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NSG2021 27th European Meeting of Environmental and Engineering Geophysics
- Conference date: August 29, 2021 - September 2, 2021
- Location: Hybrid
- Published: 29 August 2021
1 - 100 of 134 results
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3D DAS Full Waveform Inversion (FWI) Case Study for SAGD Steam Chamber Imaging
Authors H. Feng, A. Kay, A. Knudsen, W. Wang and A. AyreSummaryDistributed Acoustic Sensing (DAS) is based on Rayleigh scattering principles used to determine acoustic strain signals over long distances, effectively turning the optic fiber cable into a series of geophones. Once the data is acquired, advanced processing techniques such as Full Waveform Inversion (FWI) can be applied to obtain information for a wide variety of applications. Historically DAS is used as a complementary tool to improve understanding of flow distribution along the entire length of a producing wellbore. It is commonly used in collaboration with DTS to interpret and define flow allocation throughout the life of the well. 3D DAS Vertical Seismic Profiles (VSPs) also can be used as a cost-effective tool for the time-lapse reservoir monitoring due to the finer channel sampling and flexibility in deploying the fiber-optic cables. The application of FWI to 3D DAS is an innovative way to image the steam chamber during the steam-assisted gravity drainage (SAGD) process for Oil Sands. This successful case study demonstrates that 3D DAS FWI can provide a significantly more cost-effective and technically equivalent alternative to conventional 4D seismic programs.
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Gas Hazard and Origin: Near-Surface Zone of the Upper and Lower Silesian Coal Basins
Authors M. Kotarba and H. SechmanSummaryIn 1991–2001 the all coal mines in the Lower Silesian Coal Basin (LSCB) were closed and remediated. In the middle of the 1990s mine restructuring process started at the Upper Silesian Coal Basin (USCB). At that time 65 coal mines were operational. In 2019 22 mines were still active. The main aims of this presentation are to determine the origin of soil gases in the USCB and LSCB and evaluate gas hazard caused by secondary migration of coalbed methane and carbon dioxide from Carboniferous coal-bearing strata to near-surface zone connected with the restitution of groundwater level to the original position (“piston effect”). For evaluating gas hazard surface geochemical survey and determining the origin of soil and coalbed gases, stable isotope analyses and simulation of generation of thermogenic gases by hydrous pyrolysis experiments were performed. Comprehensive geochemical, geological and hydrogeological studies carried out in the zones of closed mines in the USCB and LSCB allowed for detecting the surface range of occurrence of anomalous concentrations of methane and carbon dioxide at the surface, the mechanism of the flow of these formation gases into the near-surface zone and establish to what an extent the local population living in post-mining areas is hazarded.
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Geoelectric Investigations with Special Measurement Geometry to Delimit Prehistoric Mining Areas in Hallstatt
Authors D. Ottowitz, B. Jochum, M. Yi, S. Pfeiler, A. Römer, K. Kowarik, D. Brandner, A. Nevosad and H. ReschreiterSummaryThe Hallstatt area in Austria has a very long history of salt mining, which was frequently interrupted by large landslides. Landslide material filled the prehistoric underground mining chambers and therefore conserved their dimensions. Within several geoelectric measurement campaigns, data from ground surface measurements and from geoelectric profiles in 2 mining tunnels from the 18th century were collected, resulting in a 3D data set of about 80,000 data points. The aim was to track a mining chamber of the Early Iron Age, which has been filled with landslide material. The already existing reconstruction of this mining chamber is based on several archeological excavation points and an interpolation in between. A collapsed mining shaft known from laser scan data has been confirmed beforehand with surface ERT measurements. Due to the high resistive salt rock and the hardness and dryness of the intruded landslide material, the Early Iron Age mining chamber could not be differentiated sufficiently. Instead, results could pinpoint that the mining chambers are following the area of the highest electrical resistivity, which is determined as “Haselgebirge” in geological maps. Therefore, indications of the location of the Early Iron Age mining chamber, where no archaeological finds exists until now, could be given.
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Transient Analysis of GPR Dipole Antenna Using Time Domain Energy Measures
Authors D. Poljak, S. Antonijevic, V. Doric, E. Miler and E.K.D. KhalilSummaryThe paper deals with a direct time domain modelling of impulsively driven GPR dipole antenna above a homogeneous lossy half-space aiming to estimate the energy stored in the near field. The space-time dependent current is obtained by numerically solving the space-time Hallen integral equation and Pocklington equation via different approaches. The obtained results are compared to the results obtained via NEC and Inverse Fast Fourier Transform (IFFT). Provided the current is known one may calculate the charge distribution along the wire by using the one-dimensional version of the continuity equation. Knowing the space-time distribution of current and charge along the dipole antenna, time domain (TD) energy measures involving spatial integrals of squared values of transient current and charge, pertaining to the energy stored in the electric and magnetic field, respectively, of dipole antenna, are evaluated. Furthermore, total power measure defined as time derivative of the total energy measure is computed. Some illustrative computational examples for transient current, TD energy and power measures are given in the paper.
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Capacitive Electrical Resistivity: An Alternative Non-Invasive Method for Permafrost Monitoring
Authors S. Bazin, S.G. Syed, G.L. Gilbert and B. EtzelmüllerSummaryCurrently, permafrost is degraded due to global warming and subsurface geophysics can contribute to characterize this degradation. Electrical resistivity tomography (ERT) is very effective in mapping frozen soils due to the strong resistivity contrast between ice and water. We present an example of 2D and 3D resistivity imaging using a capacitive coupling resistivity (CCR) survey method in Svalbard permafrost. Although little work has been published on the mapping of the active layer of permafrost (i.e. the ground layer which thaws annually) with the CCR method, this case study shows its advantage as non-invasive compared to all other investigating methods.
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Surface-Waves Extraction Using a Shot-Receiver-Time Transformation
Authors Y. Ding and A. MalehmirSummarySeismic data contain rich information carried by different types of waves, such as direct arrivals (P- and S-waves), reflections, diffractions and surface-waves. In order to utilize a specific type of wave, one needs to isolate this wanted signal from others. We propose a method to extract and/or remove mainly the dispersive surface-waves based on their geometrical property in the shot-receiver-time domain for seismic data acquired along 2D acquisition profiles. We first assemble all shot gathers into a pseudo 3D data with dimensions of shot locations, receiver locations, and time. The shot-receiver-time domain enables us to process the data along different dimensions, not restricted to the shot domain only. With the assumption of a 1D velocity model, we find that the dispersive surface-waves in a shot gather behave linearly in a time isochrone, which cuts through all shots and receivers. The linear geometrical property of the surface-waves in the time isochrone allows us to extract them efficiently and effectively using for example curvelet-based transforms. By applying the method along the time isochrones for the time samples where surface-waves are present, surface-waves can be extracted from the pseudo 3D data. We exemplify this method using a synthetic data and a field data.
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Mapping the Traps of Yakutia Kimberlite Province Using the Controlled Source Radiomagnetotellurics
Authors A. Saraev, A. Shlykov and B. TezkanSummaryResults of the controlled source radiomagnetotelluric (CSRMT) sounding method in the study of traps (dolerite sills) overlain kimberlite pipes are presented. Features of the morphology of dolerite sills are described. The pinching out of dolerite sills over pipes and the lowering of the roof of hosting rocks in the near-pipe areas can be considered as indirect signs of the presence of pipes. The CSRMT method can be used in the study of morphology of dolerite sills and mapping the roof of kimberlite hosting rocks.
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Tailing Site Characterizations Using Near-Surface Geophysical Tools in South-Central Sweden
Authors S. Tavakoli and I. KronsellSummaryGround Penetrating Radar (GPR) data with two different antenna frequencies i.e. 250 and 300 MHz were used to characterize the spatial distribution of the mine tailings and also image the ground-water level in the tailing repository area of an abandoned mine site in Yxsjöberg. GPR result, after processing provided high-resolution image of the subsurface down to maximum ca. 3.2 m depth which indicated higher for the instrument with 250 MHz antenna frequency, compared to 0.7 m effective penetration depth indicated by the instrument with 300 MHz antenna. The vertical continuation of the tailings was identified down to ca. 0.7 m in the results from both antennas and ground water table was indicated at ca. 3 m depth using the 250 MHz antenna, consistent with data from the nearby well. Self-potential (SP) data indicated steady results; however, incompatible with the topography variations, which is presumably produced due to 3D topography variations being compared with 2D SP data. The link between SP data and topography variations and thereby, ground water movement can be further studied using a 3D SP measurement to better understand the direction of the ground water movement and also check its correlation with topography variations at the site.
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Most Accurate or Fastest Possible? The Multi-Frequency SIP Excitation Enables a Choice
By T. RadicSummaryThe Spectral Induced Polarization (SIP) method measures the frequency dependence of the electrical resistivity of rocks and sediments by magnitude and phase. Usually, the resistivity spectrum is measured sequentially with mono-frequency signals. This method proves to be advantageous for large-scale measurement set-ups and high interference voltages, as it provides the highest measurement accuracy.
For small-scale field measurements or laboratory measurements, interference voltages often play only a subordinate role. Here, it is also important to achieve the highest possible measurement progress. This can be doubled by multi-frequency excitation compared to mono-frequency excitation. However, the price of faster measurement is a somewhat higher measurement error.
We have implemented and successfully tested both excitation techniques in a new 88-channel laboratory measuring instrument (SIP-LAB-FAST). The user now has the choice between the most accurate or the fastest possible measurement. He can thus optimally adapt the measurement process to the requirements of the object under examination.
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Gelation Performance of PAM/PEI Polymer-Based Mud System for Lost Circulation Control
Authors M. Shamlooh, A. Hamza, I. Hussein, M. Nasser and S. SalehiSummaryLost circulation is one of the most common problems in the drilling of oil and gas wells where mud escapes through natural or induced fractures. Lost circulation can have severe consequences from increasing the operational cost to compromising the stability of wells. Recently, polymeric formulations have been introduced for wellbore strengthening purposes where it can serve as Loss Circulation Materials (LCMs) simultaneously. Polymeric LCMs have the potential to be mixed with drilling fluids during the operation without stopping to avoid non-productive time. In this study, the significance of most common conventional mud additives and their impact on the gelation performance of Polyacrylamide (PAM) / Polyethyleneimine (PEI) has been investigated.
This work helps in better understanding the process of using polymeric formulations in drilling activities. It provides insights to integrate gelling systems that are conventionally used for water shut-off during the drilling operation to replace the conventional loss circulation materials to provide a higher success rate.
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Fiber Optic Sensing for Landslides Early Signs Monitoring and Consequences Assessment
More LessSummaryA variety of landslides such as mud flow, rock fall and rock avalanche can be detected with Distributed Acoustic Sensing (DAS) when they occur. The consequences of the event, and, most importantly, the very early signs of its development can be assessed by Distributed Strain Sensing (DSS). The combination of DAS and DSS instrumentation helps to mitigate geohazard risks along infrastructures crossing challenging environments.
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Near-Mine Exploration for Iron Ore at Malmberget Using Body-Wave Seismic Interferometry
Authors A. Stoch and H. Van Den BergSummarySince early days definition and targeting of ore bodies at Malmberget have been done with success by geophysical methods by way of ground gravimetric and geomagnetic exploration methods. Recent reworking of pre 1980s surface gravity and vertical component magnetic field measurements showed a coincident but prominent north-south oriented anomaly within the eastern limits of the exploration permit. Because the area is largely free from infrastructural development, aside from the nearby railway track running parallel to the anomaly for some length, it was considered a good site to test the passive seismic method. Collected data was processed using body-wave seismic intereferometry followed by the conventional reflection seismic procedures. The obtained seismic sections show that the anomaly identified by gravity and magnetic surveys is detected at a depth of 450 to 550m and that the causative reflector plunges approximately 40 degrees to the north. Seismic data were dependably processed to a depth of 2.5 km opening the possibility to derive important structural information.
