bluuebunny/crossref_metadata_2025_split huggingface dataset:
Locating mofettes using seismic noise records from small dense arrays and matched field processing analysis in the NW Bohemia/Vogtland Region, Czech Republic
Authors: Flores Estrella Hortencia , Umlauft Josefine , Schmidt Andreas , Korn Michael 
Published: September 2019

ABSTRACTThe NW Bohemia/Vogtland region is characterized by currently ongoing geodynamic processes within the intracontinental lithospheric mantle. Among others, this activity results in the occurrence of mid‐crustal earthquake swarms and CO2 degassing zones called mofettes. These two natural phenomena are related to each other since it is considered that fluid flow and fluid‐induced effective stress can trigger earthquake swarms. At the Earth’s surface, they appear spatially separated, but their connection could be explained by the existence of pathways within the crust that allow efficient and permanent fluid transport. However, neither the structure nor the position of such pathways has been imaged yet.With this background, we used a matched field processing analysis within the NW Bohemia/Vogtland region to locate mofettes and investigate their characteristics. Considering the CO2 degassing process as a high‐frequency noise source, we chose two different test sites: the Dolní Částkov Borehole, which is an artificial mofette that we used to validate the method, and the South Hartoušov mofette field, a natural CO2 degassing area. On both sites, we measured seismic noise in a continuous mode for several hours (7 hours to 9 hours), with a sampling frequency of 250 samples/second, in multiple campaigns using an array of about 60 × 60 m2 with approximately 30 randomly distributed stations.For the matched field processing computation, the phase velocity of the study area is required, which we obtained from active seismic experiments with vertical hammer‐blow as the source. The phase velocity varies between 200 m/s and 420 m/s for Dolní Částkov and between 100 m/s and 280 m/s for South Hartoušov, both in a frequency range of 7 Hz–60 Hz.With the matched field processing analysis at the artificial mofette in Dolní Částkov, we could relocate the noise source successfully. In the South Hartoušov mofette field, we detected one dominant vertically extended noise source, probably a fluid pathway, as well as a small matched field processing maxima at the surface that can be related to a dry mofette.
Detection of geological structures using impact‐driven piling as a seismic source
Authors: Farmani Bagher , Kitterød Nils‐Otto , Gundersen Elisabeth 
Published: September 2019

ABSTRACTSeismic imaging during piling may reduce construction costs in sensitive areas, and combined with technology to look ahead of the pile tip, the risk of stopping the piling before reaching the bedrock may be reduced. Seismic images obtained during piling can give vital information about the subsurface structure in the vicinity of the pile. In this paper, we discuss how to produce seismic images as a part of the piling procedure without using any external seismic sources. Using surface and borehole synthetic data examples, we show that it is possible to use pressure (P) waves emitted from the pile tip for imaging purposes. A pilot field study was carried out to test the feasibility of this method. The field study revealed that high‐frequency pressure waves are emitted from the pile tip. However, they are masked by strong surface and shear waves, especially at the nearer offsets. The data processing approach allowed us to achieve reasonable signal‐to‐noise ratio for pressure waves at the further offsets, while the signal was not as well recovered at the nearer offsets. This study demonstrates that it is possible to acquire fair‐quality signals without using any seismic source other than the impact on the pile itself. More field data are needed and some acquisition procedures need to be optimized before the method can be applied for practical imaging.
Using electrical anisotropy for structural characterization of sediments: an experimental validation study
Authors: Al‐Hazaimay Sadam , Huisman Johan A. , Zimmermann Egon , Vereecken Harry 
Published: September 2019

