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24th European Meeting of Environmental and Engineering Geophysics
- Conference date: September 9-12, 2018
- Location: Porto, Portugal
- Published: 09 September 2018
41 - 60 of 203 results
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Geophysics in the Prospecting Hydrogeological of Santiago
Authors J. Duarte, A. Pina, M. Cunha and F. FigueiredoSummaryThis work aims to be a further contribution to the numerous studies that have been carried out under the prospecting of groundwater in the Cape Verde archipelago ( Pina, 2009 ; Duarte, 2017 ). The application of the method has aimed to evaluate acquisition geometry in volcanic environments, to maximize and contribute to the geological, structural and hydrogeological interpretation of the area of research, and evaluation of the model of distribution of Resistivitys with lithologies distribution model (probing holes). The spatial distribution of the surveys was chosen in such a way as to coincide with the places where this drilling was effected. The work carried out was carried out in the locality of Ribeirão do Chiqueiro, the Santiago Island, where electromagnetic probes were performed 1D using the TEM-FAST. For visualization and interpretation of the geophysical models and the data of the subsurface surveys, they were integrated with high resolution aerial image.
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The Application of ANFIS in determining the Optimum Exploratory Boreholes Points by Using Exploratory Data
Authors M. Azizi, S. Nourbakhsh Bahreghani and A. MohammadiSummaryIn this research, the application of adaptive neuro-fuzzy inference system (ANFIS) was investigated in determining the optimum exploratory boreholes points, using exploratory data. The study area is Vertaveh iron deposit that is located in the south of Kashan and 8 kilometers away from south-east of Ghamsar city. The input layers to adaptive neuro-fuzzy inference system consisted geomagnetic data and iron grade that obtained from powder drilling (RC). After the preparation of input layers, 30 percent of data selected as test data and remaining 70 percent as training data. The training data was done for three function of adaptive neuro-fuzzy inference system, then these three functions were compared by two error indices and the best function with minimum error indices was selected. Finally, the best regions were identified in terms of grade and thickness and also the optimum exploratory boreholes points were determined.
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Enhanced Interpretation of Continuous Dipmeter Logs in Formation Fracture Study
Authors M. Mavaddat, Y. Mavaddat and M.G. Akbari fardSummaryAn integrated data acquisition and log interpretation method was used to achieve the fracture study of a very old cased hole abandoned well. Accordingly, by using continuous dipmeter logs and core samples; fracture dip, density, direction and fracture intervals were determined. Results were used along with seismic interpretation and drilling history as a tool to transform recently unused continuous dip meter logs from old drilled wells into valuable information that can highly help in formation fracture studies. It can also be used in combination to drilling rate of penetration, drilling cutting logs and mud losses to get a better insight into the fracture network in the reservoir formation.
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A Combined Approach of Electrical Resistivity and Physicochemical Methods for Mapping of Hydrocarbon Contaminated Site
Authors K. Ishola, O. Bamidele, K. Oyedele and L. AdeotiSummaryAn investigation of the contaminants of the environment especially groundwater with hydrocarbon leakages from pipelines has remained a major concern globally. The purposes of this study were to map the extent, analyzed the chemical composition of the plumes, and estimate the water quality index of the water samples collected in the study area using geophysical and physicochemical methods. Six traverses were occupied and a total of thirty VES data were collected. The sixth traverse was designated a “control” traverse. Some water samples from hand dug wells and boreholes were collected. For this study, four to five geoelectric layers were delineated indicating topsoil, contaminated sand, sandy clay/clayey sand, sand and clay. The physico-chemical analysis revealed that BH-1 to BH-7 have anomalously high BOD, TDS,and conductivity above the acceptable level. At present, the groundwater in the area is not potable; therefore, concerned agencies or government should embark on remediation to salvage the groundwater aquifer system in the area from a total damage. The electrical responses from the resistivity measurements using both vertical electrical soundings(VES) and 2D electrical profiling were good at imaging the contaminated zones. For this study, the electrical techniques reveals that relatively high resistivity associated with the supposed contaminated zone.
