Surface Geophysical Techniques Research Articles

ANALYSIS OF FRACTION CONTENTS IN ROCK SCRATCH TESTS FOR ESTIMATING THE ANGLE OF INTERNAL FRICTION FOR THE SEDIMENTARY COVER OF THE KOVYKTA FIELD

The article describes the possibility of using the granulometric analysis of rock cuttings formed in controlled core scratching tests to estimate the angle of internal friction.The study object is the Kovykta gas-condensate field (GCF) that occupies a wide area in the southeastern part of the Irkutsk amphitheater of the Siberian platform. This uniquely complex geological structure holds significant reserves of hydrocarbons. Its sedimentary cover is composed of the Vendian – lower Paleozoic and partly Riphean formations. Their total thickness exceeds 6000 m, as estimated from the new seismic survey data [Vakhromeev et al., 2019].The sedimentary cover of the Kovykta GCF has been studied by surface and borehole geophysical techniques, remote sensing and geostructural methods, in combination with the tectonophysical approach [Seminsky et al., 2018] based on drilling data, including standard and special core sampling data.

Applying of SP, DC-Resistivity, DC-TDIP and TDEM soundings in high saline coastal aquifer

Four surface geophysical techniques were utilized to study the geological and hydrogeological settings of highly saline a coastal aquifer system to the north-east of River Nile Delta, Egypt. These techniques include SP, DC-Resistivity, TDIP and TDEM methods. The first target was to determine the geological stratification as a differentiation among clay, clayey, sand and sandy layers of high saline water. These techniques reflect that there is a complicated lateral and vertical difference in sediments along study area. The surface layers with depth down to ~120 m have low to medium content of clay that change with depth. Then, the second target was the differentiation laterally and vertically for salinity with depth down to ~250m as an interesting hydrogeological setting. These techniques reported that the sediments consist of thin and thick clay and silts, clayey sand, and sandy clay strata. Investigation depth was up to ~210m due to high salinity and clay content effect. At shallow depths, soil texture (down to ~100m and sometimes down to ~160m) consists of clay and silt with sand intercalation. The TEM data indicate a zone of less saline water and low clay content starting from ~40 to ~100m. There may be an evidence for a significant high to medium clay content after these depths down to ~250m. All four methods were calibrated with each other. Accordingly, good matching between the inversion model of TEMSs and composite logs of new drilled well was found, especially in lithological layers identifications. Also, this calibration confirmed that the area was complicated regarding the geological and hydrogeological conditions and the TDIP and TEM are the best methods in studying the environmental, geological and hydrogeological settings as primary important engineering implications for studying coastal highly saline aquifers.

Influence of soil heterogeneity on soybean plant development and crop yield evaluated using time-series of UAV and ground-based geophysical imagery

Understanding the interactions among agricultural processes, soil, and plants is necessary for optimizing crop yield and productivity. This study focuses on developing effective monitoring and analysis methodologies that estimate key soil and plant properties. These methodologies include data acquisition and processing approaches that use unmanned aerial vehicles (UAVs) and surface geophysical techniques. In particular, we applied these approaches to a soybean farm in Arkansas to characterize the soil–plant coupled spatial and temporal heterogeneity, as well as to identify key environmental factors that influence plant growth and yield. UAV-based multitemporal acquisition of high-resolution RGB (red–green–blue) imagery and direct measurements were used to monitor plant height and photosynthetic activity. We present an algorithm that efficiently exploits the high-resolution UAV images to estimate plant spatial abundance and plant vigor throughout the growing season. Such plant characterization is extremely important for the identification of anomalous areas, providing easily interpretable information that can be used to guide near-real-time farming decisions. Additionally, high-resolution multitemporal surface geophysical measurements of apparent soil electrical conductivity were used to estimate the spatial heterogeneity of soil texture. By integrating the multiscale multitype soil and plant datasets, we identified the spatiotemporal co-variance between soil properties and plant development and yield. Our novel approach for early season monitoring of plant spatial abundance identified areas of low productivity controlled by soil clay content, while temporal analysis of geophysical data showed the impact of soil moisture and irrigation practice (controlled by topography) on plant dynamics. Our study demonstrates the effective coupling of UAV data products with geophysical data to extract critical information for farm management.

