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- Volume 17, Issue 4, 2019
Near Surface Geophysics - Volume 17, Issue 4, 2019
Volume 17, Issue 4, 2019
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Using a vibratory source at Mt. Etna (Italy) to investigate the wavefield polarization at Pernicana Fault
Authors Giuseppe Di Giulio, Michele Punzo, Pier Paolo Bruno, Fabrizio Cara and Antonio RovelliABSTRACTThe paper presents the results of a controlled‐source seismic experiment performed using a vibratory source capable of producing harmonic vibrations. The survey aimed at investigating the horizontal directional amplification mechanism observed at the Pernicana Fault (Mt. Etna in Sicily, Italy) along an N150°/N330° orientation. With the vibratory source (a shaker truck) working in shear‐wave mode in the frequency range 5–40 Hz, we recorded seismic data using both three‐component velocimeters and 4.5 Hz geophones. These two types of receivers were arranged in linear configuration along two orthogonal orientations: radial and transverse to the direction of the main polarization observed within the fault zone. The two lines of seismic stations allowed us to investigate the polarization evolution as a function of the distance from the source. Results show that when the shear excitation induced by the source is parallel to the direction of observed polarization of the ground motion, the seismic signal propagates efficiently, maintaining the same horizontal polarization induced by the active source. This polarization is well recorded up to distances as large as 300 m from the source. On the contrary, when the shear excitation is orthogonal to the predominant site polarization, the ground excitation loses its initial polarization in less than 50 m away from the source position. At larger distances, the transmitted energy propagates with the natural site polarization independently from the original source polarization. The observations in terms of polarization were combined with surface‐wave analysis, aimed at investigating the dispersion characteristics of the wavefield on the geophone lines. The data suggest that the experimental Rayleigh‐wave dispersion spectra show a more coherent signal over the frequency range along the same orientation of the observed site polarization, supporting a preferential direction in the propagation of surface waves. From inversion of the Rayleigh dispersion curves, we inferred the shear‐wave velocity (Vs) to be in the range of 120–450 m/s in the first 30 m deep of Quaternary volcanic sediments. The dispersion curves are not constant along transverse and radial directions, suggesting the presence of fracture‐induced seismic anisotropy in the near surface.
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The behaviour of near‐surface soils through ultrasonic near‐surface inundation testing
ABSTRACTSeismometers installed within the upper metre of the subsurface can experience significant variability in signal propagation and attenuation properties of observed arrivals due to meteorological events. For example, during rain events, both the time and frequency representations of observed seismic waveforms can be significantly altered, complicating potential automatic signal processing efforts. Historically, a lack of laboratory equipment to explicitly investigate the effects of active inundation on seismic wave properties in the near surface prevented recreation of the observed phenomena in a controlled environment. Presented herein is a new flow chamber designed specifically for near‐surface seismic wave/fluid flow interaction phenomenology research, the ultrasonic near‐surface inundation testing device and new vp‐saturation and vs‐saturation relationships due to the effects of matric suction on the soil fabric.
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A comparative evaluation of vertical fractures using different azimuthal electrical resistivity survey arrays
Authors S.S. Moreira, L.A.P. Bacellar and P.R.A. AranhaABSTRACTThe identification of geological discontinuities such as fractures and faults is fundamental to geotechnical and hydrogeological characterization of rock masses. It is difficult to characterize these discontinuities, even by means of drill cores. Electrical resistivity has been historically employed as an alternative tool for the characterization of discontinuities, and new equipment for data acquisition and modern interpretation techniques have increased this possibility. However, when the characterization of vertical discontinuities is to be carried out in small areas, such as urban zones or for engineering works, traditional surveying may be impracticable, because it requires larger areas for proper acquisition. In these situations, azimuthal electrical resistivity surveying can be a good option, thanks to faster data acquisition and the possibility of reaching greater depths of investigation. There are several types of azimuthal arrays, but comparative analyses of their efficiency are still scarce. The objective of this study is to compare the applicability of several types of azimuthal resistivity surveys to identify vertical discontinuities in rock masses. The study was carried out in the laboratory using a scale model (tank), where boundary conditions could be well‐defined. We tried to replicate the natural conditions of a karstic area in Brazil, known to have hydrogeological and geotechnical problems associated with subvertical discontinuities. The simulations involved a limestone rock mass crosscut by a set of discontinuities with varied apertures. The selected arrays (square, equatorial dipole–dipole, Wenner and Schlumberger) encompassed varied inter‐electrode spacings and three different thicknesses of a saturated isotropic overburden, so as to represent field conditions. The azimuthal resistivity surveys, especially the Wenner array, proved to be promising in order to detect vertical discontinuities. The non‐collinear arrays are preferable when it comes to discerning the depth of investigation (square) and detecting electric anisotropy (equatorial dipole–dipole), despite small azimuthal distortions that were observed in relation to the strike of the discontinuities.