ABSTRACTImproved characterization of subsurface heterogeneity is important to better understand a range of key processes in the hydrologic cycle, such as overland flow, infiltration into the soil, evaporation to the atmosphere, and transpiration by plants. Recently, synthetic modelling studies have shown that information on subsurface heterogeneity can be obtained from the anisotropy in electrical resistivity. The objective of this paper is to experimentally validate the findings of this synthetic modelling study. In order to do so, we developed a new measurement procedure to determine the effective complex electrical resistivity from a set of current injections and voltage measurements on a heterogeneous sample. A synthetic modelling study showed that this new measurement procedure was able to reproduce the results of the previous study that showed how the electrical properties and the correlation length ratio of bimodal distributions of two materials can be obtained from the effective complex electrical resistivity measured in two perpendicular directions. After validation of the new measurement approach, we constructed two bimodal distributions in a 2D measurement cell and applied the newly developed measurement strategy. We were able to estimate the electrical properties, the volume fraction, and the correlation length ratio with good accuracy from the complex resistivity measurements in two directions. The remaining differences were attributed to variations in sediment thickness that occurred during sample preparation. We conclude that anisotropy should not be ignored but embraced when dealing with subsurface heterogeneity and that proper interpretation of anisotropy may actually be used to characterize subsurface heterogeneity.
Seasonal saline intrusion monitoring of a shallow coastal aquifer using time‐lapse DC resistivity traversing
Authors: Sutter Eva , Ingham Malcolm 
Published: September 2019

ABSTRACTLong‐term time‐lapse DC resistivity measurements along six 145‐ to 275‐m‐long transects have been carried out to image seasonal changes in the resistivity distribution within a shallow coastal aquifer on the Kapiti Coast, New Zealand. Two‐monthly repetition of the measurements shows a continuous decrease in the bulk resistivity at depth over an extended horizontal portion of the profile lines when coming into the drier summer season. This is most distinct in more rural locations and less pronounced along urban profiles, and is interpreted to be the result of a decrease in the fluid resistivity resulting from lower freshwater pressure that allows saltwater to intrude. Coming into the wetter winter season, the bulk resistivity at depth increases again, as freshwater pressure in the aquifer increases. A 24‐hour tidal cycle monitoring along one of the northern profiles, observations of seawater level stages, and tidal ranges show that the observed resistivity changes are seasonally driven rather than tidal overprints. Time‐lapse monitoring using electrical resistivity tomography has thus proven itself as a suitable tool for imaging both the saline interface shape and extent, and the changes in the subsurface resistivity distribution in an unconfined coastal, shallow, sand aquifer over a seasonal time frame. This significantly adds to the knowledge about the dynamic behaviour near the saline interface in coastal aquifers and can be used for groundwater assessment in this and other similar geological and hydrological environments around the world.
Spectral time‐domain induced polarisation and magnetic surveying – an efficient tool for characterisation of solid waste deposits in developing countries
Authors: Dotse Wemegah David , Fiandaca Gianluca , Auken Esben , Menyeh Aboagye , Kojo Danuor Sylvester 
Published: September 2019

ABSTRACTTime‐domain induced polarisation and magnetic data were acquired to map and characterise the decommissioned and un‐engineered municipal solid waste disposal site of the Kwame Nkrumah University of Science and Technology, located in the Kumasi Metropolis of Ghana. In this survey, 13 induced polarisation profiles 500–800 m long and 26 magnetic profiles 400–800 m long were acquired. In addition, two boreholes were drilled to help in the interpretation of the geophysical data. The study was carried out with the aim of determining the risk posed by the waste deposit to the quality of the soil and the ground water system, which is the main potable water supply for the Secondary School, the University Teaching Hospital and the Veterinary School, situated within the catchment area of the site. Full‐decay 2‐D time‐domain induced polarisation inversions in terms of Cole–Cole parameters were used for interpreting the induced polarisation data. The chargeability, resistivity and normalised charge‐ability distributions, together with the magnetic results, aided in a full characterisation of the site geology, the waste and the associated pollution plume. In particular, clear contrasts in resistivity and the polarisation parameters were found between the saprolite layer and the granitic bedrock, which are the main lithological units of the area. Furthermore, it was found that the Kwame Nkrumah University of Science and Technology waste deposit is characterised by a low‐chargeability and low‐resistivity signature and that the low‐resistivity area spreads out from the waste deposit into the permeable saprolite layer, indicating the presence of a leachate plume. A fracture zone in the granitic bedrock beneath the waste deposit, which is a potential conduit for leachate contamination of the ground water system, was also identified. The study thus provides the information needed for assessing the future impact of the waste on the water quality in the area and for designing risk‐mitigation actions.
A controlled monitoring study of simulated clandestine graves using 3D ground penetrating radar
Authors: van Schoor Michael , Coenraad Nienaber Willem , Marais‐Werner Anatulie 
Published: September 2019