Time-lapse electrical resistivity tomography mapping of DNAPL remediation at a STAR field site

Time-lapse Electrical Resistivity Tomography (ERT), a surface geophysical technique, was applied for the first time to directly map in-situ DNAPL destruction at a field site. Time-lapse ERT was used to map remediation of coal tar at a site undergoing full-scale application of Self-sustaining Treatment for Active Remediation (STAR). STAR is an in situ thermal treatment based on smoldering combustion that destroys dense non-aqueous phase liquids (DNAPLs) while generating heat, water, and combustion gases. ERT was used to provide spatially and temporally resolved data to complement the conventional discrete and overview data normally collected during STAR operations. A STAR treatment cell targeting coal tar at 2.4 mbgs and below the water table was monitored with 2D surface resistivity surveys conducted before, during and after treatment. Two 36-electrode lines, each 21 m long, were employed. Relative resistivity ratio maps and discrete (nodal) resistivity signal analysis both proved useful in numerous ways. For example, ERT was demonstrated to map likely treated coal-tar zones. This was accomplished by identifying a characteristic, local, temporal resistivity signal pattern corresponding to STAR-induced subsurface temperature and liquid saturation changes. In addition, ERT identified the subsurface areas (i) effectively captured by the vapor extraction system, (ii) possibly missed by the vapor capture system where gases built-up, and (iii) where and when re-infiltration of groundwater occurred post-treatment. In addition, the overall average subsurface resistivity provided a useful metric to correlate with traditional site measures of treatment such as the cumulative amount of DNAPL mass treated. This study provides a successful proof of concept however the maximum potential of the approach is expected with a full 3D survey. Overall, the resistivity surveys provided valuable continuous mapping of the subsurface, including the locations of DNAPL mass destruction, and showed that ERT is promising for supporting evaluations of in situ remediation programs.

Detection Method of Pile Foundation on Subway Lines Based on Cross-Hole Resistivity Computed Tomography

When urban subway lines pass under buildings, it is often necessary to carry out corresponding pile foundation detection work to ensure construction safety. However, detecting the distribution of piles with conventional surface geophysical techniques is not an easy task because of space constraints. The cross-hole resistivity computed tomography (CT) method only needs two drill holes to implement the detection, which can overcome the difficulty in arranging the survey line above the pile foundation. Moreover, this method is suitable for low-resistance body detection, with the advantages of high detection accuracy and less influence from ground interference. In this paper, six types of geoelectric models are constructed for numerical simulation analysis due to two major factors that affect the detection effect of cross-hole resistivity CT: model parameters and measurement parameters. The results show that cross-hole resistivity CT provides an accurate imaging of the shape and distribution of the pile foundation between the holes. Furthermore, the technique provides a good detection of the bottom interface of the pile. The detection effect is best when the electrode spacing is 1 m, the pile is located in the detection center domain, the borehole spacing control is not more than 20 m, and the pile drill hole spacing is not less than 5 m. The research results have effectively guided practical engineering applications and obtained good detection results. The theory and practice show that the cross-hole resistivity CT is a more feasible method for detecting pile foundations on subway lines.

Near-surface site investigation and imaging of karst cave using comprehensive geophysical and laser scanning: a case study in Shandong, China

The karst development of Jiao Dong Peninsula in Shandong Province has formed a unique karst landform, and has become one of the typical karst areas in the north of China. Karst collapse occurred during the construction of high-speed railways and caused many environmental problems. To reveal the development situation of karst area in Shandong, the conventional surface geophysical techniques were used for karst exploration and location of key area. And the results of drilling verification were in good agreement with the results of geophysical exploration. At the same time, the new technology in the field of surveying and mapping 3D laser scanning technology, is cited in the engineering geophysical exploration, and the key issue is the shape and size of the karst. It makes up for the shortcomings of traditional geophysical exploration technology that can only be qualitative and cannot be quantitatively analyzed. It realizes the transformation of karst exploration from regional detection to single cave exploration, and better implements the concept of information construction. It has contributed to the smooth progress of the project and environmental protection.

