 2013 John Wiley & Sons, Ltd.     

Arctic rockwall retreat rates estimated using laboratory‐calibrated ERT measurements of talus cones in Longyeardalen,Svalbard

Holocene rockwall retreat rates quantify integral values of rock slope erosion and talus cone evolution. Here we investigate Holocene rockwall retreat of exposed arctic sandstone cliffs in Longyeardalen, central Svalbard and apply laboratory‐calibrated electrical resistivity tomography (ERT) to determine talus sediment thickness. Temperature–resistivity functions of two sandstone samples are measured in the laboratory and compared with borehole temperatures from the talus slope. The resistivity of the higher and lower‐porosity sandstone at relevant borehole permafrost temperatures defines a threshold range that accounts for the lithological variability of the dominant bedrock and debris material. This helps to estimate the depth of the transition from higher resistivities of ice‐rich debris to lower resistivities of frozen bedrock in the six ERT transects. The depth of the debris–bedrock transition in ERT profiles is confirmed by a pronounced apparent resistivity gradient in the raw data plotted versus depth of investigation. High‐resolution LiDAR‐scanning and ERT subsurface information were collated in a GIS to interpolate the bedrock surface and to calculate the sediment volume of the talus cones. The resulting volumes were referenced to source areas to calculate rockwall retreat rates. The rock mass strength was estimated for the source areas. The integral rockwall retreat rates range from 0.33 to 1.96 mm yr^–1, and are among the highest rockwall retreat rates measured in arctic environments, presumably modulated by harsh environmental forcing on a porous sandstone rock cliff with a comparatively low rock mass strength. Here, we show the potential of laboratory‐calibrated ERT to provide accurate estimates of rockwall retreat rates even in ice‐rich permafrost talus slopes. Copyright © 2012 John Wiley & Sons, Ltd.     

Fluid injection monitoring using electrical resistivity tomography — five years of injection at Ketzin,Germany

Between the years 2008 and 2013, approximately 67 kilotons of CO₂ have been injected at the Ketzin site, Germany. As part of the geophysical monitoring programme, time‐lapse electrical resistivity tomography has been applied using crosshole and surface‐downhole measurements of electrical resistivity tomography. The data collection of electrical resistivity tomography is partly based on electrodes that are permanently installed in three wells at the site (one injection well and two observation wells). Both types of ERT measurements consistently show the build‐up of a CO₂‐related resistivity signature near the injection point. Based on the imaged resistivity changes and a petrophysical model, CO₂ saturation levels are estimated. These CO₂ saturations are interpreted in conjunction with CO₂ saturations inferred from neutron‐gamma loggings. Apart from the CO₂–brine substitution response in the observed resistivity changes, significant imprints from the dynamic behaviour of the CO₂ in the reservoir are observed.     

Characterization of seasonal local recharge using electrical resistivity tomography and magnetic resonance sounding

A groundwater recharge process of heterogeneous hard rock aquifer in the Moole Hole experimental watershed, south India, is being studied to understand the groundwater flow behaviour. Significant seasonal variations in groundwater level are observed in boreholes located at the outlet area indicating that the recharge process is probably taking place below intermittent streams. In order to localize groundwater recharge zones and to optimize implementation of boreholes, a geophysical survey was carried out during and after the 2004 monsoon across the outlet zone. Magnetic resonance soundings (MRS) have been performed to characterize the aquifer and measure groundwater level depletion. The results of MRS are consistent with the observation in boreholes, but it suffers from degraded lateral resolution. A better resolution of the regolith/bedrock interface is achieved using electrical resistivity tomography (ERT). ERT results are confirmed by resistivity logging in the boreholes. ERT surveys have been carried out twice—before and during the monsoon—across the stream area. The major feature of recharge is revealed below the stream with a decrease by 80% of the calculated resistivity. The time‐lapse ERT also shows unexpected variations at a depth of 20 m below the slopes that could have been interpreted as a consequence of a deep seasonal water flow. However, in this area time‐lapse ERT does not match with borehole data. Numerical modelling shows that in the presence of a shallow water infiltration, an inversion artefact may take place thus limiting the reliability of time‐lapse ERT. A combination of ERT with MRS provides valuable information on structure and aquifer properties respectively, giving a clue for a conceptual model of the recharge process: infiltration takes place in the conductive fractured‐fissured part of the bedrock underlying the stream and clayey material present on both sides slows down its lateral dissipation. Copyright © 2007 John Wiley & Sons, Ltd.     