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Forecasting of the Development of a Landslide by Studying Its Fracture System
Authors S. Szalai, K. Szokoli, M.K. Baracza, P. Szűcs, I. Lemperger, K. Gribovszky, M. Zubair and L. SzarkaSummaryThe fracture system of a slowly moving loess landslide was studied by the well-known ERT method and by Pressure-Probe which is a simple geotechnical tool. Its application was possible because the fractures were hidden only by a few cm thick soil. Results of both methods were compared with each other and also with the reality, because exploration of the fractures was easily possible. The largest fractures which were detected by these methods served later on as sliding surfaces verifying perfectly the results of these techniques. With these methods the fracture system can be described and thus the endangered area can be delineated. They enable also prediction of future rupture surfaces and better understanding the inner structure of a landslide and thus its development.
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Verification of ERT Numerical Results of G11n and Traditional Configurations by Quasi Field Modelling
Authors S. Szalai, K. Szokoli, M.K. Baracza, M. Kárpi, P. Szűcs, I. Lemperger, K. Gribovszky, E. Prácser, M. Zubair and L. SzarkaSummaryTraditional geoelectric array configurations, such as e.g. the Wenner-Schlumberger or the Dipole-dipole, may be very effective in one-dimensional or robust two-dimensional investigations, but they are not sufficiently sensitive to SESEP inhomogeneities, which have a Small Effect on the Surface Electrical Potential distribution due to their small size and/or large depth or small resistivity contrast to the host. Their characterization is possible by applying quasi null arrays, which provide very small signals above a homogeneous half-space. Such arrays produced very good results in numerical investigations. In this paper their field applicability is demonstrated which has been heavily questioned. The quasi field analogue modelling experiments validated all of the numerical modelγling results. Many or all of the γ11n arrays could detect prisms and vertical sheets located at depths larger than those detectable by traditional geoelectric configurations. The horizontal resolution of the γ11n arrays, too, proved to be better than that of the traditional arrays.
On the basis of this quasi field analogue study, γ11n arrays are expected to be well applicable to indicate SESEP targets (e.g., caves, mines, tunnels, tubes, cables, dykes, fractures), or to follow small variations in the subsurface conditions (monitoring of e.g. dams or waste deposits).
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Integrated Geophysical Methods to Characterize Triassic Microbialites
Authors A. Urruela Garcia, M. Himi, L. Rivero, F. Pinheiro, R. Mercedes, R. Lovera, R. Garcia-Artigas, A. Sendrós and A. CasasSummaryThe recent discovery of significant hydrocarbon reserves in microbial carbonate reservoirs from the Cretaceous Pre-salt layers in the Santos Basin (Brazil) has renewed the interest in the study of modern and ancient microbialite deposits. Understand the factors controlling the facies heterogeneities and depositional processes involved in microbial carbonate formation can help to build refined predictive models of reservoir distribution and connectivity for the oil industry. Therefore, the access to well-preserved and well-exposed microbial carbonate outcrops is essential to integrate sedimentological and geophysical data to be used for the exploration and production of analogous subsurface microbialite reservoirs. Despite the increasing interest in understanding microbialite heterogeneity, few studies have used geophysical methods to characterize the complex morphologies attained by these deposits in outcrops.In the present study, we carried out a near surface geophysics survey to test the validity of geophysical methodologies in the characterization of complex stromatolite structures in outcrops from the Middle Triassic of the Catalan Basin (NE Spain).
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Imaging Clogging in a Treatment Wetland Using Time-Domain Induced Polarization
Authors R. Garcia-Artigas, M. Himi, L. Rivero, A. Revil, A. Urruela Garcia, R. Lovera, A. Sendrós, C. Abancó and A. CasasSummaryTreatment wetlands are engineered systems for small communities that have been designed to take advantage of the same processes that occur in natural wetlands, the main problem affecting treatment wetlands is the development of clogging wich leads to the exclusion of some subparts of the filter, becoming ineffective for the treatment critically reducing the lifetime of the plant. Time-domain induced polarization is used to image clogging distribution of a gravel filter from a horizontal subsurface flow treatment wetland. Experimental data performed on the laboratory demonstrates the linear relationship between the normalized chargeability and the amount of clogging in the gravel filter because of the substantial increase of the cation exchange capacity caused by clogging coating these grains. Therefore, we were able to convert the normalized chargeability tomograms obtained with the field data into a 3D distribution of the percent clogging. This method allows to identify the zones were the clogging has accumulated through the filter and therefore predict preferential flow paths and dead flow zones. This is an important task to plan preventive measures and anticipate the filter obstruction that may decreases the effectiveness of the wastewater treatment system.
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A Fractional Differential Model for the Electrical Conductivity of Clay Rocks
Authors P. Cosenza, R. Giot, A. Giraud and S. HedanSummaryA micro-macro approach is proposed to model the effects of surface phenomena occurring at the pore fluid/solid interface on the electrical conductivity of clay rocks. This new model is a generalization of the so-called differential scheme (DS) used to infer the effective properties of a composite made up of several materials. It is named the fractional differential scheme and is based on a fractional integral formulation of the DS for a porous medium considered as a two-component composite. The formulation of the fractional DS introduces two parameters: a cementation exponent m and a fractional order α. The fractional order α is assumed to account for the amplitude of the so-called surface conduction on clay minerals. Both parameters m and α are inverted from a set of electrical conductivity measurements obtained on clay rocks. The inversion results demonstrate that the fractional DS model is able to capture the dependence of the cation concentration on the effective electrical conductivity of the clay rocks under study. Our results also show that the fractional order α can be considered an indirect indicator of the amplitude of physico-chemical interactions between hydrated cations and swelling clay minerals occurring at the pore fluid/solid interface.
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A heat tracing experiment using cross-borehole time-lapse ERT
Authors B. Shariatinik, A. Bouchedda, E. Gloaguen, J. Raymond and G. Fabien-OuelletSummaryThis study contributes to an ongoing project of geothermal heating/cooling system design for an office building in Quebec City, Canada.
In this study, a heat tracing experiment using time-lapse cross-borehole ERT was performed. The time-lapse ERT inversion was conducted in order to image the spatiotemporal resistivity changes due to the heat injection.
The inverted models were compared qualitatively to the pumping/injection rate log and temperature logs at different depths in the monitoring well. These comparisons demonstrate that the inversion results show the same trend as the temperature logs, notably they both clearly show the injection and the recovery phase of the experiment.
Our results indicate that cross-borehole ERT could be used as an additional monitoring tool for heat injection tests.
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The forgotten shear-wave reflections in the compressional-wave surveys
By A. MalehmirSummaryIt is impossible to acquire a land seismic dataset that contains purely compressional (P-wave) or purely shear (S-wave) information. Different wave modes usually leak from one to another although may be dominant in one wave mode depending on the instrumentation and setup used. Following an earlier study, in this work I show a reasonable amount of shear-wave reflections can be extracted from typical the so-called P-wave type surveys. This was possible thanks to the extreme slow background shear-wave velocity (85–90 m/s) in the study area allowing a good separation of P- from S-wave reflections. It is important that this possibility is recognized and methods are developed to extract these reflections at the same time and when designing a seismic survey. The minimum is to make sure shear-waves in such an extremely slow-velocity setting are not aliased and properly sampled. The study site, in southwest Sweden, hosts quick clays hence the additional information is significant geotechnically and the added value is great for studying the safety condition of the site. It also implies that the water content is quite high (Vp/Vs=15), a by-product conclusion concerning the liquefaction potential of the site if a landslide is triggered.
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Detection of ancient mine voids by using geophysical methods: the case of Castel-Minier
Authors N. Florsch, M. Llubes, L. Seoane and F. TereygeolSummaryThe mining operations of Ouels, also named Castel-Minier, close to the village of Aulus-les-Bains in the Pyrenees, was mainly effective between the 13th and 16th centuries. It was the most important mine in the French Kingdom during the 14th century, and was operated for silver, cupper and lead. Today, it is a site for archaeological studies and the archaeologists are specially interested in the cavities left by the exploitation. It is with the aim of retrieving the voids left in a remote area that geophysical facilities are requested, following the discovery of a 20th century plan attesting to the existence of a shaft and a vein in this area.
Two geophysical methods have been implemented: Electrical Resistivity Tomography (ERT) and microgravimetry. The first has been used on two parallel profiles spaced 15 m apart, and allowed the detection of a very high resistive body that could fit a cavity at less than a depth of 5 meters. On the other side, microgravimetry revealed two anomalies. It seems to confirm the one seen by ERT, while a second anomaly may attest to a deeper cavity hypothetically lying at 15 to 20 m depth. These hypotheses are compatible with the archaeological expectations.
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3D-Underground Seismics in crystalline, salt and clay rocks
Authors R. Giese, S. Lueth, H. Richter, B. Wawerzinek, K. Jaksch and R. EsefelderSummaryThe modular seismic acquisition system at the GFZ is basically suitable for underground exploration in crystalline, salt and clay rocks. Special features of the application result from the different properties of the media such as degree of heterogeneity, damping, anisotropy and their interaction with the excavation zone (EDZ) induced by the cavity (drifts, tunnels). This manifests itself in different frequency damping behaviour for compressional, shear and surface waves, which require a specific use of impulse and vibration sources at the joint depending on the task. Seismic borehole measurements can contribute to a significant improvement of the resolution.
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Complex Electrical Conductivity of Kimberlite
Authors K. Titov, V. Emelianov, V. Abramov and A. RevilSummaryWe detected large values of the complex electrical conductivity of four kimberlite samples based on laboratory measurements carried out when saturating the samples with brines with the electrical conductivity ranging between 200 and 30 000 S/cm. Both the real surface conductivity and the quadrature conductivity were found much larger than that of common volcanic rocks and sediments. Based on measurements of petrophysical properties of these cores as well as on optical microscopic images we argued that this anomalous conductivity is explained by highly conductive and polarizable clayey matrix, which shows an abundance of the saponite, a mineral of smectite group. These new data confirms usability of electrical (resistivity, and especially Induced Polarization) and electromagnetic methods in kimberlite exploration.
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Delay Effects in Current Switching-off and Their Manifestation in the Early Time Tem Response
Authors M. Sharlov, N. Kozhevnikov and T. PestyurinSummaryIn the near-surface TEM sounding method, it is desirable to measure the transient response starting from the earliest possible time. This requires the current in the transmitter loop to be switched off quickly, which necessitates working with a low transmitter current. In this case, the loop manifests itself as a symmetric combination of two transmission lines, and the current at different points of the loop perimeter is turned off at different times. We present transient electromagnetic responses measured with offset receiving loops, which vary depending on the place where a pulse current source is connected to the transmitter loop. The survey was carried out on the ice cover of Lake Baikal. It has been shown that at early times a delay in turning off the transmitter loop current affects transient voltage induced in offset receiving loops unlike the central loop response, which remains the same,. This fact which should be taken into account when interpreting early-time TEM data .
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Impedance Network Modelling to Simulate the Chargeability of Sand-Pyrite Mixtures
Authors A. Maineult, G. Gurin and K. TitovSummaryInduced polarization method has long been used in mining geology due to its high sensitivity to the presence of disseminated ores. However, it is important to link the measured chargeability to the ore content. This can be done experimentally: for instance, Gurin et al. (2013) studied the evolution of the chargeability with ore content for sand-pyrite mixtures. Here we propose a new numerical approach, based on the resolution of the complex electrical problem using networks of complex impedances. We show that the predicted chargeability obtained by our methodology is very close to the observed one.
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Estimation and Confirmation of Electromagnetic Attenuation and Resistivity of MáTra Mountains Rock
Authors I. Lemperger, A. Novák, P. Ván, V. Wesztergom, P. Lévai, Á. Kis, S. Szalai and J. MlynarczykSummaryElectromagnetic variation’s amplitude decreases exponentially in conducting media. Higher conductance results higher attenuation of the electromagnetic variations vs. depth.
In this presentation an investigation is introduced which aims to estimate the electromagnetic attenuation coefficient and mean resistivity of the near surface rocky layers of the Mátra mountains, Hungary. The whole investigation consists of a confirmation, based on high frequency magnetotelluric sounding, too.
The direct estimation of the attenuation coefficient is performed by means of a signal excited by the global thunderstorm activity in the ground-ionosphere cavity, the so called Schumann resonance. The method basis on the direct determination of the ELF signal’s amplitude decay between a surface and the corresponding subsurface site. In case of parallel recording at the two sites, the attenuation could basically be computed from the rate of the variation’s spectral amplitude at certain frequencies.