Investigating Microtopographic and Soil Controls on a Mountainous Meadow Plant Community Using High‐Resolution Remote Sensing and Surface Geophysical Data

AbstractThis study aims to investigate the microtopographic controls that dictate the heterogeneity of plant communities in a mountainous floodplain‐hillslope system, using remote sensing and surface geophysical techniques. Working within a lower montane floodplain‐hillslope study site (750 m × 750 m) in the Upper Colorado River Basin, we developed a new data fusion framework, based on machine learning and feature engineering, that exploits remote sensing optical and light detection and ranging (LiDAR) data to estimate the distribution of key plant meadow communities at submeter resolution. We collected surface electrical resistivity tomography data to explore the variability in soil properties along a floodplain‐hillslope transect at 0.50‐m resolution and extracted LiDAR‐derived metrics to model the rapid change in microtopography. We then investigated the covariability among the estimated plant community distributions, soil information, and topographic metrics. Results show that our framework estimated the distribution of nine plant communities with higher accuracy (87% versus 80% overall; 85% versus 60% for shrubs) compared to conventional classification approaches. Analysis of the covariabilities reveals a strong correlation between plant community distribution, soil electric conductivity, and slope, indicating that soil moisture is a primary control on heterogeneous spatial distribution. At the same time, microtopography plays an important role in creating particular ecosystem niches for some of the communities. Such relationships could be exploited to provide information about the spatial variability of soil properties. This highly transferable framework can be employed within long‐term monitoring to capture community‐specific physiological responses to perturbations, offering the possibility of bridging local plot‐scale observations with large landscape monitoring.

Uncertainty Analysis of Faulting and Folding on Near Surface Aquifers

With advances in near surface geophysical techniques, notably Airborne Electromagnetics (AEM), great strides have been made in mapping near-surface (0-200m) hydrostratigraphy, including aquitards and aquifers. However, an important uncertainty in the mapping of groundwater systems and resource estimation is the potential impact of faults to; Generate fault parallel fracture transmissivity;Reduce across-fault permeability; andProvide connectivity from one aquifer to another across aquitards.It is vital to differentiate between faults and fault propagation folds associated with “blind faults”. As AEM is an evolving technology a set of criteria based on sound structural geomechanical and structural geologic has been developed to discriminate between faults and folds.Even with consistent and robust modeling there are fundamental uncertainties that require appropriate modeling using stochastic fault seal analysis. An extensive database of calibrations has been developed for the prediction of liquid hydrocarbon free water levels (FWL). Based on hundreds of hindcasting models FWL can be predicted with better than 10m accuracy.Despite this high accuracy prediction the juxtaposition area of across fault reservoir vs reservoir juxtaposition can vary by two or three orders of magnitude. The juxtaposition area is a key term in the calculation of across- fault Darcy Flow. Variation in area is strongly nonlinear and dependent on accounting for the thickness of aquitards and variation along strike of fault displacement. It is vital that geologically valid faults are analyzed with the appropriate parameter uncertainty. Examples from the Broken Hill Managed Aquifer Project and the Sydney Basin will be used to illustrate the process.

A multidisciplinary study of groundwater conditions in sedimentary strata at Thirlmere Lakes (NSW)

