- Home
- A-Z Publications
- Near Surface Geophysics
- Previous Issues
- Volume 16, Issue 1, 2018
Near Surface Geophysics - Volume 16, Issue 1, 2018
Volume 16, Issue 1, 2018
-
-
Resistivity and chargeability survey for tunnel investigation: a case study on toxic black shale in Norway
ABSTRACTIn the past few years, the focus on Alum shale hazards and the need for efficient mapping tools have increased in Norway. Alum shale is highly toxic and poses a substantial obstacle to infrastructure development such as tunnel projects. We present an evaluation of the ground‐based electrical resistivity tomography, induced polarisation, and airborne electromagnetic methods for mapping purposes using a recent case study. This evaluation is done in combination with resistivity and chargeability laboratory measurements applied to drill cores. The aim of the geophysical survey was to improve the knowledge of Alum shale occurrence to assist a tunnel project in Gran, southeast Norway. Resistivity and chargeability models derived from an electrical resistivity tomography/induced polarisation survey enabled us to map the presence of Alum shale during the tunnel investigation. The resistivity models point to geological layers that are in agreement with the rock types observed from early drillings together with subsequent geological logging during tunnelling. The time‐domain chargeability models are imperfect but nonetheless reveal the presence of polarisable minerals. These are likely due to the high levels of sulphides contained in black shale. An airborne electromagnetic survey was done close to the area of interest, which enabled us to fly some sparse lines across the tunnel alignment as a piggyback survey. Although the airborne electromagnetic resolution is lower than electrical resistivity tomography, the successful test flight lines illustrate the potential of airborne electromagnetic surveys for Alum shale mapping in Norway and affirm the promise of airborne electromagnetic in the early stages of project exploration.
-
-
-
GPR data reconstruction method based on compressive sensing and K‐SVD
Authors Juncai Xu, Zhenzhong Shen and Zhenhong TianABSTRACTMissing and irregular ground‐penetrating radar trace data resulting from sampling conditions are important issues in engineering. This study adopted compressive sensing theory to reconstruct missing ground‐penetrating radar trace data. A ground‐penetrating radar data reconstruction method was established based on compressive sensing theory and K‐singular value decomposition. The method used the sampling matrix of the missing data as the measurement matrix and the K‐singular value decomposition algorithm to obtain a complete dictionary of sparse coefficients. A traditional dictionary cannot be adaptively adjusted according to the data features; the proposed method resolved this problem. The iteratively reweighted least‐squares method was used to reconstruct the missing trace data. Two experiments on the recovery of missing ground‐penetrating radar data through a simulation and a field example were conducted to test the feasibility and effectiveness of the proposed method.
-
-
-
Measurement of geophysical parameters on clay samples at the solid–fluid transition
ABSTRACTFluidisation occurring in clay‐rich landslides poses serious threats to populations and infrastructures and has been the subject of numerous studies to apprehend its rheological origin. In parallel, noninvasive geophysical techniques for monitoring landslides have shown considerable developments as a means to obtain the in‐situ geotechnical parameters. This study investigates the influence of fluidisation on two geophysical parameters: the shear wave velocity, , and the electrical resistivity, . Both parameters are widely used in landslide monitoring as they are sensitive, respectively, to soil stiffness and water content, two key parameters for material fluidisation. Laboratory tests were carried out on soil samples collected in five flow‐like landslides occurring in very different geological conditions. A plate–plate rheometer was used to provoke fluidisation, and was measured during oscillatory tests. The rheometer was redesigned for resistivity measurements, incorporating circular electrodes in polyvinyl chloride plates. Results show that (i) all soils exhibit a dramatic drop in at the fluidisation, and (ii) the resistivity does not significantly vary at the solid–fluid transition. These last results are analysed in terms of clay particles arrangement using the laws of Archie and Waxman–Smits, and the impact on geophysical monitoring of a landslide is discussed.
-
-
-
Sparsity constraints using a Laplacian kernel to get geological structures from potential field data
Authors Zhaohai Meng, Fengting Li, XueChun Xu, Wenyue Zhou and Danian HangABSTRACTWe 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.
-
-
-
Three‐dimensional reconstruction of a masonry building through electrical and seismic tomography validated by biological analyses
Authors Ettore Cardarelli, Giorgio De Donno, Ilaria uliveti and Claudia ScatignoABSTRACTIn this paper, we present an integrated approach, for assessing the condition of an ancient Roman building, affected by rising damp and cracking phenomena. The combination of high‐resolution geophysical methods, such as seismic and electrical tomography, with biological information, allowed a more detailed evaluation of the state of conservation of the masonry building. A preliminary three‐dimensional electrical survey was conducted to detect the existing building foundations and to determine the variation of the resistivity in the ground. Then, electrical and seismic tomography investigations were carried out on an inner wall of opus caementicium, subjected to rising damp effects and cracks. This approach was adopted to obtain a high‐resolution image of the wall, which allowed to identify the inner mortar and the outer brick component from resistivity and velocity contrasts. Furthermore, the geophysical results revealed evidence of wall fractures (indicated by low velocity and high resistivity values) and a significant volume where rising of damp was taking place (resulting in a low resistivity zone). Biological analyses validated the geophysical model: in fact, the biological proliferation occurred up to a height of 0.75 m, where the interface between high and low resistivity values was recovered. This approach can be employed to reconstruct a three‐dimensional model of masonry structures in order to plan recovery actions.