As the surface and the subsurface observation could not be performed at the same time due technical reasons, an ELF reference observation site had been involved in the investigation. The attenuation can then be estimated by comparison of the transfer functions related to the ‘subsurface station - reference site’ and the ‘reference site – surface station’ relation.
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Characterization of Undercover Karst Morpholoies by 3d Geostatistical Modeling of Ert Data
Authors C. Verdet, C. Sirieix, A. Marache, J. Riss and J. PortaisSummaryThe study of undercover karst systems and their morphology is difficult to assess using punctual geotechnical tools. Here we present a method using a geostatistical 3D reconstitution based on electrical resistivity tomography (ERT) data to assess the high variability of the karst. We applied this method on the Lascaux hill site (south-west France) in order to: (1) detect karst morphologies under a detrital cover (pinnacles, shelves, karrens); (2) redefine the limit between two limestone formations; (3) define the geometry of the limit between two domains: limestone and detrital materials.
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Electrostatic Profiling and Mapping of Electrical Resistivity and Dielectric Permittivity in an Urban Context
Authors C. Schamper, A. Tabbagh, S. Flageul, C. Benech, Q. Vitale, C. Benjamin, M. Dabas, C. Parfant and L. Perruchon-MongeSummaryElectrostatic profiling and mapping have been carried out in an urban context with two different geometries allowing the prospection of the first 3–4 meters. Despite the larger size of the prospection devices, compared to ground-penetrating radar (GPR), it has been successfully applied at almost all GPR locations in parallel within the same few days’ survey. At an intermediate operating frequency of 15 kHz, the method provides information on both electrical resistivity and “low frequency” effective dielectric permittivity (mainly linked to polarization in clayey materials), and is therefore complementary to the GPR method primarily used in urban prospection.
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Monitoring Underground Heat Storages by Means of Borehole Electrical Resistivity Tomography. A Model Test.
Authors S.L. Fischer, E. Erkul, M. Gräber, B. Wang, S.A. Al Hagrey, S. Bauer and W. RabbelSummaryUnderground heat storages are one out of several possibilities to store energy on a large scale. Using the subsoil as such a storage, it is necessary to monitor the expansion of the temperature field and to look out for possible leakages. In this study, geothermal experiments are carried out on an idealised meso-scale sand pit named "GeoModel". The sample is heated up and cooled down by circulating hot and cold water in a Borehole Heat Exchanger (BHE). Simultaneously, one half of the sand body is monitored by borehole electrical resistivity tomography including several cross-hole and in-hole configurations. Additionally, the temperature of the other half of the sample is measured by numerous sensors at different depths and distances to the BHE. According to theory, the results show a strong anti-correlation between electrical resistivity values and measured temperatures. Repeated heating and cooling cycles demonstrate reproducibility of the investigations. A leakage of the BHE, which occured during an active cooling phase, could be detected at an early stage. Inversion results are shown to be suitable for a qualitative observation of three-dimensional temperature fields in the sand pit.
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Electromagnetic-Interferometric Direct-Wave Suppression for Detection of Shallow Buried Targets With Gpr
Authors F. Balestrini, D. Draganov, D. Ngan-Tillard and F. HansenSummaryOne of the most common methodologies utilised for shallow geophysical and archaeological investigations is ground penetrating radar (GPR). GPR is a popular tool due to its non-invasiveness and capacity for real-time data analysis, widely used for the study of near-surface structures and the detection of buried targets. However, the direct wave propagating along the surface can completely cover the shallowest targets and, thus, needs to be suppressed. Electromagnetic interferometry (EMI) can be utilised to retrieve an estimate of the direct wave. Subsequently, this estimate can be adaptively subtracted from the field data to effectively eliminate the direct wave. We apply this methodology to a GPR dataset acquired at the Jewish Cemetery in Naaldwijk, The Netherlands, in August 2020. The main objective of the survey is to identify locations of possible old, buried tombstones. After applying the proposed methodology, our results show the direct wave is well suppressed. The earliest diffraction events that were covered by the direct arrival are visible after this step. The survey revealed anomalies where buried tombstones might be expected. This work serves to demonstrate the applicability of the GPR survey to detect shallow targets and the utilisation of EMI for direct-wave suppression in GPR data.
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Results of Performance Tests of Electrodynamic Vibratory Seismic Sources
Authors T. Burschil, H. Buness, P. Leineweber and U. PolomSummarySmall-scale electrodynamic vibratory seismic sources (micro-vibrators) are efficient devices for near-surface investigations up to several hundreds of metres depth. Size, resolution, signal repeatability, and production rate are superior to commonly used source systems in this engineering depth range class. Compared to hydraulically driven mini-vibrators, which are magnitudes beyond in price class and logistics effort required, the micro-vibrators benefit in performance of field operation and produce less near-offset source generated noise, demonstrated by the ELVIS micro-vibrators. Even without an expensive vibratory source similarity control system, which is required for hydraulic vibrators, the simultaneous operation of several electrodynamic micro-vibrators excite improved signal energy into the subsurface and increases the signal strength significantly. A test using four source devices simultaneously shows up to 50% increased signal amplitude strength and higher signal-to-noise ratio compared to a single device.
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Reprocessing of Reflection Seismic Data to Highlight Near-Surface Glacio-Tectonic Deformations
Authors H. Buness and T. BurschilSummaryA small scaled 3-D P-wave survey (0.018 km2) in an overdeepened valley of the Rhine Glacier (Tannwald Basin) revealed glacio-tectonic structures at depths of 20–50 m, interpreted as cuspate-lobate folding. Previous 2-D lines, processed using prestack depth migration (PSDM), only displayed a region with poor imaging quality. A careful reprocessing of the 2-D data using common reflection surface (CRS) and prestack time migration (PSTM) now gives an image matching the high-resolution of the 3-D data. The glacio-tectonic structures exhibit very low velocities that were not accounted for in the PSDM processing and led to a distortion of deeper layers. Based on the PSDM migrated 2-D data, we speculated about a deep-reaching tectonic fault zone, which is not probable anymore in the light of the reprocessed data.
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Characterization of a Coastal Area From Integration of Resistivity and Active Multicomponent Seismic Data
Authors F. Da Col, F. Accaino, G. Böhm, S. Picotti, M. Giorgi and F. MeneghiniSummaryWe present an in-depth characterization of the shallow subsurface in a coastal area of the northern Adriatic. In this research, we integrated the results from the processing of active seismic data (P-, SH-, SV- wavefields, and surface waves) with those obtained from electrical resistivity tomography. Seismic data analysis provided stacked sections as well as velocity profiles from first-break tomography. Furthermore, surface-wave analysis was performed to obtain a S-wave velocity model.
The results show a highly heterogeneous subsurface, due to changes in water saturation, as well as in the properties of the sediments. This is consistent with the geology of the area, characterized by sand deposits and clay sediments.
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Preliminary Results From Real and Synthetic Data Using the Masw-Dual Streamer (Ds) Technique
Authors H. Hamdan, G. Kritikakis, M. Harb and A. VafidisSummaryThe preliminary results of a new proposed technique for the data acquisition of the 2D MASW method is discussed in this extended abstract. This technique aims at increasing the fieldwork productivity of the 2D MASW data acquisition significantly. It is estimated that this technique may increase the productivity of the filed work of more than 70% than the traditional land-streamer towed by truck technique. The basic idea is to perform two 2D MASW profiles simultaneously for each run of the truck, employing two land streamers, two seismic recording instruments, but only one seismic source for both receiver arrays, located in the middle of the two profiles. This work aims to investigate the applicability of the proposed acquisition technique and the accuracy of the acquired data, using real data taken from two different sites as well as synthetic seismic data. The dispersion curves and S-wave profile comparison between the traditional and the proposed technique for the real and synthetic data was found to be quite encouraging. The proposed technique manages to reproduce the S-wave profiles resulted from the conventional MASW deployment, in the framework of an acceptable, for the engineers, error. Further basic research concerning the MASW-DS assumptions is considered necessary.
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Assessing Groundwater-Citarum River Interaction and Groundwater Contribution to Flooding
Authors A. Ramdhan, A. Arifin and R. SuwarmanSummaryFlooding is one of the main problems in the Upper Citarum Watershed. Though it is generally accepted that it is caused by the increase of runoff during high precipitation, the contribution of groundwater discharge to the river should also be considered. However, its contribution is still little known or quantified. As a case study, groundwater and Citarum River interaction from Kertasari to Baleendah Sub-districts in West Java, Indonesia, was assessed using the SWAT-MODFLOW model. The objectives of this study are (1) to quantify the interaction between them both spatially and temporally and (2) to estimate groundwater discharge contribution to flooding. The results show that the groundwater and Citarum River interacted mostly as gaining streams with discharge rates ranged from 0 to 9,872 meter cubic per day. However, in some segments, they interacted as losing streams. Their interaction changed both spatially and temporally. The estimated groundwater discharge contribution to flooding in the Citarum River watershed is small, 2.4 to 4.1%, which confirms that the flooding is mainly caused by the high runoff. However, the small groundwater discharge contribution should still be considered in further analysis, such as flood modelling.
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Active versus Passive Seismic Monitoring of Near-Surface Arctic Thawing
Authors H.M. Stemland, T.A. Johansen, B.O. Ruud and R. RomeynSummaryTemperatures are rapidly increasing in the Arctic, but the actual impact of surface warming on the degree of freezing of permafrost is uncertain. We conducted active and passive seismic experiments on permafrost in Adventdalen, Svalbard, several times in 2019. Active seismic data can be generated at any given time and have strong signals, but the drawback is the generation of noise during acquisition. Passive seismic acquisition makes little noise and makes it feasible to acquire data over a longer period, but the signal may be weak and vary in strength throughout the year.
The resulting seismic data show strong surface waves. High-quality dispersion images can be generated from the active data at any time, and from the passive data whenever frost quakes occur. The dispersion curves from active experiments can be followed to higher frequencies than those from passive experiments, whereas the latter type of data can easier distinguish between different modes. A seasonal time-lapse effect suggests that surface seismic data may be useful for monitoring permafrost degradation, both on a seasonal scale and on a longer-term scale due to climate change.
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Investigation of Magnetic Susceptibility Effect on NMR Measurement: Case of the Volcanic Rocks
Authors N. Chibati, Y. Geraud and V. NavelotSummaryNuclear magnetic resonance (NMR) is a powerful technique for reservoir characterization. It has been largely used in the petrophysical evaluation and pore structure study of the sedimentary formation. IN contrast in the case of the materials with high magnetic susceptibility like volcanic rocks, the NMR measurement can be affected, resulting on the underestimation of the porosity from the NMR data. Here we investigate the relationship between magnitude of the internal magnetic field gradient (G) of the volcanic material and its relationship with the NMR porosity errors and the magnetic susceptibility. The obtained results shows that G in the studied samples increase (up to 217 T.m-1) due to the magnetic susceptibility increasing and induce NMR porosity error increasing.
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Multiparameter Anisotropic First-Arrival Seismic Tomography of Acoustic Laboratory Data in Carbonates
Authors M. Salcedo, S. Garambois, D. Brito and F. SanjuanSummaryActive seismic transmission data have been performed in a block of carbonates with a piezoelectric source and a laser vibrometer receiver leading to a full illumination dataset. From the picked traveltimes, the apparent velocity distribution according to angle highlighted the necessity to consider the non-point feature of the source. An appropriate source discretization leads to an apparent velocity pattern that can be approximated with an elliptical tilted transverse isotropic model. Such a model is then used in a multiparameter tomography inversion, which leads to about 7% of velocity anisotropy located in a lower velocity region of the block, interpreted as a possible fractured band.
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Regularized Gauss-Newton Iterative Scheme Applied to Shallow Subsurface Imaging
Authors Q. Didier, S. Arhab and G. Lefeuve-MesgouezSummarySubsurface imaging is tackled as a nonlinear inverse problem, since no approximation is considered on the interaction of the incident wave with the medium. A regularized Gauss-Newton iterative scheme is used for the reconstruction of the relative dielectric permittivity and electrical conductivity. The impact of the wavelength on the spatial resolution in the reconstructed medium is studied. For a given wavelength, the proposed iterative scheme is tested in terms of convergence speed and robustness to noise.