The Thirlmere Lakes include five natural wetlands within a Blue Mountains World Heritage listed national park, where there are concerns over an apparent possibility of a long term decline in water levels. Lake levels correlate with rainfall variability and are historically known to have dried several times during prolonged droughts. However, the effects of long term hydrological changes on the Lakes are unclear, as are uncertainties associated with extraction of water for local uses and the possible effects of nearby longwall coal mining.This study is part of a large multi-disciplinary research program, of which this part focuses on groundwater conditions insedimentary strata, and the possibilities of interactions with sediments below the Lakes. Surface geophysical techniques and mapping of geological structures will be combined with deep drilling, wireline logging, geological and hydrogeological investigations. Characterisation of sedimentary strata include permeability, bulk density, moisture content, porewater stable water isotopes and XRD mineral identification. New deep drillholes are planned to obtain information on hydraulic properties of formations. A staged geophysical survey program is designed to complement the geological investigation. Resistivity imaging, time-domain electromagnetics and ground penetrating radar (GPR) will be employed to define sedimentary structure within the unconsolidated alluvium (sand, clay and peat layers) and depth estimates to the underlying rock. A combination of these geophysical methods and contextural geological information, will be used to identify structural and sedimentological anomalies and their hydrogeological properties such as permeability and connection to deeper strata. The results of this work provide new data on groundwater conditions in structured rock that underlie the sediments of Thirlmere Lakes.

Tales from the outer limits: Archaeological geophysical prospection in lowland peat environments in the British Isles

AbstractIn order to systematically investigate the potential of conventional near surface geophysical techniques to locate waterlogged archaeological targets in peatlands, the authors applied four conventional geophysical methods – earth resistance, ground‐penetrating radar (GPR), magnetic gradiometry and frequency domain electromagnetics (FDEM) – to four lowland peat archaeological test sites in Great Britain. In this article we demonstrate that a Neolithic trackway was identified in the GPR data in Somerset, with likely ‘proxy’ detections of chemical changes showing up in both electrical and magnetic surveys. This was determined by a coring programme and inductively coupled plasma optical emission spectrometry (ICP‐OES) multi‐element analysis of peat samples to determine the relative concentrations of geophysically relevant chemical elements. Though no Bronze Age timbers were detected at Flag Fen, a post‐Bronze Age agricultural landscape was identified in both the GPR and gradiometer surveys. We conclude that GPR has the greatest potential for archaeological geophysical prospection in peatland environments, but that electrical and magnetic methods can usefully be employed as secondary sources of information and should not be discounted from future research. Further, this article argues that better understandings must be developed of the impacts of geochemistry on geophysical data if we are going to realistically pursue ‘whole landscape’ surveys.

High-resolution shallow reflection seismic integrated with other geophysical methods for hydrogeological prospecting in the Nylsvley Nature Reserve, South Africa

Two perpendicular high-resolution shallow seismic and resistivity profiles were acquired to assist in imaging the near-subsurface sedimentary architecture for hydrogeological prospecting in the Nylsvley Nature Reserve along the Nyl River floodplain in South Africa. We deployed 48 channels of 14 Hz geophones with 1–2 m dense source-receiver spacing providing fold coverage of 24 (line 2) and 48 (line 1). The resistivity profiles were acquired with a Schlumberger electrode configuration with 2–3 m electrode spacing and a total of 40 steel electrodes. The raw seismic shot records were characterized by low-frequency, high-amplitude source-generated noise (surface and guided waves). Reflections from bedrock were not obvious. To enhance the reflected seismic signal, we employed an extensive seismic processing workflow which enhanced the seismic reflectivity on the stacked sections. The seismic reflection interpretation was constrained and integrated with seismic refraction and resistivity tomography, and the one-dimensional multichannel analysis of surface waves to generate the model that best represents the real subsurface geological model. The integrated results show the bedrock-overburden contact at 8–12 m depth, which correlates well with boreholes drilled in the area. In addition, the reflection seismic and refraction tomography show the bedrock undulation and velocity changes associated with erosional surfaces or weathered/fracture systems. We further interpret these characteristics to be associated with groundwater movement and storage related to the fractured/weathered zone within the bedrock. The integrated data also delineate the interface between the unsaturated sand and saturated sand-gravel that represents the groundwater water table. This study demonstrates the potential of combining several surface geophysical techniques for near-surface investigations, especially for hydrogeological prospecting.