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Evaluation of resistivity anisotropy parameters in the Eastern Mitidja basin, Algeria, using azimuthal electrical resistivity tomography
Authors Roza Aissaoui, Abdallah Bounif, Hermann Zeyen and Sid‐Ali MessaoudiABSTRACTWe study electrical anisotropy using azimuthal electrical resistivity tomography (A‐ERT) for identifying geological layers and determining the preferential aquifer flow direction. The work presented in this paper aims at calculating the anisotropy coefficient from the inverted resistivity measurements, relating the geophysical results to the site geology and water flow direction, and comparing the results with those obtained from prior studies using hydrogeological approaches. The study area is located in the Eastern Mitidja basin, about 15 km east of Algiers. The work carried out includes three measurement points, totalling 24 ERT profiles. For each point, eight A‐ERT profiles, using Wenner–Schlumberger array configuration, were performed every 22.5° around a fixed central point. The data processing includes two‐dimensional inversion of each profile, representation of the inverted resistivities at the central points as a function of the azimuth in polar diagrams, and implementation of an inversion program to determine the best fitting azimuthal anisotropy parameters. The origin of electrical anisotropy is probably due to grain alignment, which is often observed in fine‐grained sediments such as clays and some sands. This alignment creates a higher electrical resistivity perpendicular to the alignment than parallel to it, which is an indicator for higher hydraulic permeability in the minimum resistivity direction, which corresponds to the preferential groundwater flow direction.
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Integrated ground penetrating radar, electrical resistivity tomography and multichannel analysis of surface waves for detecting near‐surface caverns at Duqm area, Sultanate of Oman
Authors A.M.E. Mohamed, I. El‐Hussain, A. Deif, S.A.S. Araffa, K. Mansour and G. Al‐RawasABSTRACTCaverns in hard limestone form natural karstic features. Surveying, detecting, and managing caverns are valuable as they provide an accurate and reliable understanding of the geohazards that caverns represent. With the construction of the highway network in Duqm (Duqm Port, Sultanate of Oman), near‐surface stability problem (caverns) appeared. Three geophysical tools – ground penetrating radar, electrical resistivity tomography, and multichannel analysis of surface waves – were utilized to delineate the subsurface cavities beneath an area of 10,000 m2 (200 m × 50 m). Based on interpretation of the results of two‐dimensional and three‐dimensional ground penetrating radar, electrical resistivity tomography and multichannel analysis of surface waves, the caverns are found to extend in the N–S direction. The results show two cavern systems of varying thicknesses. The caverns are separated at the top, but at some locations they become an one‐unit cavern system, especially in the northerly direction. The cavern system is not completely hollow; at some locations, they are filled with materials of different physical properties than the host rock. The depth of the cavern system varies between 0 and 5.2 m. The caverns are found only within the limestone rock unit. There is no evidence of caverns within the shale rock unit. The results from the three methods show good agreement in terms of depth and dimensions of the detected caverns.
