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- Volume 20, Issue 1, 2022
Near Surface Geophysics - Volume 20, Issue 1, 2022
Volume 20, Issue 1, 2022
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Resolution attributes for geophysical inversion models: Depth of investigation and novel measures
More LessABSTRACTAssessing the reliability of inversion models derived from geophysical measurements is crucial for a reliable interpretation. An interpretation depends critically on the interpreter being able to discern between the characteristics of the inversion model that can be trusted as more or less well resolved and the ones that are more dubious. This paper analyses the resolution measure ‘depth of investigation’ from a conceptual and a computational viewpoint and proposes two definitions that incorporate all aspects of the inversion and that are free of a user‐defined ad hoc parameter. Two more resolution attributes are introduced: a qualified depth of investigation and the depth of required structure . The first one answers the question: What is the minimum depth to a homogeneous halfspace with an interpreter‐defined conductivity that will not increase the data residual more than a certain amount? The second one is an ‘unqualified’ depth of investigation that addresses the question: What is the minimum depth to a homogeneous halfspace with any conductivity that will not increase the data residual more than a certain amount? This latter measure indicates the depth below which no structure is needed to fit the data. Finally, measures are defined that will provide estimates of the vertical resolution width as a function of depth. All of the resolution measures presented in this paper are based on the posterior model resolution matrix.
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Development of a drone‐borne electromagnetic survey system for searching for buried vehicles and soil resistivity mapping
ABSTRACTWe developed a drone‐borne electromagnetic survey system using a commercial multi‐frequency electromagnetic sensor equipped with a GPS receiver, a WiFi serial transceiver, and an ultrasonic distance sensor to measure the height of the electromagnetic sensor above the ground surface. The electromagnetic sensor was suspended from a drone with ropes. The distance between the drone and the electromagnetic sensor was adjusted to minimize the influence of electromagnetic noise generated by the drone, and to stabilize the electromagnetic sensor during flight. The system was tested at two experimental sites. The first site consisted of two buried vehicles to simulate a landslide. We assumed a scenario in which the search for the buried vehicles was urgent and accessibility to the area was limited. The second site consisted of wet and dry agricultural fields to test resistivity mapping. In the first test, we used the in‐phase component of the measured data to locate the vehicles. The shallower vehicle was identified clearly, while the deeper vehicle was located successfully, albeit less easily. In the second test, the quadrature component was used for one‐dimensional inversion after data processing, which included data smoothing, resampling and bias noise correction. The bias noise was measured while hovering the drone at a high altitude to negate the influence of ground conductivity. The results showed that the resistivity distributions could be mapped at some depths by using a five‐frequency‐processed quadrature component, and clearly showed the difference between the wet and dry fields. The crucial parameter in the evaluations of these targets was the height of the electromagnetic sensor above the ground surface, which was measured continuously during flight. The results demonstrated the potential of the survey system to search for buried metal objects and for shallow subsurface resistivity mapping over relatively large areas.
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Radio‐magnetotelluric and controlled‐source magnetotelluric surveys on a frozen lake: Opportunities for urban applications in Nordic countries
Authors Mehrdad Bastani, Shunguo Wang, Alireza Malehmir and Suman MehtaABSTRACTIn a novel approach, we have carried out controlled‐source and radio‐magnetotelluric measurements in the frequency range of 2–250 kHz on a frozen lake located over a planned major multi‐lane underground road tunnel near the city of Stockholm. The aim was to gain a better understanding of the resistivity variations above and, potentially, within the crystalline bedrock. Previous studies on the lake water using the boat‐towed radio‐magnetotelluric technique at the higher end of the frequency band lacked resolution at depth and could not provide conclusive information about bedrock level and potential fracture systems within the bedrock. Taking advantage of Nordic winters, we measured four profiles on the frozen lake complementing the previously acquired boat‐towed radio‐magnetotelluric data utilizing a double horizontal magnetic dipole transmitter that generated signals down to 1 kHz. The new resistivity models, incorporating the lower frequency data, show improvements and deeper penetrations based on a combined analysis of penetration depth, data misfits and sensitivity studies. The resistivity models also show better correlation with the available high‐resolution shallow water seismic reflection data and the geological observations. A potential fracture system within the bedrock can also be inferred better in the new models. The idea of running similar surveys on frozen lakes can be further exploited in similar conditions in countries such as Sweden, where approximately 7% of the land is covered by freshwater bodies and poorly explored for infrastructure planning projects.
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Application of crosshole electrical resistivity tomography measurements under the influence of horizontally slotted plastic cased boreholes
More LessABSTRACTIn this work, we study the performance of cross‐hole electrical resistivity tomography measurements by employing different electrode array configurations in plastic polyvinyl chloride (PVC) cased and horizontally slotted observation boreholes by inserting a multi‐electrode cable directly into the borehole. Preliminary cross‐hole electrical resistivity tomography measurements in PVC cased boreholes related to an underground tunnel construction showed poor data quality. This was attributed to the borehole‐fluid effect caused by the PVC casings. An experimental study was conducted to support this hypothesis by setting up various simulations in a water tank, using different PVC casings with various slot densities, and different electrode array configurations. We conclude that the applicability of various measurement setups depends mainly on the acquisition protocol and, to a lesser extent, on the slot density of the PVC casing. Among the different array configurations considered, the pole–dipole array with the potential measuring electrodes being placed in a separate borehole to the current electrodes provide the most robust and reliable results, even for low slot density PVC casings. Besides, denser borehole slot configurations result in better data quality, though to a different extent for the examined protocols. A minimum slot density criterion of at least six slots/electrode spacing is proposed, regardless of the electrode array. The experimental findings are finally evaluated against real field measurements associated with the construction of an underground tunnel of the new Thessaloniki Metro, verifying the pole–dipole array's superior behaviour for this type of measurement configuration. Finally, for those cases where the aspect ratio (hole depth/hole separation) is limited, we propose a modified borehole‐to‐surface configuration with the current electrodes placed outside the boreholes. The overall results indicate that slotted PVC cased observation boreholes (e.g., conventional piezometers), typically constructed as part of many infrastructure monitoring projects, can be efficiently employed for electrical resistivity tomography mapping, generating a new perspective for geoelectrical prospecting. This measuring approach exhibits a significant advantage. The use of pre‐existing boreholes reduces the overall survey costs, reliability, and effort, while also providing high‐resolution subsurface images, especially in urban environments.