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Field and laboratory Validation of ERT-Detected Subsurface Heterogeneity at the Meter Scale
Authors E. Bloem, A. Dathe, A. Nemes, P. Fernandez, H.K. French, M. Patterson and D. GimenezSummaryAgriculture, industry, airport activities all have impact on the quality of soil and water. To improve risk assessment, monitoring, design and implementation of cost-effective preventive or mitigation measures and/or policies we require a better understanding of the underlying environmental processes. Spatial heterogeneity plays a critical role for describing water and solute transport processes in the unsaturated zone. Heterogeneous water flow in soils is complex and challenging to quantify due to changes in soil hydraulic properties across different spatial scales.
We conducted an experiment involving different field and laboratory sensory techniques and measurements to examine 3D functional heterogeneity and its physical drivers in natural soil. We compare the spatial heterogeneity of the soil from ERT measurements at two scales with the observed spatial distribution of hydraulic properties calculated from soil samples and TDR data.
ERT helped to visualize the heterogeneity of the studied system. Although at first the high resistive feature observed with ERT could not be explained by tensiometer and/or TDR data, there was a distinct difference in the retention curves inside and outside the feature, which creates difference in resistivity values. These observations help us to better understand the links between soil hydraulic properties and heterogeneous water flow.
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Monitoring of In Situ Remediation with the Direct Current Time-Domain Induced Polarization Method
Authors A. Nivorlis, T. Dahlin, M. Rossi and H. WeiSummaryIn Alingsås, a dry-cleaning facility was operated for many years, and huge amounts of the solvent PCE was spilled into the ground. This contributed to an increasing concentration of PCE over the years until the use of PCE was stopped, resulting in the formation of a DNAPL plume beneath the building. Treatment of contaminated soils in Sweden often includes excavation and landfilling, however in Alingsås this is not applicable. In situ remediation methods (thermal, biological, chemical) are the only alternative however, there is a need for tools to monitor the effectiveness of those methods. One method of particular interest in this context is the Direct Current time-domain Induced Polarization (DCIP). For that purpose, a fully autonomous and automatic monitoring system was installed in Alingsås, to perform frequent automated measurements and to provide information about the changes in the subsurface. The geophysical data should be ideally acquired, analyzed and verified with automated routines as part of a larger monitoring system. It is of great importance, especially in the early stage, to verify events that appear to show interesting changes with sampling data to evaluate the level of reliability of the system.
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Differential Acoustic Spectroscopy for Low Frequency Laboratory Measurement of Compressibility and Attenuation
By J. HarrisSummaryDifferential Acoustic Resonance Spectroscopy (DARS) was developed as a laboratory method for measuring compressibility and attenuation of small samples of materials at low acoustic frequencies. The DARS concept begins with a baseline measurement of resonance frequency and resonance linewidth of a fluid-filled but otherwise empty cavity. When a small sample of rock or other material is placed inside the cavity, shifts in resonance frequency and linewidth occur. The changes in resonance can be inverted for sample compressibility and attenuation. Previous DARS studies focussed on the estimation of compressibility. In this paper, I report the first quantitative estimates of attenuation (Q) from the DARS system. The paper begins with a brief review of the DARS concept, followed by a summary of the DARS theory developed to support Q analysis, and finally measurements and analysis of non-porous and porous samples.
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Analysis of the Frequency-Time Characteristics of Seismic and Electromagnetic Fields over a Gas Condensate Field
Authors V. Potylitsyn, D. Kudinov, G. Shaidurov, E. Kokhonkova and V. RomanovSummaryThis article presents data on the analysis of frequency-time characteristics of seismic and electromagnetic fields obtained in July 2017 at the Bystryansky gas condensate field (Russia, Krasnoyarsk Territory, Minusinsk District). Spectrograms of signals measured both in the center of the deposit and at its periphery are given. The optimal frequency range for recording seismo-electric effects is obtained.