ABSTRACTA controlled three‐dimesional ground penetrating radar monitoring study over simulated clandestine graves was conducted near Pretoria, South Africa, in which the detectability of graves as a function of post‐burial interval was assessed, as this is of particular interest to local forensic investigators. It was demonstrated that the site‐specific environmental parameter (a clay‐rich loamy soil with poor drainage) and heavy seasonal rainfall (as confirmed by ground‐penetrating‐radar‐derived soil moisture estimates) drastically compromised the long‐term grave detectability, especially when adopting a three‐dimensional depth slice analysis approach. It is also seen that the disturbed burial zone is the major contributor to the total grave anomaly rather than the buried body due to the combination of environmental parameters and the absence of buried artefacts. This paper also advocates the combined use of different data representations (two‐dimensional and three‐dimensional) to increase the likelihood of detecting subtle grave anomalies.
Characterization of the sedimentary fabrics in ornamental rocks by using GPR
Authors: Rey Javier , Martínez Julián , Montiel Violeta , Cañadas Francisco , Ruiz Nicolás 
Published: September 2019

ABSTRACTIn this study, the use of ground‐penetrating radar for characterising ornamental sedimentary rocks was tested. Specifically, the ability of this non‐invasive geophysical prospecting method to identify fabrics and textures in rocks was investigated. Blocks mined from quarries were analysed, and ornamental rocks with the same lithologies as other widely utilised ornamental rocks with a variety of sedimentary fabrics and textures were selected. Rocks with clastic brechoid, cross‐laminated sparitic, massive or layered micritic, and laminar bindstone textures were analysed. Antennas that provided the maximum detail and a sufficient depth of penetration were used. The low electrical conductivity of carbonates permitted the use of high‐frequency antennas (800 MHz and 1.6 GHz), which were useful in studying the entire thickness of a boulder (up to 2 m).The cross‐laminations in the oolitic limestones, the laminar bindstones of travertines, the differentiation between massive and brecciated fabrics, and the massive and slaty fabrics of the mic‐ritic limestones were examined using these two frequencies. In micritic textures without discontinuities (neither sedimentary nor diagenetic), the radargrams could detect facies with few reflections (Crema Marfil). In addition to analysing fabrics and textures, the ground‐penetrating radar measurements could identify anisotropy in these rocks, which makes ground‐penetrating radar an effective tool for evaluating the mechanical state of a boulder prior to its cutting.
Grey wolf optimisation for inversion of layered earth geophysical datasets
Authors: Chandra Akash , Agarwal Aayush , Shalivahan  , Singh Roshan K. 
Published: September 2019