Monitoring geothermal reservoir developments with the Controlled-Source Electro-Magnetic method — A calibration study on the Reykjanes geothermal field

Surface geophysical monitoring techniques are important tools for geothermal reservoir management as they provide unique information on the reservoir development away from boreholes. For magmatic environments, electromagnetic (EM) methods are attractive monitoring tools as they allow to characterize the reservoir and hence potentially monitor changes related to fluid injection/production. Indeed, the electrical resistivity of reservoir rocks is highly dependent on the volume, chemistry and phase of the in-situ geothermal brine (e.g. liquid, vapor, supercritical).Passive EM techniques (e.g. magnetotellurics or MT) are traditionally used for geothermal exploration and a few recent studies have demonstrated its potential for monitoring reservoir development. One of the main challenges is though the presence of cultural noise and/or variability of the Earth magnetic field that can obfuscate the EM signals of interest. We have investigated the benefits and drawbacks of active EM surveying (Controlled-Source EM or CSEM) to tackle this challenge, first with a synthetic study and subsequently with an actual time-lapse survey acquired in 2016 and 2017 over the Reykjanes geothermal field in Iceland before (baseline) and after (monitor) the thermal stimulation of the supercritical RN-15/IDDP-2 geothermal well.The synthetic study showed that for geothermal fields having a resistivity structure similar to the Reykjanes field (i.e. a conductive caprock overlying a more resistive higher temperature reservoir), CSEM and MT measurements can both detect resistivity changes within the deep resistive reservoir, provided that measurement errors are small. Variations in many survey parameters (e.g. errors in receiver position/orientation, differences in recording devices, variations of near surface conditions, external noise) can create significant time-lapse CSEM measurement errors. Our actual time-lapse survey showed that when similar CSEM equipment is used during the baseline and monitor surveys and systematically d-GPS positioned, the remaining key parameter controlling the survey repeatability is the level of external noise. Since the influence of external noise on CSEM data can be artificially reduced (e.g. by increasing the transmitter dipolar moment), it offers the possibility to adapt the survey design to increase the chance of detecting the time-lapse signals of interest. On the contrary, little control is possible on the MT signal to noise ratio and hence repeatability.The time-lapse EM survey acquired over the Reykjanes geothermal reservoir showed indeed that a high CSEM survey repeatability can be achieved with electric field measurements (within a few percent) but that time-lapse MT survey is a challenging task because of the high level of cultural noise in this industrialized environment. To assess the quality of our CSEM dataset, we inverted the data and confronted the resulting resistivity model with the resistivity logged in the RN-15/IDDP-2 well. We obtained a good match up to 2-3km depth, i.e. enough to image the caprock and the liquid-dominated reservoir but not deep enough to image the reservoir in supercritical conditions. To obtain such an image, we had to jointly invert legacy MT data with our CSEM data. On the monitoring aspects, the analysis of changes in electric fields did not allow to identify any CSEM signal related to the thermal stimulation of the RN-15/IDDP-2 well. One possible explanation is the weakness of the time-lapse CSEM signal compared the achieved CSEM survey repeatability as a result of a limited resistivity change over a limited volume within the reservoir. Future reservoir developments in the supercritical reservoir (e.g. hydraulic stimulation, long-term fluid circulation) will most likely generate stronger resistivity variations over a larger volume than during the thermal stimulation of the well. This calibration study provides the basic information for deciding when and how an EM monitor survey must be performed for the monitoring of the Reykjanes geothermal reservoir but also for the definition of the monitoring strategy of similar high-enthalpy geothermal reservoirs.

Application of geophysical methods in a dam project: Life cycle perspective and Taiwan experience

There is a growing demand for using non-destructive geophysical techniques to internally image dam condition and facilitate the early detection of anomalous phenomena. Near surface geophysical techniques have advanced significantly in the last few decades, and can play a significant role in the siting, construction, and operational safety and sustainable management of dams. Application of engineering geophysics in site characterization during feasibility investigation phase is already part of the standard of practice. This paper introduces newer applications of engineering geophysics during construction phase, dam safety assessment, and sustainable management, including quality control of compacted soils, investigation of abnormal leakage in an earth dam, evaluation of an aged concrete dam, geophysical health monitoring for a newly-constructed dam, and monitoring of sediment transport for sediment management. The applications were presented with more emphases on the needs of dam engineering and adapting appropriate geophysical methods to make assessment more effective and consequential. The collage of these case studies is to broaden the view of how geophysical methods can be applied to a dam project throughout a dam's life cycle and strengthen the linkage between geophysical surveillance and engineering significance at all stages.

