Electrical resistivity and seismic refraction tomography to detect buried cavities

Near‐surface cavities can pose serious hazards to human safety, especially in highly urbanized town centres. The location of subsurface voids, the estimation of their size and the evaluation of the overburden thickness are necessary to assess the risk of collapse. In this study, electrical resistivity tomography (ERT) and seismic refraction tomography data are integrated in a joint interpretation process for cavity location in the city of Rome. ERT is a well established and widely employed method for cavity detection. However, additional information provided by seismic refraction tomography is capable of eliminating some potential pitfalls in resistivity data interpretation. We propose that the structure of the cavities defined by ERT can be used as a base to optimize seismic refraction tomography investigations within the framework of a joint interpretation process. Data integration and the insertion of a priori information are key issues for reducing the uncertainties associated with the inversion process and for optimizing both acquisition procedures and computation time. Herein, the two geophysical methods are tested on both synthetic and real data and the integration of the results is found to be successful in detecting isolated cavities and in assessing their geometrical characteristics. The cavity location inferred by geophysical non‐invasive methods has been subsequently confirmed by direct inspection.     

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.     

Deep exploration using long-offset transient electromagnetics: interpretation of field data in time and frequency domain

In the framework of the Deep Electromagnetic Soundings for Mineral Exploration project, we conducted ground-based long-offset transient-electromagnetic measurements in a former mining area in eastern Thuringia, Germany. The large-scale survey resulted in an extensive dataset acquired with multiple high-power transmitters and a high number of electric and magnetic field receivers. The recorded data exhibit a high data quality over several decades of time and orders of magnitude. Although the obtained subsurface models indicate a strong multi-dimensional subsurface with variations in resistivity over three orders of magnitude, the electrical field step-on transients are well fitted using a conventional one-dimensional inversion. Due to superimposed induced polarization effects, the transient step-off data are not interpretable with conventional electromagnetic inversion. For further interpretation in one and two dimensions, a new approach to evaluate the long-offset transient-electromagnetic data in frequency domain is realized. We present a detailed workflow for data processing in both domains and give an overview of technical obstructions that can occur in one domain or the other. The derived one-dimensional inversion models of frequency-domain data show strong multi-dimensional effects and are well comparable with the conventional time domain inversion results. To adequately interpret the data, a 2.5D frequency-domain inversion using the open source algorithm MARE2DEM (Modeling with Adaptively Refined Elements for 2-D EM) is carried out. The inversion leads to a consistent subsurface model with shallow and deep conductive structures, which are confirmed by geology and additional geophysical surveys.     

Non-invasive Geophysical Surveys in Search of the Roman Temple of Augustus Under the Cathedral of Tarragona (Catalonia,Spain): A Case Study

An integrated geophysical survey has been conducted at the Tarragona’s Cathedral (Catalonia, NE Spain) with the aim to confirm the potential occurrence of archaeological remains of the Roman Temple dedicated to the Emperor Augustus. Many hypotheses have been proposed about its possible location, the last ones regarding the inner part of the Cathedral, which is one of the most renowned temples of Spain (twelfth century) evolving from Romanesque to Gothic styles. A geophysical project including electrical resistivity tomography (ERT) and ground probing radar (GPR) was planned over 1 year considering the administrative and logistic difficulties of such a project inside a cathedral of religious veneration. Finally, both ERT and GPR have been conducted during a week of intensive overnight surveys that provided detailed information on subsurface existing structures. The ERT method has been applied using different techniques and arrays, ranging from standard Wenner–Schlumberger 2D sections to full 3D electrical imaging with the advanced Maximum Yield Grid array. Electrical resistivity data were recorded extensively, making available many thousands of apparent resistivity data to obtain a complete 3D image after a full inversion. In conclusion, some significant buried structures have been revealed providing conclusive information for archaeologists. GPR results provided additional information about shallowest structures. The geophysical results were clear enough to persuade religious authorities and archaeologists to conduct selected excavations in the most promising areas that confirmed the interpretation of geophysical data. In conclusion, the significant buried structures revealed by geophysical methods under the cathedral were confirmed by archaeological digging as the basement of the impressive Roman Temple that headed the Provincial Forum of Tarraco, seat of the Concilium of Hispania Citerior Province.     