-
-
-
Focusing on soil‐foundation heterogeneity through high‐resolution electrical and seismic tomography
Authors Michele Cercato and Giorgio De DonnoABSTRACTThe reconstruction of the current status of a historic building is essential for seismic safety assessment and for designing the retrofitting interventions since different safety and confidence factors have to be assumed, depending on the level of information about the subsoil structure. In this work, we present an investigation of the shallow subsurface below and around a historic building affected by differential settlements in order to define its geometry and to characterise its stiffness at low strain. To this end, we employed high‐resolution electrical resistivity and seismic (both P‐wave and S‐wave) tomographies. A three‐dimensional electrical resistivity tomography survey was performed to obtain more information about the type and the maximum depth of the building foundation. Electrical resistivity and seismic tomographies were carried out alongside the building, aimed at imaging the top soils and the near‐surface geometry. The corresponding inverted models pointed out a remarkable heterogeneity of the shallow subsoil below the building, which is partly founded on a weathered layer and partly on a more rigid lithotype. This heterogeneity is probably a concurrent cause of the building’s instability under both static and seismic loading. Our results demonstrate that the man‐made fillings and the top soils have to be thoroughly investigated to fully understand the soil‐structure behaviour. In this light, the integration of non‐invasive high‐resolution geophysical techniques, especially tomographic methods, has been proved to properly address the problem of imaging the shallow subsoil.
-
-
-
Applying joint traveltime and waveform inversion to image the Sichuan Basin, China
More LessABSTRACTWe apply a joint first‐arrival traveltime and early‐arrival waveform inversion method to image complex near‐surface structures in the Sichuan Basin, China. The area includes rugged topography and large near‐surface velocity variations. Due to the near‐surface effects, it is difficult to produce high‐quality reflection images of the deep subsurface. First‐arrival traveltime tomography is often applied for near‐surface imaging, but the results may not be sufficiently accurate because of ray assumptions and the limited traveltime information. Waveform inversion should allow complex structures to be resolved; however, it may fall into local minima because of cycle skipping issues and may also produce artefacts in very shallow areas that are associated with rugged topography. Therefore, we combine the advantages of the two methods and mitigate their problems by performing joint inversion of the two types of data. We demonstrate the effectiveness of the joint inversion method using synthetic and real data from Sichuan, China. In the real data example, we compare the velocity models resolved from waveform inversion alone with those resolved from the joint inversion. We calculate long‐wavelength static corrections and apply them to the data processing. The common midpoint stacking results show that the joint inversion method produces a more effective statics solution.
-
-
-
Seismic survey on an open pingo system in Adventdalen Valley, Spitsbergen, Svalbard
Authors Giuliana Rossi, Flavio Accaino, Jacopo Boaga, Lorenzo Petronio, Roberto Romeo and Walter WheelerABSTRACTWe present the results of a seismic survey on an open‐system pingo, the Innerhytta pingo, located in the Adventdalen Valley, Spitsbergen Island of the Svalbard Archipelago, Norway. To evaluate the temporal evolution of the permafrost, we need detailed knowledge of its heterogeneities, of which pingos are the most visible markers. The seismic velocity contrasts and inversions peculiar to these features can be a problem for a successful seismic survey. The present paper focusses on the limits and benefits of various seismic methods for imaging the subsurface structure of the pingo, with the aim of defining the best practice for shallow seismic exploration. We performed several tests, using classical reflection/refraction arrays, seismic tomography, and surface wave analysis. We tested different kinds of seismic sources and receivers. The combined use of several sources and receivers sets and patterns, and the use of surface wave analysis and body wave tomography allowed constructing a three‐dimensional model of the velocities. For this particular pingo, beneath an approximately 15‐m thick high‐velocity layer, low velocities are found to be present at an anomalously shallow depth. This low‐velocity layer is interpreted to host the ground water circulation that controls the pingo’s structural evolution. Such observations suggest feeding water flow from depth rather than from the near surface, as commonly supposed for open‐system pingos.
-
Volumes & issues
-
Volume 22 (2024)
-
Volume 21 (2023)
-
Volume 20 (2022)
-
Volume 19 (2021)
-
Volume 18 (2020)
-
Volume 17 (2019)
-
Volume 16 (2018)
-
Volume 15 (2017)
-
Volume 14 (2015 - 2016)
-
Volume 13 (2015)
-
Volume 12 (2013 - 2014)
-
Volume 11 (2013)
-
Volume 10 (2012)
-
Volume 9 (2011)
-
Volume 8 (2010)
-
Volume 7 (2009)
-
Volume 6 (2008)
-
Volume 5 (2007)
-
Volume 4 (2006)
-
Volume 3 (2005)
-
Volume 2 (2004)
-
Volume 1 (2003)