Borehole Geophysics: Tools to Characterize Rock Properties

Effective design and construction in rock requires characterization of the engineering properties of the rock mass, as well as any associated geological features such as joints, faults, and bedding planes. Surface geophysics methods (e.g., seismic refraction or reflection) are commonly used to locate buried rock layers, determine depth, and help uncover some information about their properties. However, these surface geophysical techniques often do not provide sufficient engineering information to conduct design analyses, particularly when the designed geo-structural feature (e.g., tunnel or underground utilities) is primarily located inside the rock layers.

Exploring Integrated Geophysics and Geotechnics as a Paleolandscape Reconstruction Tool: Archaeological Prospection of (Prehistoric) Sites Buried Deeply below the Scheldt Polders (NW Belgium)

AbstractDuring extensive construction works in Antwerp harbour, well preserved Late Glacial dune formations were discovered buried deeply below the Scheldt polders and covered by peat, organic matter (OM) rich clays and marine clayey to sandy sediments. First, coring based archaeological prospection strategies for evaluating prehistoric occupation levels in wetland landscapes are reviewed. Next, a more effective approach including near surface geophysical and geotechnical techniques is proposed and tested in Doelpolder Noord. The results indicate that high resolution electromagnetic induction survey at multiple coil spacing provides a suitable approximation of the prehistoric landscape variability but is challenged by variations in groundwater brackishness. Gridded cone penetration tests provide a solution in such cases and serve as an excellent interpretation tool for the conductivity data in general. Due to the required effort, electrical resistance imaging and shear wave land seismics were judged inefficient. Finally, a small dune with indications of paleosol conservation and estimated suitability for Final Palaeolithic to Early Neolithic occupation is sampled by Dutch hand augering and Sonic Drill Aqualock coring. Archaeological indicators for prehistoric occupation such as burnt bone and flint fragments were retrieved from these samples after sieving. Copyright © 2016 John Wiley & Sons, Ltd.

Late Cenozoic paleovalley fill sequence from the Southern Liverpool Plains, New South Wales—implications for groundwater resource evaluation

The Liverpool Plains in northern New South Wales contain some of the best agricultural land in Australia and are underlain by extensive smectite clay-dominated soils sourced from weathering the alkali basalts of the Liverpool Ranges. It had been thought that a relatively simple geological model explained the underlying Cenozoic sequence with salt-rich clays of the Narrabri Formation overlying sands and gravel aquifers comprising the Gunnedah Formation. Extensive groundwater modelling based upon this simple conceptualisation has been used in management plans proposed by the mining and agricultural industries. A 31.5 m core has been recovered using minimally disturbed triple-tube coring methods at Cattle Lane (Latitude –31.52° S, Longitude 150.47° E) to resolve uncertainty concerning the aquitard status of the upper layer. Recovered core has been examined and tested to determine grainsize, cation-exchange capacity, X-ray diffraction, X-ray fluorescence and microscopic examination of granular components. These measurements complement surface and borehole geophysical techniques, hydrogeological data and hydrochemical analysis of water samples recovered from a series of specially constructed piezometers adjacent to the cored hole. The sequence overlies a Late Cretaceous channel cut into Permian bedrock at 91 m depth with sands and clays below 31.5 m considered to represent various alluvial fill events mostly occurring since the Early Pliocene. Erosion of Late Eocene alkali basalts on the Liverpool Ranges, with the formation of smectite clays, pedogenic carbonates and with the addition of quartz from both eolian sources and locally derived from adjacent Triassic sandstone hills, provides the great majority of the sediment recovered from the cores. Late Pleistocene (114 ka) to Holocene ages were determined for the core from three optically stimulated luminescence (OSL) measurements on fine sands (13, 23 and 29 m BG). Detailed examination has failed to detect any evidence of a boundary between Narrabri and Gunnedah formations revealing rather a gradual change in dominance of clays and silts over sands and gravels embedded in a clay-rich matrix. This result challenges the conceptualisation used to conduct groundwater modelling on the Liverpool Plains.