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Geophysical Mapping of Shallow Rock Salt at Borabue, Northeast Thailand
Authors Thunyatorn Sarntima, Rungroj Arjwech and Mark E. EverettABSTRACTIn the northeast of Thailand, many swamps and lakes show evidence of rock salt dissolution, with attendant environmental problems due to saline groundwater and soil salinization. The knowledge of the depth and structure of the rock salt would be useful to achieve a better understanding of the impact of its dissolution on the environment and to facilitate effective land management strategies. Geophysical methods can provide key subsurface information. Four seismic refraction profiles and three electrical resistivity tomography profiles were carried out between Nong Bo and Nong Pong reservoirs in the salt‐affected Borabue district of Maha Sarakham province. The seismic results indicate the top of shallow rock salt beneath Nong Bo reservoir as a diagnostic zone of high P‐wave velocity >3400 m/s at depths >20 m. The electrical resistivity tomography investigation shows electrical resistivity values ≳5 Ωm at similar depths, indicative also of shallow rock salt. Zones interpreted to be saline groundwater are electrically conductive, identified by electrical resistivity values ≲5 Ωm in the electrical resistivity tomography images. The combined seismic–electrical resistivity tomography interpretation suggests that shallow rock salt lies beneath the reservoirs and ponds in the Borabue district but does not appear to form a continuous body between the two largest reservoirs.
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Modelling ground‐penetrating radar wave propagation using graphics processor unit parallel implementation of the symplectic Euler method
Authors Hongyuan Fang, Jianwei Lei, Juan Zhang, Jie An and Fuming WangABSTRACTInversion of ground‐penetrating radar signals requires accurate and efficient forward modelling. The symplectic Euler method promises good results when simulating ground‐penetrating radar wave propagation in substructures, but its computational efficiency is limited by the same Courant–Friedrichs–Lewy stability condition as the finite‐difference time‐domain method. A two‐dimensional graphics processor unit–accelerated parallel symplectic Euler algorithm is used to simulate ground‐penetrating radar wave propagation. We compared the reflection waveforms as well as the simulation time of the complex underground structure models simulated by the parallel symplectic Euler method with traditional finite‐difference time‐domain method. Results show that the parallel symplectic Euler algorithm achieves the same level of accuracy as the standard finite‐difference time‐domain method. Moreover, it significantly improves the computational efficiency, as the calculation speed is improved by more than 21 times. We verify the performance of the proposed algorithm through a map of the single‐track radar data for a three‐layered pavement model and a simulation wiggle map for a structural damage pavement model. This provides a theoretical basis for accurately interpreting ground‐penetrating radar detection data and efficient forward modelling for the next step of inversion imaging.
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Investigation of spatial and temporal variability of site response in the Arunachal Himalaya using ambient seismic noise and earthquake waveforms
Authors A.P. Singh, M. Ravi Kumar, Abhay Pandey and Ketan Singha RoyABSTRACTThe Arunachal region that constitutes the easternmost segment of the Himalaya has experienced two great earthquakes in the past. This portion is geologically and seismo‐tectonically unique compared to the other parts of Himalaya. In this study, the ambient seismic noise and earthquake waveforms registered at 34 broadband seismograph stations with a flat frequency response between 120 and 0.02 seconds are analysed to understand the stability and reliability of the horizontal‐to‐vertical spectral ratio method. Also, we use the bootstrapping method to estimate the uncertainty and variations in the horizontal‐to‐vertical spectral ratio in different seasons. Although the predominant frequency does not show any seasonal variation, the amplitudes reveal a slight dependence. The predominant frequencies in the area mainly vary from 1.1 to 7.0 Hz. At a few sites, two peaks well separated in frequency, indicate shallower and deeper structures with impedance contrast. Our results show that the average predominant frequencies are around 3.5 Hz for the undifferentiated granite gneiss, 4.3 Hz for the Bomdila granite gneiss, 2.1 Hz for the Bomdila group of rocks (Paleoproterozoic), 2.8 Hz for the Gandise batholiths, 2.6 Hz for the Gondwana succession and 1.8 Hz for the Abor formation. The amplification estimated by the site‐to‐reference method varies between 1.5 and 6.2, which is lower than that obtained from the HVSR method. Validation of the results from ambient seismic noise was performed by comparing them with those from the earthquake waveforms and correlation with local geology.
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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)