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3D image of the subsoil from complementary seismic methods
Authors M. Saade, A. Dechamp and S. RobertSummaryThis study consists of a geophysical survey, aiming to characterize the soil from a mechanical point of view by determining seismic velocities and to propose a geomechanical soil model over the first 50 meters. The survey consists of complementary seismic methods:
- Passive tomography using ambient seismic noise cross-correlations (Sissterra method, Sixense)
- H / V method
- Seismic refraction and MASW
The results of the different methods are coherent with each other and with geotechnical data obtained from boreholes. The survey allowed a complete geophysical investigation down to 80m and the highlight of the main geological features. In particular, the results of the passive tomography and H/V highlight a deepening that could be related to a sedimentary paleo-valley. The results from the active and passive seismic methods show a good correlation in velocity contrasts. They mainly indicate a horizon with strong lateral variations in Vp and Vs, probably related to water level and to the base of lithological contrast, presenting unstructured passages in the form of Quaternary paleo structures. The 3D measurement made it possible to delimit the most probable locations of these structures and highlight a velocity inversion at depth, probably due to clay passages.
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Geophysical investigations of a landslide to interpret the distortion of a railway tunnel
Authors J. Lataste and J. BruneauSummaryThe Aspe Valley (Pyrénées Atlantiques, France) is an old glaciated valley of the Würm period. Steep rock escarpments alternating with slipped less marked slopes are presented along the valley. The Peilhou rock slide is an example of gravitating instability related to glacial erosion. A consequence is the deformation of a railway tunnel, unexploited and instrumented since 1979. Geological and geophysical approaches allow observations to be correlated to disorders affecting masonry, then to interpret sliding mode of the slope. Schist and sandstone basement is fractured, folded and partly covered by moraine. The slide affects primarily a rock material, and mobilized superficial debris. Various zones are distinguished according to their geomorphological, geological and geomechanical behaviours. Use of electric profiles allows the in-depth extension of the surface observations, and complete investigations. These zones testify to the structural heterogeneity of the rock mass, in conformity with deformation observed of the railway tunnel.
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Mapping the Far Western Bushveld Complex Using Legacy 2D Seismic Reflection and Petrophysical Data
Authors T. Nadan, M. Manzi and S. Scheiber-EnslinSummaryThe Bushveld Complex is very well known for its significant contributions to the world’s Platinum-Group Element’s and chromium reserves. The Far Western Limb is an extension of the complex and has not been extensively explored due to the Quaternary sedimentary cover in the area which covers the contacts between the different lithological units. Outcrops of the chromium within the Far Western Limb have already been identified and mined. This study aims to identify the shallow subsurface lithologies and evaluate the potential for future mining of chromium ore in the Far Western Limb. The results show that the bedrock is very shallow (< 80 m) which implies that the Quaternary cover in the Far Western Limb is thin. This allows for further investigation through near-surface geophysics and drilling as the subsurface lithologies that host the chromium ores can be found close to the surface.
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Drone Geophysics for Forecasting and Monitoring Natural Hazards
Authors B. Dupuy, A. Tobiesen, A. Grøver, A. Einbu and A. RomdhaneSummaryWe present an innovative approach to combine different sensors on a flexible drone platform. The goal is to record repeatable data that can be used to forecast and monitor natural hazards such as snow avalanches and landslides.
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The Abra Xcite AIP Modelling Case Study
Authors A. Viezzoli and A. MenghiniSummaryThe role of AEM in mineral exploration is ever increasing, both internationally (e.g., AusAEM) and within the European context (Smart exploration, Infact). Acquisition with lower base frequency and modelling IP effects stand out among the recent different technical developments in AEM. We present a case study from the Abra sedimentary-hosted deposit from Western Australia, with IP modelling of an Xcite dataset, accompanied by extensive ancillary geophysical information.
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Data Fusion of ERT and Infiltration Tests, Using Bayesian Maximum Entropy to Mapping Saturated Hydraulic Conductivity
Authors S. Rabouli, M. Serre, V. Dubois, J. Gance, H. Henine, P. Molle, C. Truffert and R. ClementSummarySoil infiltration is used as an outlet for the discharge of domestic treated wastewater. The sizing of these infiltration areas in a heterogeneous soil context is difficult, due in particular to the spatial measurement of saturated hydraulic conductivity, which is costly and time-consuming to implement. Nowaday, there is no adequate technique for evaluating the variability of satured hydraulic conductivity for a heterogeneous soil. The development of a methodology for the spatial characterization of soil properties is essential. We demonstrate here the benefit of using Bayesian Maximum Entropy data fusion to obtain a 2D representation of the infiltration capacity of heterogeneous soils. This tool opens up prospects for improving the sizing of infiltration areas, in context of heterogeneous soil.
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Inversion of VLF Data Using a Non-Linear Smoothing Operator
Authors M.A. Uge, G. Karcioglu, A. B.Tekkeli and M.S. ArslanSummaryInversion of electromagnetic induction data, including VLF, is generally realized using smooth inversion methods. The smoothness of the recovered models and the regularization of the ill-conditioned problem is ensured with smoothing matrices. Smoothing matrices are simple linear derivative matrices penalizing the resistivity differences between adjacent cells. Since these matrices are linear operators, they are calculated once at the beginning of the inversion process. Considering its structure, smoothing matrices can be considered similar to low-pass Gaussian filters. Similarly, it’s possible to define a non-linear smoothing operator based on rank order filtering. We have defined a non-linear smoothing constraint based on these filters and penalized the differences from the cells corresponding to the desired rank value. Since the defined constraint is non-linear it is re-calculated as the model parameters change. The defined constraint is tested on synthetic data and its results are compared to the results obtained with a traditional smoothing matrix. Accordingly, the defined non-linear rank order smoothing constraint can provide relatively focused, amplified structures, and can increase blockiness.
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Seismic Monitoring of a Maritime Pine Root-System Failure During Its Overturn: A Field Experiment
Authors V. Allègre, A. Denis, A. Cointe and J. CoureauSummaryThe mechanical response of a few maritime pines was investigated by a series of experiments at the field scale. During the overturning of two trees, a seismic array was deployed at the ground surface to continuously monitor the micro-seismicity associated with the failure of their root-system. We identified seismic events during the traction process applied to each tree. A threshold force value and a significant duration of the traction process seem to be needed in order for cracks to happen, and for seismic events to be recorded. We identified tens of seismic events, especially during the most advanced stages of the experiments, prior to complete failure of the root-system. However, a few events were observed during early stage of the experiment, when the mechanical solicitation was still in the elastic regime. Using pairs of geophones and seismic intensity ratios, we localized the epicenters of most of the events within the seismic network. According to the sensitivity of the array, all epicenters are roughly localized within a 1 square-meter area. The estimation of epicenters show that the events are aligned to the direction of traction, on the opposite side of the tree.
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EASYMAG: A Versatile Solution for High-Quality UAV Magnetic Acquisitions
Authors J. Mercier de Lepinay, T. Fréville, B. Gavazzi, B. Kiemes, L.M. Sanabria and H. ReillerSummaryTerremys has sought to develop EASYMAG, an unmanned aerial system fully dedicated to magnetic surveying in a variety of contexts ranging from small and shallow objects or pipelines detection to large-scale geological mapping. Four major requirements are identified if one wishes to efficiently acquire quality magnetic data at such variable scales: 1- the sensitivity of the magnetic acquisition must be greater than the lowest expected signal according to the application (usually a few nT or lower); 2- The navigation and positioning must be precise enough to compete with ground operators with centimetric accuracy; 3- the flight flexibility must allow contrasting flight domains with topographic draping at any height; 4- the efficiency on the field must compete with existing vectors. Each of these aspects are investigated to demonstrate Terremys’ ability to perform quality magnetic surveys. A case study of drone magnetic acquisition for archaeological purposes is presented in comparison with a ground survey acquired in the same area. Ground and UAV surveys are of comparable quality and they can lead similar interpretations.
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New Insights into a Very-Large, Slowly Moving Landslide (Hell-Bourg, Reunion) from High-Resolution Seismic Surveys.
Authors C. Rault, K. Samyn, B. Aunay, A. Bitri and M. DelatreSummaryIn November 2020, a passive and active seismic investigation was applied to the large slow-moving landslide of Hell-Bourg, Réunion Island. The objective of this survey is to provide relevant information for the characterisation of the internal structure and hydrogeology of the landslide. The first joint application of geophysical methods, geomorphological surveys, field observations and measurement has already led to the general characterisation of this complex landslide up to 200 m depth. The initial results from the seismic survey obtained from the P-wave refraction tomography allow confirming the existence and continuity of the main sliding surface, which had previously pointed out by resistivity model from aerial electromagnetic data. Furthermore, thanks to this data discontinuities that affect both the landslide and the substratum up to 450 m deep have been detected, suggesting a strong relation between the dynamic of the landslide and the substratum geometry. These initial results are promising and thus suggest that the seismic data we are currently processing will reveal new insights to improve our knowledge of the structure and dynamics of the Hell-Bourg landslide.
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Is It Redundant to Use Model-Based Subtraction Together With the Reference Noise Cancellation?
Authors L. Liu, M.P. Griffiths, M.Ø. Vang, D.J. Grombacher and J.J. LarsenSummarySurface NMR is a competitive method in hydro-geophysical investigation due to its directive sensitivity to the water content, permeability and hydraulic conductivity of the aquifer. One of the main obstacles to expand its application in many area of interest is the low signal-to-noise ratio (SNR) of measurement. To improve the SNR by mitigating noise, numerous approaches has been proposed, for example remote reference noise cancellation (RNC) and model-based powerline harmonics subtraction (MB). In this abstract, the question whether MB should be applied before RNC is investigated. First, the theoretical length of Wiener filter (WF) in RNC is analysed by observing harmonics relationship between primary and reference channels. Then the benefits of applying MB before RNC is given within multiple noise sources are present. Last, the noise level of field measurements processed by RNC and MB followed by RNC are compared.
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Correlation between Distributed Rayleigh Sensing (DRS) and Moisture Sensors as Indicators of Slope Instability
Authors P. Clarkson, R. Crickmore, A. Godfrey, C. Minto, J. Chambers, B. Dashwood, D. Gunn, L. Jones, P. Meldrum, D. Morgan, A. Watlet and J. WhiteleySummaryThis paper describes the verification of Distributed Rayleigh Sensing (DRS), traditionally associated with acoustic sensing, for monitoring low frequency activity on a slope prone to landslides that is used as the British Geological Survey’s landslide observatory at Hollin Hill, North Yorkshire, U.K. The observatory is monitored using a variety of geological survey instruments and provides a unique opportunity to compare measurement systems that have very different principles of operation.
Previous studies of the slope have shown good correlation between the low frequency strain and temperature measured using the fibre with prior knowledge of the geology of the site and longer-term measurements made on more established geological survey instruments. This paper presents a more detailed comparison of measurements made on the DRS system over the winter of 2020/2021, with measurements of soil moisture content made on point sensors and estimates of ground movement measured using GPS marker posts.
The DRS system is sensitive to multiple important indicators of slope instability and can monitor ground movement effectively. Areas of unstable ground can be clearly identified by the larger changes observed in the fibre output in those regions.
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Cross-Borehole ERT: Sensitivity, Model Resolution, and Field Data Quality
Authors L.M. Madsen, A.K. Kühl, L. Levy and A.V. ChristiansenSummarySensitivity, resolution, and data quality are important parameters to consider when designing an ERT cross-borehole survey. We present an open-source algorithm for computing 2D and 3D sensitivity patterns of any borehole setup and use this to compare single-borehole and different cross-borehole electrode configurations, which both show complex patterns. To study the resolution capability of an entire survey, the model resolution matrix is computed for a field dataset. The dataset is split into different electrode configurations and the model resolution matrices of the different configuration are compared. The results show that the sensitivity and resolution decreases very quickly away from the electrodes, especially for the single-borehole configurations. In the studied field case, this means cross-borehole configurations are needed to correctly image the area between the boreholes, even though the cross-borehole data often are associated with a lower signal-to-noise ratio due to near-zero potential measurements and generally more outliers. The study concludes that in a production mode, where acquisition time and thus the number of possible data points are limited, the combination of electrode configurations must be carefully considered and a trade-off between resolution and data quality must be evaluated.