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First Land Application of the Differential Electrical Dipole Method together with Transient Electromagnetics
Authors K. Lippert, B. Tezkan, J. Boekmann and A. HaroonSummaryThe newly developed DED-transmitter consists of two horizontal electrical dipoles that share a common central electrode which has one polarity, while the outer ones have the opposite polarity. Thus, the current in each dipole flows in opposite directions. The differential signal is recorded by electrical receivers at a certain distance. This feasibility study, realized along the beach of de Panne in Belgium, is the first land-based application of the DED system of its kind. The freshwater distribution has a thickness variation of 8–15m in a depth of up to 30m. 24 DED measurements were conducted at the beach at low tide, utilizing two 75m transmitter dipoles in an inline configuration parallel to the beach. Six transmitter locations were assembled parallel to the beach with a spacing of 50m along with four electrical receiver stations at different offsets. Additionally, 40 inloop TEM stations were measured. Previously performed 1D modelling studies suggested an interpretation of the DED Step-On signal as more suitable compared to the DED Step-Off signal. A joint inversion algorithm is for DED and TEM data was developed. Based on these results a freshwater distribution underneath the Belgian coastline is confirmed with a varying thickness between 8 and 30m.
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Feasibility of 3D Random Seismic Arrays for Subsurface Characterizations in Urban Environments
Authors B. Brodic, A. Malehmir, M. Svensson and J. JonssonSummaryTo test new and cost-effective methods for 3D subsurface characterization in urban and near-surface environments, along with mapping bedrock morphology and zones of weaknesses at a contaminated site, a seismic survey was conducted in the city of Varberg, southern Sweden. The survey consisted of 12 active-source landstreamer seismic profiles, 89 randomly distributed wireless seismic recorders along an area of 350×300 m and 46 wireless recorders deployed along the train tacks crossing the site. In this study, we analyzed the properties and potential of two datasets obtained only from the wireless recorders for seismic imaging. One dataset consists of recordings of shots done along the active-source profiles, while the other one consists of 5 days of passive seismic data recording. The first dataset was used for first-break traveltime tomography with the picked data indicating uniform source-receiver azimuth-offset distribution and minor acquisition footprints. P-wave velocity model obtained from the inversion was successful in imaging bedrock surface and indicated two preferential directions from which contaminants may migrate away from the main contaminated site. Preliminary analysis indicates that the site’s noisy nature limits 3D reflection seismic potential of the dataset with active source recordings while the passive dataset shows promising results for site characterization.
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Three-Dimensional Ambient Noise Tomography Based on Common Midpoint Spatial Autocorrelation
Authors K. Hayashi and C. KonishiSummaryWe developed a passive surface wave method that delineates 2D or 3D near-surface S-wave velocity (VS) structure from ambient noise obtained from dozens of receivers on the ground surface. A new cableless (nodal) seismograph that includes CPU, A/D converter, GPS clock, Wi-Fi, batteries and SD card was developed and used for data acquisition. The GPS clock permits units to be synchronized over any distance without cables. Ambient noise was recorded using a receiver spacing small enough to avoid spatial aliasing. A common midpoint spatial autocorrelation (CMP-SPAC) method was used to process ambient noise. First, coherencies were calculated for all pairs of receivers. Next, coherencies having common midpoint (CMP) were grouped together and a dispersion curve was calculated at each CMP. A one-dimensional inversion with horizontal constraint was used to estimate a 2D or 3D VS model. This paper summarizes the equipment and method and provides a field example.
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A New Towed Ground-Based TEM-System for 3D Mapping of the Top 50 Meters of The Subsurface
Authors P.K. Maurya, E. Auken, A.V. Christiansen, N. Foged and T.T. EiskjærSummaryThe top 0–50 m of the subsurface are critical for infrastructure, water supply, artificial infiltration, farming, waste deposits, construction, etc. Yet, the tools for imaging this zone are limited to boreholes or geophysical methods such as electrical resistivity imagining (ERT) with a limited applicability for area-covering surveys larger than a few hectares. We present a new highly efficient towed transient electromagnetic system, tTEM, which bridges the gap in coverage and resolution between point or line measurements and airborne electromagnetics. The system yields images in full 3D with a lateral resolution down to 10 × 10 m. The system is towed by and All-Terrain Vehicle, uses a 2 × 4 m2 transmitter coil and has a z-component receiver 9 m offset from the transmitter. The first bias-free gate is as early as 4 µs from ramp beginning (1.4 µs after ramp-end). The turn-off time is 2.6 µs. Data are processed and inverted using methods directly adopted from airborne electromagnetics. We discuss the system design and present a case study where the system has been used to map complex subsurface geology in large detail to build 3D geological/hydrogeological models.