Characterizing hydrological processes in a semiarid rangeland watershed: A hydrogeophysical approach

The complex ecohydrological processes of rangelands can be studied through the framework of ecological sites (ESs) or hillslope‐scale soil–vegetation complexes. High‐quality hydrologic field investigations are needed to quantitatively link ES characteristics to hydrologic function. Geophysical tools are useful in this context because they provide valuable information about the subsurface at appropriate spatial scales. We conducted 20 field experiments in which we deployed time‐lapse electrical resistivity tomography (ERT), variable intensity rainfall simulation, ground‐penetrating radar (GPR), and seismic refraction, on hillslope plots at five different ESs within the Upper Crow Creek Watershed in south‐east Wyoming. Surface runoff was measured using a precalibrated flume. Infiltration data from the rainfall simulations, coupled with site‐specific resistivity–water content relationships and ERT datasets, were used to spatially and temporally track the progression of the wetting front. First‐order constraints on subsurface structure were made at each ES using the geophysical methods. Sites ranged from infiltrating 100% of applied rainfall to infiltrating less than 60%. Analysis of covariance results indicated significant differences in the rate of wetting front progression, ranging from 0.346 m min^?1/2 for sites with a subsurface dominated by saprolitic material to 0.156 m min^?1/2 for sites with a well‐developed soil profile. There was broad agreement in subsurface structure between the geophysical methods with GPR typically providing the most detail. Joint interpretation of the geophysics showed that subsurface features such as soil layer thickness and the location of subsurface obstructions such as granite corestones and material boundaries had a large effect on the rate of infiltration and subsurface flow processes. These features identified through the geophysics varied significantly by ES. By linking surface hydrologic information from the rainfall simulations with subsurface information provided by the geophysics, we can characterize the ES‐specific hydrologic response. Both surface and subsurface flow processes differed among sites and are directly linked to measured characteristics.     

2D cooperative inversion of direct current resistivity and gravity data: A case study of uranium bearing target rock

Interpretation of a single geophysical data set is not sufficient to get complete subsurface information. Cooperative or joint inversion of geophysical data sets is the preferred method for most case studies. In the present study, we present the results of the cooperative inversion approach of direct current resistivity and gravity data. The algorithm uses fuzzy c-means clustering to determine the petrophysical relationship between density and resistivity to obtain the similarity. Synthetic data set has demonstrated that the cooperative inversion approach can produce more reliable and better resistivity and density models of the subsurface as compared to those obtained through individual inversions. To utilize the presented cooperative inversion algorithm, the number of geologic units (number of clusters) in the study region must be known a priori. As a field study, the cooperative inversion approach was used to identify the extension of uranium-bearing target rock around the Beldih open cast mine. We noted the inconsistencies in both resistivity and density models obtained from the individual inversions. However, the presented cooperative inversion approach was able to produce similar resistivity and density models while maintaining the same error level of the respective individual inversions. We have considered four geologic units in the presented cooperative inversion as a field case study. We have also compared our cooperative results with drilled borehole and found to be a reliable tool to differentiate between the target rocks (kaolinite and quartz–magnetite–apatite rocks) and the ultramafic rock (host rock quartzite/alkaline granite). However, this study is subject to certain limitations such as the inability to differentiate between closely spaced kaolinite and quartz–magnetite–apatite rocks.     