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Discrete Cosine Transform Reparameterization for Bayesian Time-Lapse ERT Inversion
Authors A. Vinciguerra, M. Aleardi, A. Hojat and E. StucchiSummaryTime-Lapse electrical resistivity tomography (TL-ERT) is used to monitor dynamic processes through mapping the resistivity variations in the subsurface. Inversion of TL-ERT data is a highly non-linear and ill-conditioned problem characterized by non-unique solutions. For this reason, an accurate uncertainty appraisal is essential to quantify the ambiguity affecting the estimated resistivity model. We present a probabilistic TL-ERT inversion in which the Differential Evolution Markov Chain (DEMC) algorithm samples the posterior probability density function, while the Discrete Cosine Transform (DCT) is used to compress the model space. The model compression aims at mitigating both the ill-conditioned nature of the inversion problem and the curse of dimensionality issue. On the other hand, the DEMC combines principles coming from metaheuristic optimisation methods and Markov Chain Monte Carlo algorithms to speed up the probabilistic sampling. To draw essential conclusions about the reliability and applicability of the implemented algorithm, we focus on synthetic inversion experiments in which we simulate two data acquisitions at different time instants (t0 and t1) and we jointly estimate the resistivity model at t0 along with the resistivity changes at t1. The results demonstrate that the implemented method provides accurate model predictions and uncertainty estimations with an affordable computational cost.
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Ensemble-Based Time-Lapse ERT Inversion with Model and Data Space Compression Through Deep Variational Autoencoders
Authors A. Vinciguerra and M. AleardiSummaryTime-lapse electrical resistivity tomography (TL-ERT) aims to image resistivity changes in the subsurface. This is an ill-posed and non-unique inverse problem and hence the estimation of the model uncertainties is of crucial importance. To reduce the computational cost of the probabilistic inversion, model and data can be re-parameterized into low-dimensional spaces where the inverse solution can be computed more efficiently. Among the many compression methods, deep learning algorithms based on deep generative models provide an efficient approach to reduce model and data spaces. Here, we propose a TL-ERT probabilistic inversion where the data and model spaces are compressed through deep variational autoencoders, while the optimization procedure is driven by the ensemble smoother with multiple data assimilation, an iterative ensemble-based algorithm that performs a Bayesian updating step at each iteration. This method provides multiple realizations for the quantification of the uncertainty by iteratively updating an initial ensemble of models that we generate according to previously defined prior model and spatial variability pattern. A finite-element code constitutes the forward operator. We test the method on synthetic data computed over a schematic subsurface model. Our tests demonstrate the applicability and the reliability of the proposed TL-ERT inversion.
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Low-Frequency Drone-Borne GPR for Soil Conductivity Mapping
Authors K. Wu, E. Jacquemin, L. Palt, L. Ory, T. Parizel, V. Vincent Dienst and S. LambotSummaryIn this study, we proposed a low-frequency drone-based GPR for soil conductivity mapping. The lightweight radar system consists of a handheld VNA, a microcomputer, a differential GPS and a power bank. A 5-meter half-wave dipole antenna is used to provide the operating frequency range of 15-45 MHz. For the data processing, antenna-drone effects are filtered out with the antenna transfer functions generated from calibration, and 3-D LUT is used for fast and robust inversion. The soil conductivity is obtained from point measurements using kriging interpolation. The result shows a good spatial correlation, and reasonable conductivity values comparing with EMI, which initially proves the concept of drone-borne GPR for soil conductivity mapping.
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Transept Foundations of a 12th Century Chapel Revealed by Geophysical and Photogrammetric Prospection
Authors C. Fauchard, L. Aillaud, A. Legrand, R. Antoine, V. Guilbert, C. Ledun and B. BeaucampSummaryThis study presents the work carried out at the Notre-Dame-du-Val chapel located in Sotteville-sur-Mer (Normandy, France). Local authorities are considering restoration work and have requested a geophysical survey of the surroundings. First, aerial photography by drone yields a Structure from Motion (SfM) model of the exterior of the chapel, completed by a 3D interior model. Geophysical prospecting consists in imaging shallow surface of surroundings, with help of Ground Penetrating Radar (GPR) and Electrical Resistivity Imaging (ERI).
GPR measurements clearly reveal former apses on both sides of the chapel, corresponding to the transepts meets in perfect agreement with the presence of filled-in openings visible in the 3D interior photogrammetric model. The ERI measurements underline the presence of a very resistive substratum, probably corresponding to the local chalky formation where the chapel was built. Besides, resistive formation underlines the potential earthworks carried out to support the transepts meets, correlated with GPR signals. Several GPR and ERI anomalies have not been interpreted and deserve deeper investigation: archaeological excavations remains the only solution to confirm their nature.
The restitution within the photogrammetric model allows a very explicit display of the results, integrating the architecture of the chapel and geophysical measurements.
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Preliminary Evaluation of Geothermal Energy Potential in Western Part of Dahomey Basin, Southwestern Nigeria
Authors E.A. Ayolabi, O.B. Balogun, M.O. Okunubi and R.P. AkinwaleSummaryGeothermal Energy, Dahomey Basin, Curie Point Depth, Temperature Gradient, Heat Flux
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Application of Satellite Imagery Landsat-8 to Identify Surface Hotspot as Preliminary Survey on Ranau
Authors T. Taufiq and M. MaharaniSummaryRanau Geothermal Prospect Area is located at borderline South Sumatera and Lampung Province on the Southwest Sumatera Island, bordered by three clusters of young volcanic, namely Mt. Pugung on Southwest, Mt. Seminung on South, and Mt. Raja on Northeast. With this condition, based on geological map, Ranau Area is dominated by volcanic breccia, tuff, and andesite-basaltic lava (Qhv) from Seminung Volcanoes, Andesite-Basaltic (Qv) with thickness 300 m from Pugung Volcanoes, contained by sulphide minerals and quartz veins (Tomh), and Rhyolitic tuff, pumiceous tuff, welded tuff with carbonaceous claystones intercalations (QTr). On surface mapping and based on reference found three geothermal manifestation such as Kota Batu hotspring on northeast of Seminung Mountain, Ujung hotspring on the west seminung, and wahid hotspring on northwest area. Based on Land Surface Temperature Map, we obtain the high temperature indicated by dark-red color with value is 35-50ºC. In this research, we would to correlate the result of landsat-8 satellite imagery (processed into land surface temperature map) with geothermal manifestation (three hotspring) based on reference to know the hotspot zone as a preliminary study of geothermal and geo-tourism prospect area.
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Integrating Electrical Resistivity and Seismic Refraction Tomography at an Active Landslide Site
Authors J. Whiteley, A. Watlet, S. Uhlemann, P. Wilkinson, J. Boyd, C. Jordan, M. Kendall and J. ChambersSummaryThis study demonstrates a procedure for preparing co-acquired ERT and SRT data from an active landslide for joint quantitative interpretation. We show the results of integrating these co-located datasets using a simple clustering approach to improve the interpretation of the inverted models. The results identify the major lithological units forming the structure of the landslide, and provide an example of the benefits of quantitative integration and interpretation.
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Geophysical Investigations to Study the Celtic Open Settlement of la Peyrouse (Dordogne, France)
Authors J. Hantrais, V. Mathé, P. Corfmat, G. Sheehan, C. Chevillot, R. Chapoulie and E. HiriartSummarySince 2019, geophysical archeological surveys conducted in the Celtic and Gallo-roman site of La Peyrouse (Dordogne, France) within the collective regional program RAPSODIE and a PhD research. The aim of this study is to explore, investigate and document an unfortified economic center dated from the 3rd century BC setting extensive near-surface techniques, particularly geophysical surveys. While excavating large-scale sites such as open Celtic settlements is a challenge for archaeologists, this protocol provides a quick understanding of the site. After five campaigns, the total area mapped in La Peyrouse with a manual geophysical system amounts to 12 ha. Numerous archaeological structures such as a craft production area, a religious building, a presumably public place, and other buried remains of the settlement were detected and mapped. Excavations on high-potential areas previously selected by geophysical surveys thus allowed an integrated archaeological interpretation. The compilation of data has provided new information on this large open agglomeration. This considerably increases our knowledge of this type of habitat still poorly known in Celtic Europe.
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Marine Gas Hydrate Deposits Study: Temperature Inversion Advantages
Authors A. Vasilev, E. Kozhuharov, N. Botoucharov, I. Genov, P. Petsinski and R. PehlivanovaSummaryThis work demonstrates the possibilities of the marine geothermy (heat flow methods) to reconstruct the formation process of heat-related geological objects with an example of 2D inversion of data from in-situ temperature measurements in seafloor sediments for gas hydrate deposit study in Danube paleodelta, Black Sea.
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Active and Passive 3d Seismic Survey Around the Scrovegni Chapel Using Autonomous Nodes
Authors I. Barone, R. Deiana, A. Ourabah, J. Boaga and G. CassianiSummaryA dense 3D seismic survey, including both active and passive surface wave measures, has been performed around the Scrovegni Chapel in Padua (Italy), in order to provide a deeper understanding of the archaeological setting of the area. In particular, ambient noise has been characterized both in terms of amplitude and direction of propagation. A cross-correlation analysis of 22 hours of continuous recording allowed to reconstruct the virtual source gathers, which are symmetric and comparable to active gathers despite the directional character of noise, typical of an urban environment. These preliminary results demonstrate the potential of this dataset to bring new insights about the area.
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Seismic Monitoring With Low-Cost MEMS Sensor Arrays in Italy
Authors V. Cascone, J. Boaga and G. CassianiSummaryMicro-Electro-Mechanical System (MEMS) sensors could efficiently integrate the use of common accelerometers to moderate/strong seismic events. In this study we present a new prototype of low-cost MEMS accelerometer, showing the results of a calibration experiment on the shake table apparatus. We install 16 new prototype of MEMS based accelerometers in two seismogenic sectors in Italy (Northern and Central Italy) in order to build distributed arrays. We find that these sensors are capable of recording seismic events (ML > 1.7) at distances in the order of tens of km. Moreover, MEMS seismic records are compared against the national accelerometric network data. In particular we compare the Peak Ground Accelerations (PGAs) and Spectral Accelerations. We find a good match of these two fundamental ground-motion parameters, demonstrating that these low-cost sensors could be an interesting choice for increasing the spatial density of stations and provide more accurate wavefield recordings.
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Calibration of multi-frequency EMI data: example at a test site in Rouen (France)
Authors C. Finco, F. Rejiba, C. Schamper and L.H. Cavalcante FragaSummaryElectromagnetic induction (EMI) is an efficient method, widely used to characterize the ground a large range of applications from hydrogeology, archeology to pedology. It is particularly interesting as it is not only sensitive to electrical properties of the ground but also magnetic and in certain condition dielectric as well. However, to quantitatively interpret EMI measurements, the data needs to be calibrated. The calibration process usually requires additional measurements, often a DC resistivity sounding or profile. Multi-frequency EMI devices, like the GEM-2 (Geophex Ltd) do not allow depth soundings like multiple offsets devices but multi-parameters interpretation is possible more often as the contribution of the different parameters to the measurement varies with frequency. Using the consistency of the measurements at different frequencies, should allow the calibration of the out-of-phase part of the data without additional measurements. In particular the interpretation of the out-of-part of the lowest frequency data should theoretically give a very similar apparent conductivity.
An example from a well characterized test-site in Rouen is presented here, showing the apparent resistivity calculated from the data before and after this self-calibration process. The confrontation with an inverted DC resistivity profile, show a good agreement between the methods after the calibration.
