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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 - 50 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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