ABSTRACTGeophysical observables using optimisation algorithms are interpreted in terms of physical properties defining the Earth. The field of optimisation is a dynamic field as no single algorithm can solve all optimisation problems. We implement the grey wolf algorithm, an apex predator‐based method, in optimizing geophysical datasets over a layered earth. The grey wolf optimiser is a swarm‐based meta‐heuristic algorithm and has two extremely interesting social practices, viz., social leadership hierarchy and hunting behaviour. The leadership hierarchy is simulated by employing different types of grey wolves where hunting strategies are implemented as optimisation methods. Global minimum from the grey wolf optimiser has been obtained with a pack of 7 wolves and 1500 iterations. To evaluate the efficacy of the grey wolf optimiser, we performed inversion of noise‐contaminated vertical electrical sounding (apparent resistivity), induced polarisation sounding (apparent chargeability), and magneto‐telluric apparent resistivity data. Subsequently, we implemented grey wolf optimisation on field apparent resistivity, apparent chargeability, and magnetotelluric apparent resistivity data adapted from published literature. Both noise‐contaminated synthetic and field data have been compared with popular population‐based algorithms, i.e., particle swarm optimisation and ant colony optimisation and with a local optimisation algorithm—ridge regression. The sensitivity analysis was performed by inverting the noise‐contaminated datasets using the grey wolf optimiser, particle swarm optimisation and ant colony optimisation with six different search space. It is observed that the grey wolf optimiser is least sensitive to the varied search space. The results obtained from the grey wolf optimiser as compared with other techniques are relatively more stable and the obtained normalised RMS deviation is either less or equal with ridge regression, particle swarm optimisation, or ant colony optimisation. This is due to the fact that the grey wolf optimiser does not converge prematurely and avoids getting trapped in a local minimum, as a balance between exploration and exploitation is maintained. We also compared the grey wolf optimiser using L1‐norm and L2‐norm as misfit functions for field data examples. The grey wolf optimiser using L1‐norm resulted in a more stable solution. The execution time of the grey wolf optimiser is least as compared with other population optimisation techniques. Grey wolf optimisation could be applied for routine interpretation of geophysical datasets.
Airborne mapping of sensitive clay—stretching the limits of AEM resolution and accuracy
Authors: Anschütz Helgard , Bazin Sara , Kåsin Kristoffer , Pfaffhuber Andreas A. , Smaavik Tone F. 
Published: September 2019

ABSTRACTDue to postglacial uplift, lowlands in Canada, Norway, Sweden and Russia are prone to formation of highly unstable, sensitive, and leached marine clay (quick clay). Quick‐clay failures are dramatic due to its high water content, resulting in liquefaction. It thus poses a major hazard for society and construction projects in particular, and knowledge of its extent is of vital importance. Quick‐clay assessment is usually undertaken in geotechnical boreholes having the disadvantage of giving only information at the borehole location. To overcome this limitation, geophysical ground‐based methods like electrical resistivity tomography have been used successfully. However, when a larger area has to be investigated, electrical resistivity tomography surveys become costly and time consuming. We show results from an airborne electromagnetic survey aiming at detection of different clay units for a road project in southeastern Norway. Airborne electromagnetic data clearly show structures within the sediment layer that correspond well with results from geotechnical boreholes. While a clear distinction between clay and quick clay cannot be derived from airborne electromagnetic alone, our study shows that this method has high‐enough resolution and accuracy to map differences in clay units, which can subsequently be probed at specified locations. Thus, by using airborne electromagnetics to target borehole locations, the costs for the geotechnical drilling program can be reduced significantly.
An ultra‐high‐resolution 3D marine seismic system for detailed site investigation
Authors: Monrigal Olivier , de Jong Ivan , Duarte Henrique 
Published: September 2019

ABSTRACTCurrently, marine ultra‐high‐resolution 3D surveys tend to be characterized by signal frequency ranges of 0–600 Hz and bin sizes of the order of 3 to 6 metres. This may be acceptable in industry geohazard studies for top‐hole well drilling, but it does not have sufficient resolution for the very detailed information required by geotechnical engineers for the design and positioning of the turbine foundations in offshore wind farms.We present a newly developed ultra‐high‐resolution 3D system, which takes the 3D‐resolution and 3D‐imaging one step further: to frequency ranges up to 2.5 kHz and 1‐m bin sizes. This system is based on an ultra‐high‐resolution sparker source utilising negative discharge technology, thus guaranteeing a stable and repeatable source signature. The high‐fidelity multi‐channel recording system is composed of four 24‐trace streamers with GPS positioning on both sources and receivers. Sophisticated navigation processing guarantees decimetre precision and the resultant 1‐m bin sizes. In addition, slant streamer technology is applied to ensure a broadband seismic response. Processing techniques, including critical corrections for wave motion and tides, have been developed.Trial surveys were conducted in Thailand to validate the method and later in Croatia to study the shallow geology for the foundations of a 2.5‐km bridge across a sea inlet. The resultant 3D cubes gave very detailed views of the subsurface allowing the imaging of stratigraphic structures and small‐scale features, such as individual boulders in a boulder bed, that had hitherto not been identified.
Sparsity constraints using a Laplacian kernel to get geological structures from potential field data
Authors: Meng Zhaohai , Li Fengting , Xu XueChun , Zhou Wenyue , Hang Danian 
Published: September 2019