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Using 1C nodes in a 3C combination - benefits, and inconveniences
Authors A. Ourabah, L. Petronio, A. Affatato, L. Baradello, N. Goujon and Z. SongSummaryHigh-density multicomponent acquisition is perceived as the holy grail of land seismic, however, 3C is rarely considered when surveys are planned mainly because of the cost and bulkiness of 3C nodes and also the lack of land processing examples that could justify such spent, especially when it can be directed towards more proven benefits like increased trace density. Ironically, the availability and affordability of 3C data play a crucial role in developing the processing technology that could promote the acquisition of more 3C land surveys. The recent evolution in onshore nodal technology has brought to the market a variety of nodes of different sizes, shapes, and sensor types, some are small and light enough that even if combined in a 3C configuration remain smaller and lighter than a purpose-built 3C node. Acquiring a 1C, 2C or 3C survey with the same node’s inventory could be very attractive and could incite more users to attempt a 3C acquisition. In this paper, we demonstrate how piezoelectric nodes can be used in a 3C combination by comparing them to established geophone 3C sensors in a 2d S-wave survey. We also discuss the benefits and inconveniences of a such approach in land acquisition.
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Airborne and ground-based TEM mapping in polar regions — Antarctica cases
Authors N. Foged, L. Meldgaard Madsen, S. Tulaczyk and D. GrombacherSummaryWe present transient electromagnetics (TEM) mapping results from different geological targets in polar environments, primary from two airborne TEM surveys and one ground -based TEM mapping campaign conducted in the McMurdo Dry Valleys and on the Ross Ice Shelf in Antarctica. With the different cases, we demonstrate that the TEM methods are well suited to image different subsurface structures in polar regions as they can provide valuable information on ice thicknesses and discriminate between e.g. glacier ice/sea ice, frozen/unfrozen sea water, frozen/unfrozen water systems (permafrost), and bedrock/glacier ice. We further demonstrate that a high data and instrumentation quality, as the ground-based and airborne TEM data produces consistent results.
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The Application of Distributed Acoustic Sensing for Shallow Marine Investigations an Intertidal Case Study
Authors A. Trafford, S. Donohue, R. Ellwood, A. Godfrey and L. WacquierSummaryTypically, there is a paucity of geotechnical information available to designers of offshore foundations for marine renewable structures, which often results in highly conservative designs.
With an increased recognition of the value of shear wave velocity measurements as a critical design parameter in the marine geotechnical industry, there is a real requirement to develop a cost effective shear wave seismic data acquisition methodology.
This case study documents field trials at Dollymount Strand, Dublin using fibre optic cables as continuous seismic sensors for the collection of surface wave data. Data were collected using an OptaSense ODH4 interrogator connected to a 1000m long CST armoured single mode fibre optic cable. A Sercel Mini G airgun, operated from a survey vessel, was used as the seismic source.
The collection of data from an intertidal site allowed for direct comparison of different acquisition methodologies in both dry and flooded conditions.
The findings of the study showed that Distributed Acoustic Sensing (DAS) was effective at collecting shallow marine seismic data for analysis using the Multichannel Analysis of Surface Waves (MASW) method. Using DAS in this environment has great potential due to the rapid data acquisition compared to other currently available technologies.
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Anomalous geomagnetic variations associated with the Etna volcanic activity during February 2021
By S. RiabovaSummaryA series of paroxysmal events separated by relative calm periods at Etna volcano (37.75 N; 14.99 E) began on February 16, 2021 and continued to February 28, 2021. The magnetic effect caused by the eruption of the Etna volcano were analyzed. The data of registration of three components of the magnetic field of Mikhnevo observatory and a number of INTERMAGNET observatories, located at different distances from the volcano, were used. The result of data analysis reveals significant anomalous changes in the Earth’s magnetic field during paroxysmal Etna volcanic activity. The morphology of anomalies is similar at different distances from the volcano. In general, amplitudes of geomagnetic anomalies do not exceed tens of nT. Anomalies are most pronounced in the horizontal component of the Earth’s magnetic field.
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Modeling Hydrocarbon Bearing Reservoirs Using Fuzzy SVR and Electrofacies Analysis
Authors N. Moosavi and M. BagheriSummaryPermeability is one of the key petrophysical parameters of hydrocarbon bearing formations. One of the crucial roles of this parameter is to estimate production rate in oil bearing reservoirs. Permeability is usually measured by core plugs in laboratory. Although the measured value by this method is very precise, it is very expensive and time consuming. Furthermore, core data is not always available for all the wells. Over the last years, regression methods have been widely used to predict permeability in areas with core missing data. Among various regression approaches we selected SVM which works due to SRM and its produced model is less prone to over-fitting problems. Petro-physical properties in heterogeneous reservoirs vary in vertical displacements. So predicting them is difficult due to their abrupt changes. We exploited Electrofacies Analysis using Multi Resolution Graph-Based Clustering to partition formations due to their similarity. We also modified Support Vector Regression to FSVR by giving each data point a membership function to reduce effect of noise and outliers on data.The predicted outputs correlated with core data in the test well. The results show that predicted permeability from FSVR has a notably better correlation with real data compared to correlation outputs from SVR.
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Imaging a near-vertical structure with seismic refraction tomography: an offshore study
Authors B. Benjumea, F. Bohoyo, C. Morales, M. Druet, A. Maestro, C. Rey-Moral and C. EscutiaSummarySeismic refraction tomography (SRT) can be a useful tool for detecting lateral velocity changes in the subsurface. In this work, we leverage a traditional seismic reflection offshore dataset to image a high velocity structure imbedded in sediments. The studied profile is located in the western edge of the South Orkney Microcontinent (SOM). Seafloor depth in this sector ranges between 268 and 304 m. The multichannel streamer of 120 channels and 1.4 km length allowed to detect refraction arrivals from the subsurface and used them as input for SRT to reconstruct a P-wave velocity model of the near–structure. A layered initial model provided the best result retrieving a sharp image of the velocity anomaly. Interpretation of this anomaly was supported by the comparison between the SRT velocity model and the multichannel seismic reflection image. Shallow subparallel layers of sediments characterized by velocities between 1500 and 2000 m/s overly a zone with older deformed sedimentary layers related to velocities ranging between 2500 and 3000 m/s. Within this second layer, a high-velocity anomaly (more than 4000 m/s) is delineated. Geological context supports interpreting this anomalous structure as metamorphic rocks or gabbroic bodies from PMA (Pacific Margin Anomaly) as part of the basement.
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Typical Effects of the Registration Technology Implemented in the Gpr Receiver
Authors O. Gulevich, L. Volkomirskaya, A. Reznikov and V. VarenkovSummaryThe method of signal registration implemented in GPRs using frequency or time gating or nonlinear amplification of amplitudes may significantly impact the GPR data, in fact, distorting the reactions of rocks to the applied electromagnetic field of the probing signal. This distortion of the field occurs under various experimental conditions and may affect the ability to draw reliable inferences about rock characteristics if it is not taken into account when interpreting the GPR data.
Numerically calculated waveforms obtained in the approximation of geometric optics or significant simplifications of the models of propagation of electromagnetic signals cannot serve as a standard in GPR and be directly applied to a wide range of experimental results obtained by different GPRs, which differ in design.
We describe some of the typical effects of the GPR design on waveforms and some interpretational errors, which occur when the effect of the registration technology implemented in the receiver is not taken into account.
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Evolution of the Orange Basin; Cretaceous Deepwater Fold-and-Thrust Belts to Cenozoic Mass Transport Systems
More LessSummaryThe focus of this study is on the deepwater Orange Basin, offshore SW Africa, in which several DWFTB systems are found. Previous studies have mainly focused on the 2D seismic interpretation of the Orange Basin, which is naturally limited. In this study, the availability of high-resolution, 3D seismic reflection data will allow us to constrain the strato-structural architecture of the deep-water Orange Basin from a Cretaceous DWFTB system to the overlying Cenozoic deposits using Schlumberger’s Petrel E & P software package for seismic interpretation. Understanding the architectural elements of southern Africa’s passive margin, and the tectonic evolution of the DWFTB systems contained within, is important in building on the scientific knowledge known of what occurs in these settings worldwide and in further constraining prospective sites for petroleum exploration in similar settings.
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Complex Resistivity Imaging Using Controlled Source Electromagnetic Data
Authors J. Porté, J. Girard and F. BretaudeauSummaryControlled Source Electromagnetic (CSEM) method is an imaging technique using a multi-frequency electromagnetic signal to obtain the electrical resistivity of the underground, up to 3 km depth. This medium property is usually considered as a real and constant value in the quasi-static limit. Nevertheless, in some Earth materials, induced polarization (IP) phenomena are occurring when an electric perturbation is applied. These mechanisms are described by a frequency dependent complex resistivity (CR). CR is usually studied using electrical method with a direct current (DC) hypothesis, neglecting by the way electromagnetic induction.
We implemented a frequency dependent CR in the 3D Finite-Difference CSEM modelling and inversion code, POLYEM3D. A multi-stage approach is defined to undertake the multi-parameters problem according to their sensitivity. Furthermore, to manage an increasing number of parameters, second order polynomials are used to describe frequency variations of CR.
We show through synthetic inversions that we are able to recover a 3D complex resistivity image and its frequency variations from CSEM data in the IP/EM coupling domain, where IP signal is sufficiently large compared to EM induction.
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Experiments and Modelling of Seismoelectrics in the Ultrasonic Range: A Comparison With Electrokinetic Theory
Authors V. Martins Gomes, D. Brito, S. Garambois, C. Bordes and H. BarucqSummaryAcoustical wave propagation in a fluid-filled porous media creates electromagnetic (EM) disturbances through a physical process referred in the literature as seismoelectric (SE) conversion. Two EM fields result from this conversion, one accompanies the seismic wave in the form of a transient electric field and the other is generated at interfaces and propagates independently with an electromagnetic wave behaviour. The latter has an arguably high potential of application in near-surface exploration, possibly comparable to other indirect EM methods. In what follows, an experimental setup conceived to measure both SE fields mentioned above was employed in the study of a porous saturated homogeneous medium with (1) and without (2) a thin layer inside. It was seen that measuring absolute potentials is more efficient for detecting the EM-wave generated at interfaces than using electric fields estimated from measured potential differences. Using a numerical code and an experimental source, the measured data was compared with the modelling showing there is a fair agreement between experiments and electrokinetic theory. Our results are promissing and open the path for a more in depth investigation of the SE conversion phenomena and its relation with physical properties.
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Tectonic Evolution of the Deepwater Orange Basin (South Africa) Using 3d Reflection Seismic Data
Authors V. Mahlalela, M. Manzi and Z. JinnahSummaryThe application of seismic attributes to high-resolution 3D reflection seismic data acquired for the deepwater exploration block in the Orange Basin, offshore South Africa, provided insight into the mechanisms controlling deformation in hydrocarbon systems. The seismic interpretation was carried out on high-resolution, prestack time-migrated 3D seismic data that image post-rift sequences from the Upper Cretaceous to the Quaternary. Three post-rift sequences were identified in the seismic sections; post-rift sequences II, IV and V. Two slumping events caused by gravity failure of the margin were also identified in the Coniacian to Santonian and Late Cenozoic sequences. The seismic data also image a thrust fault system, which is interpreted to have been formed by episodic gravity failure from the Coniacian to Santonian. The thrust fault system soles into two detachment levels which coincide with the Cenomanian-Turonian boundary source rocks and top of the Turonian marine shale unit. The Late Cenozoic slumping event creates S to WSW and N to ENE dipping normal faults. The thrust faults exploit the S to WSW dipping normal faults to transport hydrocarbons to the seafloor to form pockmarks. Large submarine canyons were interpreted to have formed by a combination of the two slumping events.
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Impact of the Dem’s Resolution on 2d and 3d-Eri Common Pratice
Authors Y. Fargier, T. Dezert, R. Antoine, A. Tonnoir and C. FauchardSummaryERI is commonly applied to structures and sites whose complex geometry can affect the measurement considerably. Photogrammetry is a recognized method for obtaining dense and precise measurements of a surface. However, in the worst scenario, this topographic information is not or too little taken into account during the inversion process of ERI leading to artifacts. In the opposite scenario, topographic point clouds are too dense to be fully used by inversion software. In this paper, we present the effect of these various scenarios based on the inversion of synthetic data simulated on a complex geometry with an embedded anomaly. The results show the limitations associated with certain current practices as well as the quality of inversion obtainable with a limited amount of topographic information.