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Cross-Hole ERT Measurements in Slotted PVC Cased Boreholes - A New Perspective in Geoelectrical Prospection
Authors A. Almpanis, P. Tsourlos, G. Vargemezis and C. PapazachosSummaryIn this work we study the case of performing cross-hole ERT measurements in plastic PVC cased slotted observation boreholes by inserting a multi-electrode cable directly into the borehole. We present results from an experimental study by setting up simulations in a water tank using different PVC casings, with various number of slots and by testing them using different electrode array configurations.
The findings of the experimental test were also tested by field data. We conclude that the applicability of this measurement set-up depends on the density of the slots on the PVC casing, with denser slot configurations leading to a better data quality. Among the tested configurations, the pole-dipole array is shown to be the most preferable array. The previous findings are verified by the field data results, giving a new perspective for geoelectrical prospecting, since the obtained results suggest that slotted PVC cased observation wells can be efficiently employed for ERT mapping, reducing the overall survey costs, reliability and effort, especially in urban environments.
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Estimation of Anisotropy Parameters in VTI Media Using the Ray-Based Tomography
By G. BohmSummaryIn this paper a new procedure to estimate the Thomsen parameters ε and δ in VTI (Vertical Tranversely Isotropic) media is presented. The approach used in this work exploits the angles of the ray segments computed by the anisotropic ray tracing in travel time tomography using direct, reflected and refracted arrivals. The reliability of this technique is demonstrated by two synthetic examples related to transmission tomography (cross-hole) and reflection/refraction tomography.
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Shallow Cavern Detection through the Acquisition and Interpretation of Airborne Full Tensor Gradiometry
Authors A. Morgan, S. Payton, G. Jorgensen and K. StaffordSummaryEvaporite sediments within the Delaware basin of west Texas are known to be significantly karsted through the action of surface and subsurface water. Surface geomorphology offers indications of subsurface karsting, but such features may be occluded by more recent sedimentary cover. Recent drilling efforts have penetrated shallow caverns resulting in the loss of drilling fluids and the abandonment of the borehole after mitigation methods failed to stem further fluid loss. Airborne Full Tensor Gradiometry (FTG) is acquired to locate the known karst penetrated by the wells and to assess further risk in the adjacent leased acreage. FTG acquisition is rapidly acquired, processed and interpreted in order to allow risk assessment to take place within the strict time frame imposed by drilling commitments. Such data acquisition could not take place through ground methods due to the extent of the acreage and the time required to secure multiple permits which are not required by airborne data acquisition.
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Geological Modelling Implications - Different Inversion Strategies from AEM Data
Authors A. Rapiti, F. Jørgensen, A. Menghini, A. Viezzoli and G. VignoliSummaryData from AEM survey, carried out in the Peace Region of NE British Columbia to support groundwater investigation, were used in this study. An understanding of the geological structures can have significant implication in groundwater management decisions. Previous studies have demonstrated the presence of shallow aquifers specifically those within paleovalleys. In this study we present different inversion strategies and their geological modelling implications. The aim was to obtain geophysical models that have better resolved electrical resistivity units. This, therefore, should allow more straightforward geological interpretation.
Different types of geophysical inversions on synthetic data have been performed. Testing different settings, for each inversion strategy permitted to study how the final geophysical model could change. This workflow was applied to real data over a subset collected in the south-western part of the Peace Region, an area characterized by different buried valleys.
Sharp Laterally Constrained inversions have been thoroughly investigated. The sharp result is very sensitive to settings choice: sharp inversion with tight settings allows to recognition of the basic electrical resistivity blocks of the model, on the other hand, sharp-inversion with loose settings allows to obtain resistivity variations within the blocks. We present a rough geological interpretation on a representative cross section.