Calibrating a Salt Water Intrusion Model with Time‐Domain Electromagnetic Data

Salt water intrusion models are commonly used to support groundwater resource management in coastal aquifers. Concentration data used for model calibration are often sparse and limited in spatial extent. With airborne and ground‐based electromagnetic surveys, electrical resistivity models can be obtained to provide high‐resolution three‐dimensional models of subsurface resistivity variations that can be related to geology and salt concentrations on a regional scale. Several previous studies have calibrated salt water intrusion models with geophysical data, but are typically limited to the use of the inverted electrical resistivity models without considering the measured geophysical data directly. This induces a number of errors related to inconsistent scales between the geophysical and hydrologic models and the applied regularization constraints in the geophysical inversion. To overcome these errors, we perform a coupled hydrogeophysical inversion (CHI) in which we use a salt water intrusion model to interpret the geophysical data and guide the geophysical inversion. We refer to this methodology as a Coupled Hydrogeophysical Inversion‐State (CHI‐S), in which simulated salt concentrations are transformed to an electrical resistivity model, after which a geophysical forward response is calculated and compared with the measured geophysical data. This approach was applied for a field site in Santa Cruz County, California, where a time‐domain electromagnetic (TDEM) dataset was collected. For this location, a simple two‐dimensional cross‐sectional salt water intrusion model was developed, for which we estimated five uniform aquifer properties, incorporating the porosity that was also part of the employed petrophysical relationship. In addition, one geophysical parameter was estimated. The six parameters could be resolved well by fitting more than 300 apparent resistivities that were comprised by the TDEM dataset. Except for three sounding locations, all the TDEM data could be fitted close to a root‐mean‐square error of 1. Possible explanations for the poor fit of these soundings are the assumption of spatial uniformity, fixed boundary conditions and the neglecting of 3D effects in the groundwater model and the TDEM forward responses.     

Joint interpretation of hydrological and geophysical data: electrical resistivity tomography results from a process hydrological research site in the Black Forest Mountains,Germany

The use of electrical resistivity tomography (ERT; non‐intrusive geophysical technique) was assessed to identify the hydrogeological conditions at a surface water/groundwater test site in the southern Black Forest, Germany. A total of 111 ERT transects were measured, which adopted electrode spacings from 0·5 to 5 m as well as using either Wenner or dipole‐dipole electrode arrays. The resulting two‐dimensional (2D) electrical resistivity distributions are related to the structure and water content of the subsurface. The images were interpreted with respect to previous classical hillslope hydrological investigations within the same research basin using both tracer methods and groundwater level observations. A raster‐grid survey provided a quasi 3D resistivity pattern of the floodplain. Strong structural heterogeneity of the subsurface could be demonstrated, and (non)connectivities between surface and subsurface bodies were mapped. Through the spatial identification of likely flow pathways and source areas of runoff, the deep groundwater within the steeper valley slope seems to be much more connected to runoff generation processes within the valley floodplain than commonly credited in such environmental circumstances. Further, there appears to be no direct link between subsurface water‐bodies adjacent to the stream channel. Deep groundwater sources are also able to contribute towards streamflow from exfiltration at the edge of the floodplain as well as through the saturated areas overlying the floodplain itself. Such exfiltrated water then moves towards the stream as channelized surface flow. These findings support previous tracer investigations which showed that groundwater largely dominates the storm hydrograph of the stream, but the source areas of this component were unclear without geophysical measurements. The work highlighted the importance of using information from previous, complementary hydrochemical and hydrometric research campaigns to better interpret the ERT measurements. On the other hand, the ERT can provide a better spatial understanding of existing hydrochemical and hydrometric data. Copyright © 2009 John Wiley & Sons, Ltd.     

浅地表地球物理技术在岩土工程中的应用与挑战

地球物理勘探方法在岩土工程的应用已有很长一段时间,但其成效与工程师的期待往往有不小的落差,以致于在一般的工程应用上仍不普遍.近年来浅地表地球物理技术有显着的进展,特别是在走时速度层析成像(Traveltime Tomography)、电阻率层析成像法(Electrical Resistivity Tomography)及多道瞬态面波法(Multichannel Analysis of Surface Wave).本文首先介绍这些方法在台湾岩土工程的应用,主要包括地层土壤液化潜能评估、坝体的安全检测、土壤与地下水污染调查及地基改良的质量管控等,应用案例以台湾常使用的地球物理勘探方法逐一介绍.虽然许多成功案例与新的应用方向对于浅地表地球物理技术在岩土工程应用的推广起了鼓舞作用,本文从工程师的角度提出地球物理勘探工程大量应用的挑战与瓶颈,包括如何提升探测数据的客观性、数据反演非唯一性问题、探测深度与分辨率的限制、实际条件违背反演基本假设的情况、以及地物性质与工程性质链接的不确定性问题,并进一步针对这些问题说明相关研究的进展与实务对策.希冀透过上述探讨,降低物探师与工程师认知上的差距,提升地球物理勘探在工程的应用的合理性与普及性.     