Combined electrical and electromagnetic imaging of hot fluids within fractured rock in rugged Himalayan terrain

Integrated electrical resistance tomography (ERT) and short-offset transient electromagnetic (TEM) measurements were carried out to investigate a geothermal area in the Main Central Thrust (MCT) zone of Garhwal Himalayan region, India. The study area is located around Helang on either side of Alaknanda River and it is dotted with hot water springs with water temperature of 45°–55 °C emerging at the surface.To assess the geothermal potential and its lateral and vertical extension in and around the hot water springs in the study area, 7 ERT profiles and 21 TEM stations on 7 profiles were established around the hot water spring and at far distant locations. The 2D inversion of ERT data indicates a low resistivity (< 50 Ωm) zone in the vicinity of hot springs, which appears to be associated with an underground water channel through the fractured rock. The bedrock resistivity is very high (> 1000 Ωm) whereas the resistivity of the weathered near surface soil at a far distant location from the hot spring is low (< 100 Ωm) again. A common feature of all TEM data is the sign reversal observed at roughly 10 μs. The consistent sign reversal in all TEM data indicates the existence of the multi-dimensionality of the geoelectrical structure. Therefore, the TEM data were treated by using the SLDM (Spectral Lanczos Decomposition Method) 2D/3D forward modeling code based on the finite difference algorithm. The resistivity structure obtained from ERT data was used as an input for the modeling of TEM data. Based on the joint analysis of the ERT and TEM data it can be inferred that geothermal anomalies associated with the hot spring in the MCT zone are a local feature appearing as a low resistivity zone (< 50 Ωm) at shallow depth (< 100 m) in the vicinity of the hot spring region.     

高密度电阻率法在果香峪溶洞勘查中的应用效果分析

地下岩溶的发育影响着地面基础工程的安全使用,所以,我们要对岩溶地区发育特征进行全面的了解,以采取相应的措施保证该地区地面建筑的安全。溶洞的空间赋存状态一般是不规则的三维地质体,它的发育与地下水系及该地区的地质构造有紧密联系,造成溶洞的洞室可能含有高阻或低阻填充物,导致溶洞与围岩之间存在物性差异。高密度电阻率法正是利用物...     

Characterization of a dismissed landfill via electrical resistivity tomography and mise-à-la-masse method

Electrical resistivity methods are widely used for environmental applications, and they are particularly useful for the characterization and monitoring of sites where the presence of contamination requires a thorough understanding of the location and movement of water, that can act as a carrier of solutes. One such application is landfill studies, where the strong electrical contrasts between waste, leachate and surrounding formations make electrical methods a nearly ideal tool for investigation. In spite of the advantages, however, electrical investigation of landfills poses also challenges, both logistical and interpretational. This paper presents the results of a study conducted on a dismissed landfill, close to the city of Corigliano d'Otranto, in the Apulia region (Southern Italy). The landfill is located in an abandoned quarry, that was subsequently re-utilized about thirty years ago as a site for urban waste disposal. The waste was thought to be more than 20 m thick, and the landfill bottom was expected to be confined with an HDPE (high-density poli-ethylene) liner. During the digging operations performed to build a nearby new landfill, leachate was found, triggering an in-depth investigation including also non-invasive methods. The principal goal was to verify whether the leachate is indeed confined, and to what extent, by the HDPE liner. We performed both surface electrical resistivity tomography (ERT) and mise-à-la-masse (MALM) surveys, facing the severe challenges posed by the rugged terrain of the abandoned quarry complex. A conductive body, probably associated with leachate, was found as deep as 40 m below the current landfill surface i.e. at a depth much larger than the expected 20 m thickness of waste. Given the logistical difficulties that limit the geometry of acquisition, we utilized synthetic forward modeling in order to confirm/dismiss interpretational hypotheses emerging from the ERT and MALM results. This integration between measurements and modeling helped narrow the alternative interpretations and strengthened the confidence in results, confirming the effectiveness of non-invasive methods in landfill investigation and the importance of modeling in the interpretation of geophysical results.     