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The Heavy Mineral Indicator Adds a New Dimension to Airborne Radiometrics
Authors I. Hooshyari-Far, A. Kepic and M. CarsonSummaryFor many years the output of radiometric surveys are maps of total count gamma activity and estimations of K, U, Th. Our work puts the focus upon using the long neglected aspects of the radiometric spectrum: the scattered gamma ray radiation. The heavy mineral indicator (HMI) is computed form a ration of low energy versus high energy scattered gamma rays. It is in effect the ratio of Photoelectric effect versus Compton effects. The Elashgin case history demonstrates that low-level and high resolution full spectrum radiometrics provides good data for the HMI technique. HMI values are dominated by Fe concentration due the abundance of Fe over other high atomic number elements. Areas composed of higher atomic numbers, mostly due to the iron content, such as ferricretes and laterite show up clearly in the Elashgin data without the need to be “interpreted” with respect to geological context. With the high resolution of modern radiometrics on planes and future drones this technique works well for remote sensing. An exciting aspect of HMI analysis for airborne radiometrics is not just the future, but the past: many high quality full spectrum data sets already exist and may be easily reprocessed to reveal new geological insights.
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A Drone as Platform for Airborne Gamma-Ray Surveys to Characterize Soil and Monitor Contaminations
Authors S. Van Der Veeke, R.L. Koomans, F.M. van Egmond and J. LimburgSummaryGamma-ray spectrometers are an invaluable tool in the field of geophysics where they are used for geological mapping and mineral exploration. Recent technological advances introduce the emergence of a new platform for gamma-ray surveys: Unmanned Airborne Vehicles (UAVs) otherwise known as drones. This new platform enables the fast and efficient collection of environmental radiometric data in otherwise inaccessible areas. An overview is given of the technological and data-analysis developments to realize the use of a spectrometer under a drone.
As a result of weight and power optimization a spectrometer system based on a 1.0L NaI crystal can be used under a drone. The system collects gamma spectra with sufficient statistics to perform full spectrum analysis and determine variation in geophysical soil parameters. The platform can autonomously measure and process data giving real-time insight in the collected data and results. Two test cases are discussed in which the use and accuracy of this platform is validated for precision farming as well as locating and monitoring radioactive contaminations. It is concluded that a self-contained gamma-ray measurement system under a drone combines the best of two worlds by maintaining high resolution while increasing the ease of use.
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Near Surface Conductive Zones in Hessdalen Valley (Norway) Inferred by Preliminary VLF Surveys
Authors G. Vargemezis, J. Zlotnicki, B. Gitle Hauge, A. Kjøniksen and E. StrandSummaryDuring the last four years VLF surveys have been performed in Hessdalen valley during four wide geophysical field campaigns including Total Magnetic Field surveys, time recordings of the electrical field, and magnetotelluric soundings. VLF measurements have been carried out on a 20 m average spacing along a large number of traces totaling 70 km length. The covered area is now about 100 km2. Hereafter, we focus on the results of the VLF upshots. A part of the conductive zones have been found to be related with mineral deposits (mainly sulphites). The distribution of these conductive zones could suggest that they draw an ellipse of 6×12 km dimensions related with the anticline in the area. These results combined with the other geophysical results will contribute to understand if and how these, very superficial, structure could participate to the genesis of the so-called Hessdalen lights.
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Geophysical Investigation of the Malga Archaeological Park (Carthage, Tunisia)
Authors A. Jrad, C. Jallouli, Y. Quesnel and P. RochetteSummaryA non-invasive multitechnical geophysical survey, using magnetic cartography, seismic and electrical resistivity tomographies, was carried out at the Malga archaeological park of Carthage in Tunisia. The goals were to understand the geological stratigraphy of the subsurface and to locate buried archaeological relics at the high-potential archaeological area of El Malga site. A potential linear magnetic anomaly was observed at the eastern-north side of the prospected area and which can be related to ruins probably made of sandy bricks like those of the Carthage-Zaghouan Aqueduct. Numerous bipolar magnetic anomalies are also characterized suggesting the presence of magnetized bodies in the subsurface. Seismic tomography profiles crossing magnetic anomalies show four main stratigraphic layers which are from the top to the bottom; the weathered zone with the lowest seismic velocity, a more compacted layer which deposited above the layers with the higher seismic velocity. The enhancement of the velocity can be related to enrichment with limestone or the impact of the phreatic table water. A depth of about 1 m was defined for potential archaeological features with strong electrical resistivity contrasts.
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