采空区地震与电法探测数据的联合反演研究及应用

综合地球物理技术在采空区的探测中发挥了重要作用.目前通常采用单方法反演、仅对不同方法反演结果进行对比解释的综合勘探方式,单方法反演的多解性严重降低了其探测精度.如何提高采空区的探测精度,对采空区进行有效探测一直被认为是地球物理技术面临的首要难题.为了提高地震与电法技术的探测精度,基于交叉梯度联合反演理论,设计了地震初至折射走时数据和高密度电法数据的联合反演算法流程,对采空区理论模型和野外实际数据进行了联合反演处理.结果发现通过两者的联合反演,不仅可以提高采空区电阻率反演模型的成像效果,而且能够获得地震单方法反演难以成像的采空区低速异常体,从而提高了地震与电法技术对采空区的探测精度.表明地震与电法探测数据联合反演是一种提高采空区探测精度的有效方法.     

Electrical resistivity tomography survey for delineating uncharted mine galleries in West Bengal,India*

The history of subsidence, fires, flooding and other kinds of environmental hazards related to shallow coal workings in India goes back to colonial times some 300 years ago. As coal production accelerated in modern times, so did the environmental and socio-economic drawbacks related to exploitation. In the mid-1980s, a hydropneumatic sand-stowing method was developed to fill in abandoned galleries but their exact location had to be known. Unfortunately, most of these old workings are uncharted and consequently large tracts of land cannot be stabilized. A research programme making use of integrated surface, borehole and cross-hole geophysical methods was undertaken over a five-year span to try to solve this problem. Surface geophysical methods, being cheaper and faster than their cross- and downhole counterparts, were used to cover larger areas on an exploratory basis, while cross-hole methods were employed to locate more accurately one or a network of galleries to be perforated by drillhole(s) and used as a conduit for sand stowing. We report the results of one of the cross-hole geophysical methods: electrical resistivity tomography (ERT). A pole–dipole configuration is used and both cross-hole and surface-borehole methodologies are tested. Forward modelling and inversion of synthetic data making use of downhole and surface physical and geometrical parameters are presented first. This phase is followed by the inversion of real data. It is concluded that ERT is not applicable for the detection of dry voids, but is effective in a waterlogged environment which is estimated to represent 85–90% of the cases. In waterlogged galleries, ERT is applicable in both cross-hole and surface-downhole modes, the latter allowing a larger surface coverage at low cost. ERT is thus a reliable geophysical tool to image water-filled voids and an adequate technique to address environmental and geotechnical problems.     

Out‐of‐plane effects in 2D borehole‐to‐surface resistivity tomography and applications in mineral exploration

In this paper, we discuss the effects of anomalous out‐of‐plane bodies in two‐dimensional (2D) borehole‐to‐surface electrical resistivity tomography with numerical resistivity modelling and synthetic inversion tests. The results of the two groups of synthetic resistivity model tests illustrate that anomalous bodies out of the plane of interest have an effect on two‐dimensional inversion and that the degree of influence of out‐of‐plane body on inverted images varies. The different influences are derived from two cases. One case is different resistivity models with the same electrode array, and the other case is the same resistivity model with different electrode arrays. Qualitative interpretation based on the inversion tests shows that we cannot find a reasonable electrode array to determine the best inverse solution and reveal the subsurface resistivity distribution for all types of geoelectrical models. Because of the three‐dimensional effect arising from neighbouring anomalous bodies, the qualitative interpretation of inverted images from the two‐dimensional inversion of electrical resistivity tomography data without prior information can be misleading. Two‐dimensional inversion with drilling data can decrease the three‐dimensional effect. We employed two‐ and three‐dimensional borehole‐to‐surface electrical resistivity tomography methods with a pole–pole array and a bipole–bipole array for mineral exploration at Abag Banner and Hexigten Banner in Inner Mongolia, China. Different inverse schemes were carried out for different cases. The subsurface resistivity distribution obtained from the two‐dimensional inversion of the field electrical resistivity tomography data with sufficient prior information, such as drilling data and other non‐electrical data, can better describe the actual geological situation. When there is not enough prior information to carry out constrained two‐dimensional inversion, the three‐dimensional electrical resistivity tomography survey is the better choice.     