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Time-Lapse Monitoring of Moisture Induced Landslide Using Surface Waves at Hollin Hill Landslide Obsevatory
Authors L. Wacquier, J. Whiteley, D. Gunn, B. Dashwood, J. Chambers, A. Watlet, A. Trafford and S. DonohueSummaryThe Hollin Hill site has been used as a landslide research site to test different geophysical characterization and monitoring methods, to assess temporal and spatial stability. This abstract presents the use of surface waves content and its dispersive properties, in the context of moisture induced landslide monitoring of seasonal variations, using 16 seismic datasets acquired between October 2016 and August 2019. For each dataset, a 2D pseudo section of the landslide is assembled, and the relative difference between each dataset is calculated. The statistical distribution of each difference indicates a seasonal variation control or a soil moisture control on surface waves velocity measurements with time. Specifically, this approach shows potential for monitoring seasonal variability in elastic properties linked to changes in soil moisture, which has a direct impact on periods of more active slope movement. These results are currently being investigated further and will be discussed in future work.
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Porosity of Near-Surface Soil Layers From 3d Elastic Full-Waveform Inversion: Tests on Synthetic Data
Authors Y. Kawasaki, R. Ghose and S. MinatoSummaryThe porosity of the shallow soil layers is a crucial information in hydrogeology, soil science, agriculture, and construction engineering. To resolve the spatial distribution of porosity in soil, 3D elastic full waveform inversion (FWI) can be a promising tool. In this research, we have tested 3D FWI on a realistic subsoil model constructed from field-measured data. Different source types, dominant frequency of the seismic source, and the inversion frequency have been examined on the synthetic dataset. Our results suggest that (1) the use of SH and Love waves leads to a higher accuracy compared to P-SV or Rayleigh waves, and (2) the use of a broader frequency range in inversion and/or a source wavelet of higher dominant frequency leads to more accurate and better resolved Vs and bulk-density models. Finally, the porosity values for the different layers have been estimated from the inverted bulk density profile. Our results indicate that 3D FWI has the potential to estimate reasonably accurately the Vs and density profiles, which can in turn offer 3D porosity distribution.
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Monitoring Settling and Consolidation of Fluid Mud in a Laboratory Using Ultrasonic Measurements
Authors I. Fadel, A. Kirchek, M. Buisman, K. Heller and D. DraganovSummaryUltrasound measurements are routinely used to evaluate the safe depth for ships navigation - nautical depth - at waterways and ports using single-beam dual-frequency echo-sounders. The nautical depth is routinely defined by suspension density in the range of 1100–1300 kg/m3 in the mud layer. While ultrasound measurements have a weak sensitivity to density variations, calibration is always needed to convert ultrasound measurements into reliable indicators for nautical depth levels in the mud layers using densely distributed density rheological in-situ measurements.
We present a laboratory ultrasonic transmission experiment to monitor the fluid mud’s settling and consolidation processes using a sample from the Port of Rotterdam. We use P- and S-wave ultrasonic transducers in the frequency range between 200 to 1000 kHz. Our results show that the P-wave velocities slightly increase during the consolidation and settling process while the P-wave amplitudes decrease. On the other hand, we observe a high S-wave velocity that increases together with amplitudes over time. The P- and S-wave amplitude and S-wave velocity variation over time correlate well with the mud average density variation. The presented results can be very useful for fluid-mud monitoring at a lab scale, besides possible utilization for large-scale monitoring field campaigns.
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Diffraction Recognition Using Deep Learning
Authors M. Markovic Juhlin, R. Malehmir and A. MalehmirSummaryDiffractions are important features in seismic data that carry information on small-scale geological entities. Because of their low-energy signal compared to the reflected waves, in order to image them, diffraction signal needs to be separated from the rest of the wavefield including ambient noise. Several methods have been developed for diffraction signal separation, and the newest trends include deep learning algorithms. Using deep learning, as a tool for diffraction recognition and separation has been promising particularly in sedimentary settings. In this study we use deep learning algorithm for diffraction classification in hardrock settings dominated with much lower signal-to-noise ratio and much higher background velocity. Staring with simple synthetic seismic sections and in order to make more complex case, we add more features such as different noise levels and reflection features with different dip directions. Results show good potential, taking into account a limited amount of training data. We also employed the developed algorithm on a real GPR dataset with strong diffractions to showcase the workflow. Our longer-term plan is to build a workflow enabling deep learning algorithms for hardrock seismic datasets in order to automatize feature extractions including diffractions.
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Seismic SH Full Waveform Inversion: A tool for high-resolution near-surface characterization
Authors D. Köhn, M. Zolchow, R. Mecking, D. Wilken, T. Wunderlich, D. De Nil and W. RabbelSummaryIn recent years, Seismic Full Waveform Inversion (FWI) has been established as a high-resolution prospection method. Since 2014, the Applied Geophysics working group at Christian-Albrechts-University Kiel applied FWI to seismic SH data with different degrees of complexity for near-surface characterization tasks related to archaeological prospection and engineering geophysics. In this talk, we will review three of our field data applications which demonstrate the potential of FWI to resolve near-surface structures. The close collaboration with archaeologists allowed the unique opportunity to compare FWI results with the true subsurface structures. In the first example, SH-FWI is applied to field data from a transect over a medieval canal structure in southern Germany. The FWI is able to resolve the definition of the canal shape and small-scale structures within the canal. Significant surface topography poses another challenge for FWI. An example with significant surface topography is the Danewerk fortification wall in northern Germany. In this case, the FWI is able to resolve structures related to the construction history of the Danewerk. Finally, we applied a 2D SH-FWI workflow to resolve a partially collapsed excavation tunnel embedded deep inside a burial mound in Pergamon (Turkey).
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A physical model to study deep contaminated sites: ERT study with surface-downhole electrode configuration.
Authors M.V. Bongiovanni, V. Grünhut and E. LópezSummarySurface electrical resistivity tomography is a widely used tool to map the subsurface. One of its limitations is the decrease in resolution as depth increases. Another limitation is that the electrodes planted on the surface can be heavily influenced by temperature, weather, and water saturation changes over time. Consequently, the data can be easily contaminated by noise and therefore, unreliable for long term monitoring. Borehole DC electrical surveying allows to extend the anomaly detection capability beyond the limits of surface electric surveying. Even more, with two wells, the cross-hole DC electrical surveying provides detailed information on the variation of electrical resistivity between the boreholes, but just in a very limited zone near them. The implementation of borehole to surface electrical resistivity tomography allows to reduce this limitation. Such an arrangement is expected to provide an increase in detection capability in the area in-between the boreholes and surface. In the present work we studied the feasibility of surface-downhole measurements to detect and estimate the dimensions of a contamination plume in a deep aquifer, performing a physical model at laboratory scale. We conclude that the detection and consequently the monitoring of contaminated deep aquifers with two wells using surface-downhole ERT is possible.
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Characterization of a complex fault system by 2D acoustic Random Objective Waveform Inversion
Authors D. Köhn, M. Thorwart, D. De Nil and W. RabbelSummaryOne key ingredient of a successful seismic full waveform inversion (FWI) is the choice of the objective function. Robust FWI approaches combine objective functions with different weights of the phase- and amplitude-mismatch between modelled and field data. Typically, the phase-mismatch is corrected first and the amplitude-mismatch later. However, the weighting of one waveform information over another can result in convergence issues. In contrast, the recently proposed Random Objective Waveform Inversion (ROWI) randomly applies different objective functions at each stage of the FWI. In this study, we created a ROWI based on the combination of the robust Global Correlation Norm (GCN) and an AGC-weighted l2-norm. While the GCN is able to fit waveforms of the direct wave and dominant reflections, the AGC-norm enhances weak reflections with the drawback of destroying the amplitude information. Therefore, both objective functions can complement each other. The proposed ROWI approach is applied to land-seismic data acquired above the Glücksstadt Graben in Northern Germany. The ROWI result reveals a complex, heterogeneous system of thrust faults and disturbed layers within the Graben, while a much simpler layering is present outside of the Graben. The ROWI result is consistent with a time-migrated reflection seismic image and borehole logs.
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Comparison between classical and geostatistical regularization methods for ERT crosshole imagery
Authors K. Tsakirmpaloglou and O. KaufmannSummaryERT imaging between boreholes is a method of imaging and monitoring that has many applications. In stratified environments with little deformation, geostatistical regularization is reputed to lead to geologically more realistic results than approaches based on classical smoothing constraints with anisotropy. Using inversions carried out on a synthetic case, we examined the impact of different regulations on the recovered model.
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Monitoring a Spatial Drilling Trajectory Deviation Using a Drill-Bit Signal as a Source.
Authors Z. Wilczynski and A. KaslilarSummaryDeviation from the desired drilling trajectory may cause operational problems. Real-time monitoring of the drill bit position helps in mitigating these issues. In this study, we suggest a seismic-while-drilling (SWD) method to detect the (near) real-time drill bit position. We use a homogenous model typical for hard rock environments and conduct 3D finite-difference modeling of elastic wave propagation. We introduce a receiver array at the surface and a drill bit source at different depths deviating from its vertical trajectory. By cross-correlating the records in respect to a virtual source location, we obtain differences of arrival times at a given trace versus arrival times at the virtual source location. The correlation image presents a hyperbola where the apex indicates the horizontal location of the source, i.e. the closes drill-bit receiver path. Considering a 2D orthogonal array, and a 3D array, we show the possibility of monitoring and estimating the horizontal location of the drill bit.
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Three decades of reflection seismic surveying at Neves-Corvo, Portugal
Authors G. Donoso, A. Malehmir, J. Carvalho and V. AraujoSummaryThe Neves-Corvo is arguably the largest and one of the most important volcanogenic-hosted massive sulphide (VMS) endowments in the world. In the last three decades three seismic acquisition programs have been carried out in the area. In 1996, six profiles covering the area, with a special interest in the tier-1 Lombador deposit. Reprocessing of this dataset was done in 2019 and showed the relevance of revisiting legacy data using current tailored processing workflows. In 2011, a 24 km2 3D survey was done with the intention of producing new targets and in particular with the discovery of the Semblana deposit in 2010, the 3D dataset showed its potential beyond the earlier 2D surveys. In 2019, new technologies for seismic exploration were tested, the new seismic survey consisted of two perpendicular 2D surface profiles positioned above the known Lombador deposit, additionally source points were activated inside the exploration tunnels and simultaneously recorded on the surface, which provided an improved velocity model for migration and time-to-depth conversion. The objective of this study is to provide an overview of these surveys and present the latest results of revisiting the 2011 3D seismic data.
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Tomographic seismic imaging of a carbonate core at the laboratory scale
Authors D. Brito, C. Shen, J. Diaz, C. Bordes, J. Virieux and S. GaramboisSummaryCarbonate rocks studies are of particular interest in exploration. Such rocks are generally heterogeneous, anisotropic, porous and fractured and imaging their petrophysical properties are usually challenging with classical techniques. We present here a laboratory study where seismic tomography, a widely used method in Earth interior imaging, is tested on a decimetric carbonate core. The goal is to obtain a high-resolution Vp tomography of the interior of the carbonate rock using innovative experimental approaches and inversion workflows, the processing being accompanied by direct numerical modeling of the wave propagation within the carbonate sample.
A point-like pulsed-laser and a piezoelectric transducer are alternatively used as seismic sources and a laser interferometer records the travelling waves reaching the periphery of the carbonate core. Tomographic images of Vp are obtained and analyzed with both sources and compared to an X-ray CT-scan tomography.