ABSTRACTWe propose a new method for including sparsity constraints into potential field data inversion using a Laplacian kernel. The method obtains three‐dimensional density distributions, which are at best a proxy for geological structures. Compressive sensing has been used in many domains to recover the original data from the acquisition data. Compressive sensing is an inversion philosophy where sparsity constraints are applied, and thus, we use this principle into geophysical inversion. Here, we extend this algorithm to potential field data inversion. We introduced the Laplacian smoothed L0 norm into the stabiliser function as a sparse constraint, and the density constraint function has been introduced into our inversion method to guarantee the inverted density to be in the geological and physical meaning range. Compared to the traditional smooth inverse algorithm, our proposed method can obtain accurate geological structures with sharp boundary and sub‐surface “block” structures. This method permits reconstruction of (non‐smooth) density functions that represent a blocky geological structure. Our results using synthetic gravity data show that the Laplacian smoothed L0 norm inversion method with sparsity constraints predicts more focused and accurate depth and density anomalies than smooth inversion method. Application of this Laplacian smoothed L0 norm sparsity constraints method to the inversion of gravity data collected over the Humble salt dome, Harris County, near Houston, United States, leads to improved interpretation of geological structures. These results confirm the validity of the proposed method and its potential application for other potential field data inversions to explore geological structures.
Spectral induced polarization in a sandy medium containing semiconductor materials: experimental results and numerical modelling of the polarization mechanism
Authors: Abdulsamad Feras , Florsch Nicolas , Camerlynck Christian 
Published: September 2019

ABSTRACTInduced polarization is widely used for mineral exploration. In the presence of sulfides (more generally speaking, semiconductors), the charge carriers inside the particles are electrons and electron gaps. The mechanism of induced polarization is not fully understood in those cases. In order to improve our knowledge about the mechanisms controlling induced polarization in such media, we carried out spectral induced polarization measurements on unconsolidated mineralised medium and performed a numerical modelling based on the solution of the Poisson–Nernst–Planck equations set. Different types of semiconductors (graphite, pyrite, chalcopyrite, and galena) have been included in the experiments. The polarization effects of grain radius, semiconductor content, and electrolyte salinity and type have been investigated at the lab scale. The experimental results showed that the chargeability or frequency effect of the medium is a function of the mineral volume and is independent of electrolyte salinity and type. Furthermore, the time constant is highly dependent on grain radius and electrolyte salinity, whereas it is slightly dependent on mineral type. The observed dependence of the chargeability and time constant on salinity could be explained by considering the semiconductor grain as an electric dipole impacting the potential and consequently the charge distribution in its vicinity. This dipole is generated inside the particle to compensate the primary electrical field. Since the Poisson–Nernst–Planck equations are coupled, the potential depends in return on the resulting ions distribution. Therefore, it could be used to explain the induced polarization phenomena. Using the finite element method, we computed the solution of the Poisson–Nernst–Planck equations for a grain of pyrite surrounded by an ionic solution, where the charge diffusions in both grain and electrolyte were considered. Regarding the electrolyte salinity, the dependence of the relaxation time observed from the experimental study is qualitatively consistent with that extracted from the numerical simulation.