Two-Dimensional Magnetotelluric Modelling of Ore Deposits: Improvements in Model Constraints by Inclusion of Borehole Measurements

A combination of magnetotelluric (MT) measurements on the surface and in boreholes (without metal casing) can be expected to enhance resolution and reduce the ambiguity in models of electrical resistivity derived from MT surface measurements alone. In order to quantify potential improvement in inversion models and to aid design of electromagnetic (EM) borehole sensors, we considered two synthetic 2D models containing ore bodies down to 3000 m depth (the first with two dipping conductors in resistive crystalline host rock and the second with three mineralisation zones in a sedimentary succession exhibiting only moderate resistivity contrasts). We computed 2D inversion models from the forward responses based on combinations of surface impedance measurements and borehole measurements such as (1) skin-effect transfer functions relating horizontal magnetic fields at depth to those on the surface, (2) vertical magnetic transfer functions relating vertical magnetic fields at depth to horizontal magnetic fields on the surface and (3) vertical electric transfer functions relating vertical electric fields at depth to horizontal magnetic fields on the surface. Whereas skin-effect transfer functions are sensitive to the resistivity of the background medium and 2D anomalies, the vertical magnetic and electric field transfer functions have the disadvantage that they are comparatively insensitive to the resistivity of the layered background medium. This insensitivity introduces convergence problems in the inversion of data from structures with strong 2D resistivity contrasts. Hence, we adjusted the inversion approach to a three-step procedure, where (1) an initial inversion model is computed from surface impedance measurements, (2) this inversion model from surface impedances is used as the initial model for a joint inversion of surface impedances and skin-effect transfer functions and (3) the joint inversion model derived from the surface impedances and skin-effect transfer functions is used as the initial model for the inversion of the surface impedances, skin-effect transfer functions and vertical magnetic and electric transfer functions. For both synthetic examples, the inversion models resulting from surface and borehole measurements have higher similarity to the true models than models computed exclusively from surface measurements. However, the most prominent improvements were obtained for the first example, in which a deep small-sized ore body is more easily distinguished from a shallow main ore body penetrated by a borehole and the extent of the shadow zone (a conductive artefact) underneath the main conductor is strongly reduced. Formal model error and resolution analysis demonstrated that predominantly the skin-effect transfer functions improve model resolution at depth below the sensors and at distance of \(\sim \) 300–1000 m laterally off a borehole, whereas the vertical electric and magnetic transfer functions improve resolution along the borehole and in its immediate vicinity. Furthermore, we studied the signal levels at depth and provided specifications of borehole magnetic and electric field sensors to be developed in a future project. Our results suggest that three-component SQUID and fluxgate magnetometers should be developed to facilitate borehole MT measurements at signal frequencies above and below 1 Hz, respectively.     

Study of water tension differences in heterogeneous sandy soils using surface ERT