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Convolutional neural networks for the characterization of magnetic anomalies
Authors J. Cárdenas Chapellín, C. Denis, H. Mousannif, C. Camerlynck and N. FlorschSummaryDuring the last decade, deep learning architectures demonstrated to be very promising in geophysics, notably in the field of seismic interpretation. Their great power and flexibility may produce valuable contributions to finding solutions to a diversity of geophysical constraints, for example, the limitations of inversion algorithms or the fine-tuning often required on the different noises that affect the data. In our paper, we use convolutional neural networks to characterize magnetic geological bodies. This characterization consists of counting the number of dipolar magnetic anomalies produced by these bodies and predicting their corresponding parameters. After several experiments with deep learning architectures, we found that the combination of YOLO and DenseNet achieves excellent performance by reaching an accuracy of over 90% in their respective metrics. We also applied visualization tools to explain our results. We managed to visualize the discriminative area of our network with Grad-Cam, and we observed the "logic" of the predictions by using t-SNE. In future studies, we will evaluate the robustness of the proposed approach using real data, for example, in the context of pyrotechnical detection for unexploded ordnance exploration.
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On the applicability of 2D SH-FWI for high-resolution imaging of 3D subsurface structures
Authors D. Köhn, M. Thorwart, D. De Nil, J. Albert, W. Rabbel and F. SirockoSummarySeismic full waveform inversion (FWI) has been established as a tool for the characterization of near-surface structures in a wide-range of applications from engineering/archaeogeophysics to hydrocarbon exploration. To reduce computational costs, most FWI applications are using 2D medium approximations to describe seismic wave propagation in the framework of the inverse problem, even if the subsurface structures would demand a 3D medium description. In this study, we investigate the applicability of a 2D SH-FWI to map complex, 3D subsurface structures of a potential thrust fault system, located east of the Great Plön lake in Northern Germany. During two extensive, geophysical field surveys in 2017/2018, four intersecting, seismic SH-profiles were acquired, covering the study area. The independent FWI of the SH-profiles with respect to the shear-wave velocity and density model reveals complex, heterogeneous subsurface structures. Despite the simplified 2D medium approximations used in the FWI, similar features are visible at the intersection points of the SH-profiles. The near-surface area down to a depth of 15 m is dominated by thin, alternating, layers disturbed by low-velocity anomalies. The deeper subsurface consists of high-velocity anomalies dipping in SW direction. The general trends of FWI density models can be correlated with borehole density logs.
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Seismic Characterization of a historical heritage building
Authors M.A. Martinez, C. García, M. Vásconez, E. Aracil, U. Maruri and P. MartínezSummaryHistorical heritage building, Passive MASW, Vs30
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In-Mine Seismic Method for Platinum Orebody Exploration in Maseve Platinum Mine, South Africa.
Authors M.K. Rapetsoa and M. ManziSummaryIn-mine seismic experiments were conducted to delineate geological structures and possibly the platinum orebody (UG2) that lies beneath the incline shaft. Three 115 m segments of seismic lines were collected, with a spread of 355 m. The seismic lines were designed to image geological structures along the incline shaft using 10 Hz receivers and a sledgehammer. Preliminary results are presented in this study and reflections identified on the raw shot gathers are interpreted to originate from the slightly dipping Merensky reef that is few meters below the tunnel. This study demonstrates and encourages the use of in-mine seismics for future mine development and planning.
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Classification of Vineyard Soil Physicochemical Zones Using Non-Invasive Frequency-Domain Electromagnetic Induction and Ndvi Methods.
Authors P. McLachlan, M. Schmutz, J. Cavailhes and S. HubbardSummaryIn this work, a novel method for obtaining maps about the physiochemical nature of soils from electromagnetic induction (EMI) data is investigated for a French vineyard. Specifically, K means clustering is used to produce maps from both the quadrature and in-phase components of EMI measurements. The commonly discarded in-phase component of EMI measurements exhibits a higher correlation with plant vigor, as measured by NDVI. In addition to the non-intrusive characterization, extensive soil samples are used to explore the correlations and causations that link soil physiochemical properties with EMI and NDVI data. It is demonstrated that the in-phase component of EMI is related to metal concentrations at the site, these properties co-vary with crucial properties related to plant vigor, e.g. cation exchange capacity and organic matter content.
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Geophysical Imaging of the Root Zone: Methods, Implications and Outlook.
Authors G. Cassiani, B. Mary, J. Boaga, I. Barone and V. IvanSummaryThe use of non-invasive geophysical methods has been demonstrated to be of high value in root zone characterization. The potential for further development of these methods is very large and will provide indispensable information for understanding and modelling of SPAC / ECZ (Earth Critical Zone) processes.
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Sparse Data Transformation for Unsupervised Clustering for the Exploration Ahead of Tunnel Face
Authors A. Sapronova, P.J. Unterlas, J. Hecht-Méndez, T. Dickmann and T. MarcherSummaryIn collaboration with Amberg Technology, 1Institute of Rock Mechanics and Tunnelling at Graz University of Technology is developing a model to predict geological conditions ahead of the tunnel face. The model employs a cascade of unsupervised and supervised machine learning algorithms and uses the seismic data and available geological documentation at the underground construction site. At first, we use unsupervised methods to cluster the entire dataset into an arbitrarily defined number of clusters so that each cluster contains a unique label (representing geological conditions). We then train supervised classifiers to predict the label(s) for each cluster.
In this work, we elaborate on developing two critical parts of this model: pre-processing and unsupervised clustering of the dataset. We tested several methods for sparse data de-noising and clustering to answer the following question: ""given a sparse structured mesh or a graph (unstructured), what method can be used for pre-processing and unsupervised clustering, to reveal major characteristics (features) of the dataset at the low dimensional space."" Four methods that showed the most robust results were selected and used further in the model’s versioning.
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Geophysics for Cultural Heritage
By R. DeianaSummaryThe main characteristic of geophysical methods is the non-invasiveness and their consequent ability to investigate and, therefore, know a system providing information on anomalies, in a specific time or time-lapsed, studying the non-direct way the variations of one or more physical parameters. This feature is undoubtedly crucial for all investigations in cultural heritage, for a particular system or artefact under protection.
The development of new technologies, sensors, and software has led to new geophysical applications in cultural heritage analyses in recent decades. Today, thanks to geophysical methods at different scales, besides the more systematic and well-known exploration, geological, hydrogeological, and engineering applications used over the times, we can provide crucial information on the state of ancient systems and objects, fundamental for restoration and conservation strategies, then opening new frontiers for the application of these non-invasive methods in cultural heritage applications.
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Georadar Investigations in the Church of San Paolo (San Giacomo Dei Militari, Palermo).
Authors P. Capizzi, M. Marrone, C. Aleo Nero, A. Bonfardeci, A. Canzoneri, A. Carollo, R. Martorana and F. RomanoSummaryAs part of a research project aimed at drawing up a restoration project for the church of San Paolo, the monument was investigated with georadar surveys. The investigations were followed in a targeted manner by archaeological excavations, under the supervision of the Superintendence of Cultural and Environmental Heritage of Palermo.
The San Paolo church is located in the oldest part of the city. The geophysical investigations will be useful to identify any pre-existing structures on which the church could be founded. The georadar investigations focused on the central nave and one of the side aisles to identify buried structures and crypts. The data showed several anomalies. One of these, located in the middle of the central nave, has been verified with an excavation. The entrance to a crypt, not yet explored, and several bone fragments were then found.
Finally, some high-frequency profiles were acquired in the walls and columns of the church, which highlighted numerous anomalies that will be verified with exploratory tests.
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Perspectives in Ground-Penetrating Radar at High Latitudes: From Occasional Imaging to Automated Continuous Monitoring
Authors A. Saintenoy, E. Léger, C. Grenier and N.M. ThiérySummaryAmong different geophysical methods employed for glacier and permafrost characterization, Ground-Penetrating Radar has been employed many times since its first developments.
However these GPR characterization were and are still mostly performed during field campaigns which can be sparse in time, expensive and human resource demanding, and can be limited by the difficulty to access areas.
The purpose of this short paper is to present two approaches for northern latitudes features imaging during field mission and discuss the benefits brought by hardware and software improvements to develop the field of long-term monitoring. The main objective of this paper is to highlight the new paradigm that the internet of things brought to the field of surface GPR characterization of high latitude environment.
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Three-Dimensional Time-Lapse Inversion of Tem Data With Application in an Icelandic Geothermal Site
Authors L. Xiao, G. Fiandaca, P. K. Maurya, A. Vest Christiansen and L. LévySummaryThe transient electromagnetic (TEM) method is widely used in resistivity mapping but rarely applied in monitoring experiments. In this study, we present an algorithm to invert time-lapse TEM data, with inversion of both synthetic data and field data. Three main novelties distinguish this new inversion algorithm: i) a multiple-mesh approach is used for the definition of model parameters and forward modelling, ii) the forward and jacobian computations are carried out in 3D and iii) two datasets, each composed of several TEM soundings, are inverted simultaneously with a generalized minimum support norm for time-lapse changes. In the synthetic example, dense and coarse acquisition layouts are modelled, to study the effect of data coverage on model retrieval. Coarse data coverage allows to retrieve the time-lapse anomaly, thanks to the 3D sensitivity of TEM data. However, dense data coverage over the anomalies gives better resolution, as expected. In the field example, we present the time-lapse inversion results of data collected at an Icelandic geothermal powerplant in 2019 and 2020. The TEM data were acquired for defining the baseline in a monitoring an experiment of Hydrogen sulphur sequestration planned for 2021. As expected, no variations were imaged by the time-lapse inversion.
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A Theoretical Approach to Near Surface Pedophysical Permittivity Models
Authors G. Mendoza Veirana, P. de Smedt, W. Cornelis, D. Hanssens and J. VerheggeSummaryDespite well-established knowledge about the relationships between the electrical conductivity or relative permittivity of rocks, ambiguity remains as to how these relationships and knowledge can be transferred to soils. In general, there are three problems that hinder the robust application of available pedophysical models. First, these models often rely on soil properties and attributes that cannot be directly quantified in the laboratory or in the field, such as pore connectivity and depolarisation factor, which reduces their applicability. Secondly, the introduction of tuning parameters (such as the cementation exponent and Roth's exponent) tends to reduce the theoretical significance of the physical process in question. Finally, oversimplifying pedophysical relationships by considering only one soil attribute (such as moisture content) effectively ignores the effect of other relevant properties (clay content or bulk density). Here we present a theoretical approach that shows how to sort out this issues while both fitting parameters and anisotropy indicators can be derived from soil properties that are easy to measure in the laboratory.
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Interpretation of Legacy 3d Seismic Data for Underground Platinum Mines: Implication for Mine Safety
Authors S. Matloga, M. Manzi and G. BybeeSummaryMethane gas pockets are intersected during normal mining activities and this causes explosions. Methane gas migrates from deeper sources to mine working environments through faults and dykes. High-resolution 3D reflection seismic method was employed in the western Bushveld Complex to a depth of 1500 m to delineate geological features in subsurface mining environment. Strong seismic reflections were tracked on the seismic cube to create seismic horizons of the platinum-bearing horizons. Preliminary results show faults at various orientations crosscutting the UG2 and Merensky Reef located between 400 – 900 m below surface. Iron-rich ultramafic pegmatoids and potholes were mapped, the former as vertical and chaotic seismic reflection features and the latter as slump structures along the seismic reflections. Spatial correlation between these seismically detected features and methane occurrences influences this research to encourage the implementation of mitigation strategies for future mine planning and development.
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Combining Soil Sampling, EM38 and 3D GPR Techniques to Map Key Water Distribution Parameters
Authors E. Bloem, J. Sala, H. Johansen Lindgaard, I. Sturite and Ø. AustadSummaryTo understand the drainage capacity of agricultural soils in Norway, we need to understand the dynamics of water distribution in the subsoil. We need a better overview of existing drainage systems in combination with the soil types on which agriculture is performed. We have investigated an agricultural field using a combination of traditional soil sampling, EM38 and 3 dimensional Ground Penetrating Radar (3D GPR).
Combining EM38 with the soil sampling made it possible to map the lateral heterogeneity of the soil types at the experimental site. The 3D GPR added a depth resolution otherwise not available with the EM38. We have cross analysed the results from the EM38 and the depth slices from the 3D GPR and we can conclude that there is a positive match between the images obtained from the two methods. The variations in apparent conductivity tie in very well with the soil texture, in terms of scattering and attenuation, that can be appreciated in the 3D GPR data. Also, we were able to map the existing drainage system. The methods provide complementary information that can be used to better understand the dynamics of water distribution in the subsoil.
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