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Joint inversion of seismic and electrical data in saturated porous media
Authors Flora Garofalo, Laura Valentina Socco and Sebastiano FotiABSTRACTJoint inversion strategies and physical constraints on model parameters may be used to mitigate equivalence problems caused by solution non‐uniqueness. This strategy is quite a common practice in exploration geophysics, where dedicated rock physical studies are usually carried out, while it is not so frequent in near surface geophysics. We use porosity as a constraint among seismic wave velocities and electrical resistivity in a deterministic joint inversion algorithm for surface wave dispersion, P‐wave traveltimes and apparent resistivity from vertical electrical sounding. These data are often available for near surface characterization. We show that the physical constraint among model parameters leads to internally consistent geophysical models in which solution non‐uniqueness is mitigated. Moreover, an estimate of soil porosity is obtained as a relevant side product of the procedure. In particular, we consider a clean sand deposit and hence the appropriate formulations for the computation of porosity from seismic velocities and resistivity are implemented in the algorithm. We first demonstrate how the non‐uniqueness of the solution is reduced in a synthetic case and then we applied the algorithm to a real‐case study. The algorithm is here developed for one‐dimensional condition and for granular soils to better investigate the physical constraint only, but it can be extended to the two‐dimensional or three‐dimensional case as well as to other materials with the adoption of proper rock physical relationships.
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Diagnostics of asphalt pavement using elastic standing waves
Authors L. Ngomayezwe, Yu. I. Kolesnikov and К. V. FedinABSTRACTTo ensure the safe operation of roads, it is necessary to periodically monitor the condition of road surfaces. Lately, near‐surface geophysics methods have been actively used to solve such problems. The seismoacoustic methods that are used to diagnose road surfaces are typically dynamic methods based on the excitation of body or surface elastic waves in the road surfaces. The purpose of this work is to assess the possibilities of using elastic standing waves for diagnosing a hard road surface. This article presents the results of field experiments that demonstrate the possibility of detecting cavities under an asphalt pavement using flexural standing waves. Such waves, like vibrations of a membrane fixed or partially fixed at its edges, can be formed on the hard surface cover above the cavity as a result of exposure to acoustic noise. In this article, the accumulation of amplitude spectra of a large number of noise records was used to extract standing waves from the acoustic noise recorded on the surface of the pavement. It is shown that the joint visualization of the averaged spectra obtained by profile observations over the cavity makes it possible to confidently identify several modes of flexural standing waves. Based on the areal measurements, a map of the distribution of the amplitudes of one of the modes of flexural standing waves in the asphalt pavement over the cavity was constructed. At a qualitative level, this distribution is consistent with the results of computer simulations using the finite element method. The fact that, under the influence of acoustic noise in the pavement, the flexural standing waves are formed which are absent at other places indicates the absence of rigid contact at its lower boundary. Thus, the horizontal dimensions of the cavity can be estimated from the size of the area on which flexural standing waves are formed. In addition, the article shows that the analysis of vertical compressional (but not flexural) standing waves arising in the pavement under the influence of noise allows us to investigate the thickness of the pavement and to qualitatively assess the ratio of acoustic stiffness of the pavement and the underlying layer.
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Imaging subsurface geological complexity (2D/3D) beneath the Greater Srinagar region of the Kashmir basin, Northwest Himalaya
Authors S. Vishal Gupta, Imtiyaz A. Parvez and Prosanta K. KhanABSTRACTA high‐resolution microtremor measurement in Greater Srinagar city of the Kashmir valley has been analysed to image 2D and 3D subsurface geological complexities. This region is located in the highly seismogenic Himalayan belt and sits atop a deep sedimentary lake bed with a laterally varying thickness of soft sediments. Srinagar region is a major economic centre and the capital city of the Kashmir valley with 2 million inhabitants living at high seismic risk. To assess the subsurface complexity beneath the city, we present: (1) high‐resolution subsurface shear wave velocity Vs structure using the horizontal‐to‐vertical spectral ratio inversion; (2) shear wave velocity for top 30 metres of soil column (VS30) map with National Earthquake Hazards Reduction Program site classification; (3) comparison of VS30 maps calculated from horizontal‐to‐vertical spectral ratio inversion and topographic slope methods; and (4) azimuthal behaviour of horizontal‐to‐vertical spectral ratio peaks, all of which unravel the subsurface spatial heterogeneity and suitability for the building of engineering structures in the study area. In addition, a new matlab code is applied to generate 3D subsurface Vs slices in the study region in different directions using its pre‐generated 2D Vs profile data. The presented potentiality of microtremor horizontal‐to‐vertical spectral ratio technique in Srinagar region, which lies on the eastern edge of the basin with significant topographic irregularities, indicates an uneven distribution of local site effects (primary and secondary) in the case of strong ground motion. The comprehensive results can be promising in engineering analyses of local ground and structural responses in order to mitigate the impact of earthquake occurrence and seismic risk in the city and adjoining regions.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)