Assessment of an ancient bridge combining geophysical and advanced photogrammetric methods: Application to the Pont De Coq,France

A high resolution geophysical survey was carried out on the Pont De Coq, a medieval stone arch bridge located in Normandy (France) in 2011 and 2012. Two complementary methods are used: Electrical Resistivity Tomography (ERT) and Ground PenetratingRadar (GPR). They allow to evaluate the structural state of the bridge and to characterize the subsurface around and beneath the bridge. An excellent correlation is obtained between the geophysical methods and the geological data obtained around the bridge. In order to improve the restitution of the geophysical data, an advanced photogrammetric method is performed, providing a high resolution 3D Digital Terrain Model (DTM) of the Pont de Coq. The advanced photogrammetry enhances the presentation of the GPR and ERT data. This approach is an easy-to-use, rapid and cost-effective tool for stakeholders. Finally, it is a promising and original method for improved interpretations of future geophysical surveys.     

Laterally constrained inversion of helicopter-borne frequency-domain electromagnetic data

Helicopter-borne frequency-domain electromagnetic (HEM) surveys are used for fast high-resolution, three-dimensional resistivity mapping. Standard interpretation tools are often based on layered earth inversion procedures which, in general, explain the HEM data sufficiently. As a HEM system is moved while measuring, noise on the data is a common problem. Generally, noisy data will be smoothed prior to inversion using appropriate low-pass filters and consequently information may be lost.For the first time the laterally constrained inversion (LCI) technique has been applied to HEM data combined with the automatic generation of dynamic starting models. The latter is important because it takes the penetration depth of the electromagnetic fields, which can heavily vary in survey areas with different geological settings, into account. The LCI technique, which has been applied to diverse airborne and ground geophysical data sets, has proven to be able to improve the HEM inversion results of layered earth structures. Although single-site 1-D inversion is generally faster and — in case of strong lateral resistivity variations — more flexible, LCI produces resistivity — depth sections which are nearly identical to those derived from noise-free data.The LCI results are compared with standard single-site Marquardt–Levenberg inversion procedures on the basis of synthetic data as well as field data. The model chosen for the generation of synthetic data represents a layered earth structure having an inhomogeneous top layer in order to study the influence of shallow resistivity variations on the resolution of deep horizontal conductors in one-dimensional inversion results. The field data example comprises a wide resistivity range in a sedimentary as well as hard-rock environment.If a sufficient resistivity contrast between air and subsurface exists, the LCI technique is also very useful in correcting for incorrect system altitude measurements by using the altitude as a constrained inversion parameter.     