Herbaceous vegetation in the Sahel grows almost exclusively on sandy soils which preferentially retain water through infiltration and storage. The hydrological functioning of these sandy soils during rain cycles is unknown. One way to tackle this issue is to spatialize variations in water content but these are difficult to measure in the vadose zone. We investigated the use of Electrical Resistivity Tomography (ERT) as a technique for spatializing resistivity in a non-destructive manner in order to improve our knowledge of relevant hydrological processes. To achieve this, two approaches were examined. First, we focused on a possible link between water tension (which is much easier to measure in the field by point measurements than water content), and resistivity (spatialized with ERT). Second, because ERT is affected by solution non-uniqueness and reconstruction smoothing, we improved the accuracy of ERT inversion by comparing calculated solutions with in-situ resistivity measurements. We studied a natural microdune during a controlled field experiment with artificial sprinkling which reproduced typical rainfall cycles. We recorded temperature, water tension and resistivity within the microdune and applied surface ERT before and after the 3 rainfall cycles. Soil samples were collected after the experiment to determine soil physical characteristics. An experimental relationship between water tension and water content was also investigated. Our results showed that the raw relationship between calculated ERT resistivity and water tension measurements in sand is highly scattered because of significant spatial variations in porosity. An improved correlation was achieved by using resistivity ratio and water tension differences. The slope of the relationship depends on the soil solution conductivity, as predicted by Archie's law when salted water was used for the rain simulation. We found that determining the variations in electrical resistivity is a sensitive method for spatializing the differences in water tension which are directly linked with infiltration and evaporation/drainage processes in the vadose zone. However, three factors complicate the use of this approach. Firstly, the relation between water tension and water content is generally non-linear and dependent on the water content range. This could limit the use of our site-specific relations for spatializing water content with ERT through tension. Secondly, to achieve the necessary optimization of ERT inversion, we used destructive resistivity measurements in the soil, which renders ERT less attractive. Thirdly, we found that the calculated resistivity is not always accurate because of the smoothing involved in surface ERT data inversion. We conclude that further developments are needed into ERT image reconstruction before water tension (and water content) can be spatialized in heterogeneous sandy soils with the accuracy needed to routinely study their hydrological functioning.     

Application of ERT,Saline Tracer and Numerical Studies to Delineate Preferential Paths in Fractured Granites

Accurate quantification of in situ heterogeneity and flow processes through fractured geologic media remains elusive for hydrogeologists due to the complexity in fracture characterization and its multiscale behavior. In this research, we demonstrated the efficacy of tracer-electrical resistivity tomography (ERT) experiments combined with numerical simulations to characterize heterogeneity and delineate preferential flow paths in a fractured granite aquifer. A series of natural gradient saline tracer experiments were conducted from a depth window of 18 to 22 m in an injection well (IW) located inside the Indian Institute of Technology Hyderabad campus. Tracer migration was monitored in a time-lapse mode using two cross-sectional surface ERT profiles placed in the direction of flow gradient. ERT data quality was improved by considering stacking, reciprocal measurements, resolution indicators, and geophysical logs. Dynamic changes in subsurface electrical properties inferred via resistivity anomalies were used to highlight preferential flow paths of the study area. Temporal changes in electrical resistivity and tracer concentration were monitored along the vertical in an observation well located at 48 m to the east of the IW. ERT-derived tracer breakthrough curves were in agreement with geochemical sample measurements. Fracture geometry and hydraulic properties derived from ERT and pumping tests were further used to evaluate two mathematical conceptualizations that are relevant to fractured aquifers. Results of numerical analysis conclude that dual continuum model that combines matrix and fracture systems through a flow exchange term has outperformed equivalent continuum model in reproducing tracer concentrations at the monitoring wells (evident by a decrease in RMSE from 199 to 65 mg/L). A sensitivity analysis on model simulations conclude that spatial variability in hydraulic conductivity, local-scale dispersion, and flow exchange at fracture-matrix interface have a profound effect on model simulations.     

Electrical resistivity tomography to investigate geological structures of the earth's upper crust

It is important to have detailed knowledge of the electrical properties of the earth's crust in order to recognize geological structures and to understand tectonic processes. In the area surrounding the German Continental Deep Drilling Project (KTB), we have used DC dipole–dipole soundings to investigate the electrical conductivity distribution down to a depth of several kilometres. We have adapted the electrical resistivity tomography (ERT) technique, a well-established near-surface method, to large-scale experiments. Independent transmitting and receiving units were used to realize the concept of simultaneous multichannel registration of the scalar electrical potential at 44 dipoles. The measured data yielded apparent resistivities which were inverted to a 2D resistivity model ranging from the surface down to a depth of 4 km. Two highly conductive structures with steep inclination were detected. They are expected to be major fault zones embedded in a metamorphic body. The rather low resistivity ( ρ  < 10 Ωm) can be explained by the existence of graphitic minerals and/or electrolytic fluids.     