Combined geophysical techniques for detailed groundwater flow investigation in tectonically deformed fractured rocks

<p>In this paper we present a combination of several near surface geophysical investigation techniques with high resolution remote sensing image interpretations, in order to define the groundwater flow paths and whether they can be affected by future seismic events. A seasonal spring (Amvrakia) located at the foot of Meteora pillars near the village of Kastraki (Greece) was chosen as a test site. The Meteora conglomeratic formations crop out throughout the study area and are characterized by large discontinuities caused by post Miocene till present tectonic deformation [Ferriere et al. 2011, Royden and Papanikolaou 2011]. A network of groundwater pathways has been developed above the impermeable marls underlying the conglomeratic strata. Our research aims to define these water pathways in order to investigate and understand the exact mechanism of the spring by mapping the exposed discontinuity network with classic field mapping and remote sensing image interpretation and define their underground continuity with the contribution of near surface geophysical techniques. Five Very Low Frequency (VLF) profiles were conducted with different directions around the spring aiming to detect possible conductive zones in the conglomeratic formations that the study area consists of. Moreover, two Electrical Resistivity Tomography (ERT) sections of a total length of 140m were carried out parallel to the VLF profiles for cross-checking and verifying the geophysical information. Both techniques revealed important conductive zones (<200 Ohm m) within the conglomerate strata, which we interpret as discontinuities filled with water supplying the spring, which are quite vulnerable to displacements as the hydraulic connections between them might be easily disturbed after a future seismic event.</p>

Time-lapse cross-hole electrical resistivity tomography monitoring effects of an urban tunnel

Tunnel construction in urban areas has recently become a topic of interest and has increased the use of tunnel boring machines. Monitoring subsurface effects due to tunnel building in urban areas with conventional surface geophysical techniques is not an easy task because of space constraints. Taking advantage of the construction of a new metro line in Barcelona (Spain), a geoelectrical experiment, which included borehole logging and time-lapse cross-hole measurements using permanent electrode deployments, was designed to characterise and to study the subsurface effects of the tunnel drilling in a test site.

The application of magnetic gradiometry and electromagnetic induction at a former radioactive waste disposal site

A former radioactive waste disposal site is surveyed with two non-intrusive geophysical techniques, including magnetic gradiometry and electromagnetic induction. Data were gathered over the site by towing the geophysical equipment mounted to a non-electrically conductive and non-magnetic fibre-glass cart. Magnetic gradiometry, which detects the location of ferromagnetic material, including iron and steel, was used to map the existence of a previously unknown buried pipeline formerly used in the delivery of liquid waste to a number of surface disposal trenches and concrete vaults. The existence of a possible pipeline is reinforced by historical engineering drawing and photographs. The electromagnetic induction (EMI) technique was used to map areas of high and low electrical conductivity, which coincide with the magnetic gradiometry data. The EMI also provided information on areas of high electrical conductivity unrelated to a pipeline network. Both data sets demonstrate the usefulness of surface geophysical surveillance techniques to minimize the risk of exposure in the event of future remediation efforts.

Simulation of Electrical Conduction in Geomaterials by SPICE

Electrical responses of the subsurface can be used to identify geologic strata, locate anomalies, detect and delineate contamination, among many other applications. All these applications depend on the spatial variations of electrical properties in the subsurface and the resulting flow pattern of electric current. Due to the heterogeneity of the subsurface and complex boundary conditions, three-dimensional electric current flow problems are not easy to analyze, in particular when the response is frequency- and/or time-dependent. In this paper, a method of electric circuit analogy is proposed to simulate the electrical responses of geomaterials using the circuit simulator SPICE. The technique will allow simulation of more complex electrical conduction behavior of geomaterials without much extra effort. The excellent agreement between simulated results and analytical solutions developed for surface geophysical techniques establishes the viability of the method. Limitations of the approach and potential solutions to relax these limitations, and other potential applications of the technique in geosciences are also discussed.