Time‐Lapse Electrical Geophysical Monitoring of Amendment‐Based Biostimulation

Biostimulation is increasingly used to accelerate microbial remediation of recalcitrant groundwater contaminants. Effective application of biostimulation requires successful emplacement of amendment in the contaminant target zone. Verification of remediation performance requires postemplacement assessment and contaminant monitoring. Sampling‐based approaches are expensive and provide low‐density spatial and temporal information. Time‐lapse electrical resistivity tomography (ERT) is an effective geophysical method for determining temporal changes in subsurface electrical conductivity. Because remedial amendments and biostimulation‐related biogeochemical processes often change subsurface electrical conductivity, ERT can complement and enhance sampling‐based approaches for assessing emplacement and monitoring biostimulation‐based remediation. Field studies demonstrating the ability of time‐lapse ERT to monitor amendment emplacement and behavior were performed during a biostimulation remediation effort conducted at the Department of Defense Reutilization and Marketing Office (DRMO) Yard, in Brandywine, Maryland, United States. Geochemical fluid sampling was used to calibrate a petrophysical relation in order to predict groundwater indicators of amendment distribution. The petrophysical relations were field validated by comparing predictions to sequestered fluid sample results, thus demonstrating the potential of electrical geophysics for quantitative assessment of amendment‐related geochemical properties. Crosshole radar zero‐offset profile and borehole geophysical logging were also performed to augment the data set and validate interpretation. In addition to delineating amendment transport in the first 10 months after emplacement, the time‐lapse ERT results show later changes in bulk electrical properties interpreted as mineral precipitation. Results support the use of more cost‐effective surface‐based ERT in conjunction with limited field sampling to improve spatial and temporal monitoring of amendment emplacement and remediation performance.     

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.     

频率域航空电磁数据变维数贝叶斯反演研究

传统的梯度反演方法已经广泛应用于频率域航空电磁数据处理中,然而此类方法受初始模型影响较大,且容易陷入局部极小.为解决这一问题,本文采用改进的变维数贝叶斯反演方法实现航空电磁数据反演.该方法根据建议分布对反演模型进行随机采样,并依据接受概率筛选合理的候选模型,最终获得反演模型的概率分布和不确定度信息.为解决贝叶斯反演方法对深部低阻层反演效果不佳的问题,本文通过引入合理加权系数,调整对反演模型约束强度,在很大程度上改善了反演效果.通过对模型统计方法进行改进,在遵循原有模型采样方法和接受标准的基础上,将满足数据拟合要求的模型纳入统计范围,削弱不合理模型对统计结果的干扰.本文最后通过对含有高斯噪声的理论数据和实测数据进行反演,并与Occam反演结果进行对比,验证了该方法的有效性.     

Lithological classification assisted by the joint inversion of electrical and seismic data at a control site in northeast Mexico

In this paper, evidence is presented that the combination of geospectral images and geophysical signatures (resistivity–velocity cross-plots) is a good tool to provide a natural visualization of the distribution and variations of lithological features in a test site. This was confirmed by the correlation between the electrical resistivity and seismic velocity values obtained after cross-gradient joint inversion at two profiles and geotechnical information provided by shallow boreholes in a site located in the Earth Sciences School grounds in Linares, Northeastern Mexico. The results obtained from this study show how the cross-gradient joint inversion facilitates the analysis of hydrological estimates and assists in lithological classification of subsurface materials.     

Rapid inversion of data from 2D resistivity surveys with electrode displacements

Resistivity monitoring surveys are used to detect temporal changes in the subsurface using repeated measurements over the same site. The positions of the electrodes are typically measured at the start of the survey program and possibly at occasional later times. In areas with unstable ground, such as landslide‐prone slopes, the positions of the electrodes can be displaced by ground movements. If this occurs at times when the positions of the electrodes are not directly measured, they have to be estimated. This can be done by interpolation or, as in recent developments, from the resistivity data using new inverse methods. The smoothness‐constrained least squares optimisation method can be modified to include the electrode positions as additional unknown parameters. The Jacobian matrices with the sensitivity of the apparent resistivity measurements to changes in the electrode positions are then required by the optimisation method. In this paper, a fast adjoint‐equation method is used to calculate the Jacobian matrices required by the least squares method to reduce the calculation time. In areas with large near‐surface resistivity contrasts, the inversion routine sometimes cannot accurately distinguish between electrode displacements and subsurface resistivity variations. To overcome this problem, the model for the initial time‐lapse dataset (with accurately known electrode positions) is used as the starting model for the inversion of the later‐time dataset. This greatly improves the accuracy of the estimated electrode positions compared to the use of a homogeneous half‐space starting model. In areas where the movement of the electrodes is expected to occur in a fixed direction, the method of transformations can be used to include this information as an additional constraint in the optimisation routine.