井间电磁成像系统应用研究

井间电磁（EM）成像系统的技术目标是实现井间电学特性的直接测量,并提供井间电阻率的二维及三维图像.从1997年开始,胜利油田与美国EMI公司合作,开展了EM技术的应用与研究,主要是通过大型、系统的现场试验,在大井间距的条件下,进一步验证仪器的性能,进行油藏研究的适用性和可行性分析;以系统的现场实测数据为依据,加快成像处理方法和软件的开发;开展穿透金属套管井的EM试验,探讨金属套管条件下的电阻率成像方法。两年来,利用EMI提供的XBH2000型测量系统,对胜利油田典型的低电阻率砂泥岩剖面,成功地进行了3对井、10个井次的试验。试验证明了仪器性能的可靠,取得了测量重复性好、系统完整的井间EM数据.并反演得到电阻率成像,在分析井间油气分布和油层开采动态方面,取得了良好的地质效果.     

Using 2D inversion of magnetic resonance soundings to locate a water-filled karst conduit

A new methodology for magnetic resonance sounding (MRS) data acquisition and interpretation was developed for locating water-filled karst cavities. This methodology was used to investigate the Ouysse karst system in the Poumeyssens shaft in the Causse de Gramat (France). A new 2D numerical MRS response model was designed for improved accuracy over the previous 1D MRS approach. A special survey performed by cave divers confirmed the accuracy of the MRS results. Field results demonstrated that in favourable conditions (a low EM noise environment and a relatively shallow, large target) the MRS method, used with a coincident transmitter/receiver loop, can be an effective tool for locating a water-filled karst conduit. It was shown numerically that because an a priori orientation of the MRS profile with the karst conduit is used in the inversion scheme (perpendicular for instance), any error in this assumption introduces an additional error in locating the karst. However, the resulting error is within acceptable limits when the deviation is less than 30°. The MRS results were compared with an electrical resistivity tomography (ERT) survey. It was found that in Poumeyssens, ERT is not able to locate the water-filled karst. On the other hand, ERT provides additional information about heterogeneities in the limestone.     

Characterization of an unstable rock mass based on borehole logs and diverse borehole radar data

Unstable rocky slopes are major hazards to the growing number of people that live and travel though mountainous regions. To construct effective barriers to falling rock, it is necessary to know the positions, dimensions and shapes of structures along which failure may occur. To investigate an unstable mountain slope distinguished by numerous open fracture zones, we have taken advantage of three moderately deep (51.0–120.8 m) boreholes to acquire geophysical logs and record single-hole radar, vertical radar profiling (VRP) and crosshole radar data. We observed spallation zones, displacements and borehole radar velocity and amplitude anomalies at 16 of the 46 discontinuities identified in the borehole optical televiewer images. The results of the VRP and crosshole experiments were disappointing at our study site; the source of only one VRP reflection was determined and the crosshole velocity and amplitude tomograms were remarkably featureless. In contrast, much useful structural information was provided by the single-hole radar experiments. Radar reflections were recorded from many surface and borehole fracture zones, demonstrating that the strong electrical property contrasts of these features extended some distance into the adjacent rock mass. The single-hole radar data suggested possible connections between 6 surface and 4 borehole fractures and led to the discovery of 5 additional near-surface fracture zones. Of particular importance, they supplied key details on the subsurface geometries and minimum subsurface lengths of 8 of the 10 previously known surface fracture zones and all of the newly discovered ones. The vast majority of surface fracture zones extended at least 40–60 m into the subsurface, demonstrating that their depth and surface dimensions are comparable.