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- Volume 16, Issue 3, 2018
Near Surface Geophysics - Volume 16, Issue 3, 2018
Volume 16, Issue 3, 2018
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Data acquisition, processing and filtering for reliable 3D resistivity and time‐domain induced polarisation tomography in an urban area: field example of Vinsta, Stockholm
Authors Matteo Rossi, Tbrleif Dahlin, Per‐Ivar Olsson and Thomas GüntherABSTRACTThere is an increasing demand for trustworthy engineering geological conceptual models in urban areas due to an increasing trend in the underground infrastructure construction. Good‐quality site investigations can reduce the risk of encountering unexpected geological conditions during construction. Geoelectrical measurements can be used as a tool for providing an overview of the site conditions and serve as a base for planning a geotechnical drilling program and for integration of the results. Geophysical surveys in urban environments may encounter problems due to strict logistical constraints and may be severely affected by electric and electromagnetic noise. Careful processing of the data is necessary to obtain a reliable estimation of the electrical properties of the ground, both electrical resistivity and chargeability.
A large three‐dimensional dataset was acquired in the suburban area of Stockholm (Sweden), with the aim of investigating a weak zone in the crystalline bedrock, which had been pointed out by prior geological and geotechnical surveys. Full waveforms of potential dipoles were recorded and processed for removing harmonic noise and background drift. Moreover, a statistical algorithm for handling the quality of the full‐waveform shapes has been proposed. The goodness‐of‐fit test identifies full waveforms with noise that derives from direct current injections, caused by grounding spots of the adjacent metro line.
The processed dataset is inverted for electrical resistivity and integral chargeability. The results image a large three‐dimensional volume of the underground. The inverted distribution of geophysical quantities marks out the presence of a wide zone of weak rock, which was not identified by geotechnical probing in the site investigation but documented during the construction phase. Such zones can potentially cause severe problems during the construction of underground infrastructure.
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Underwater ERT surveying in water with resistivity layering with example of application to site investigation for a rock tunnel in central Stockholm
Authors Torleif Dahlin and Meng Heng LokeABSTRACTWe present an approach to underwater electrical resistivity tomography surveying under conditions with several water layers with different resistivity in the water above the electrode layout. The approach is verified against a synthetic model example and tested in full scale on data from a field survey. The field survey was carried out in central Stockholm as part of pre‐investigations for a new metro train (T‐bana) tunnel planned to pass under seawater. The water passage is associated with major tectonic zones that can potentially be very difficult from a tunnel construction point of view. The aim was to identify variations in depth of the bottom sediments and variations in rock quality including the possible presence of weak zones in the rock. Survey conditions are complicated by boat traffic and electrical disturbances from the power grid and train traffic. The water depth was mapped using sonar combined with recording pressure transducers, and water resistivity as a function of water depth was recorded using geophysical borehole logging equipment. Water resistivity as a function of depth was integrated in the inversion model. The results show that the rather difficult survey conditions could be handled in a satisfactory way thanks to adequate equipment, careful planning, and attention to details. The measured data contain information that is relevant for creating coherent models of the variation in depth to rock, which corresponds well with data from drilling. The results also indicate that information in variation in rock quality that can be of critical importance for planning of underground construction can be derived from the data.
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Different techniques for the assessment of geoelectrical data errors to improve the electrical images obtained at an industrial plant
Authors Patricia Martinelli, Ana Osella, Matias de la Vega and Alejandro PinioABSTRACTThe objective of this work was to characterise the bases of columns belonging to an industrial plant. We had to design a proper methodology for characterising these structures, in a place with limited available space and time for the deployment of sensors, and with other environmental constraints, such as high electromagnetic noise level, soil vibrations due to operating machines, and complex soil composition. In particular, the floor had a stainless steel cover about 1‐cm thick, below which there was first a layer of concrete about 30‐cm thick and then clay material used to absorb the oils of the machines. Finally, there was the natural soil on which the column bases stood.
Given these characteristics, we performed dipole–dipole geoelectric profiles. Holes were drilled through the metallic cover and the concrete layer, with fixed spacings, to insert the electrodes. Because of this and the space and time restrictions, only one profile could be performed for each column. Since we knew that the data could have high noise levels and other errors negatively impacting the data quality, and also because there was a limitation to the length of the profiles, we optimised the measurement configuration by jointly using several electrode apertures, to ensure the best possible resolution and sufficient penetration depth. To obtain adequate error estimations, we performed direct and reciprocal measurements. As an alternative technique, we also carried out a second set of direct measurements, after removing and reinstalling the electrodes.
Then, for each profile, we performed 2D inversions of the three datasets separately, without considering data errors. Besides, we inverted the datasets obtained by combining the first direct and the reciprocal measurements, and the first and second direct measurements, considering the two obtained error estimates. In addition, numerical simulations were made, combining direct 3D modelling with 2D inversions. To analyse the results, we took into account the knowledge we had in advance about the main characteristics of the structures.
The first conclusion obtained is that, at least for this type of structures, inverting each dataset without considering data errors provided more realistic images. However, although the best results were obtained by inverting without considering errors, it was still fundamental to have different sets of data, for evaluating the reliability of the images and for discarding possible spurious artefacts. This is especially important in environmentally complex urban sites, where errors could be particularly high. Finally, through the numerical simulations, a more rigorous interpretation could be made, and also, the effect of data errors in the quality of the images could be assessed.
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Geophysical pre‐investigation for a Stockholm tunnel project: joint inversion and interpretation of geoelectric and seismic refraction data in an urban environment
Authors Mathias Ronczka, Roger Wisén and Tbrleif DahlinABSTRACTUnderground constructions for public traffic purposes are becoming increasingly important for urban areas in order to use the limited space more efficiently. Several electric resistivity tomography and seismic refraction tomography measurements were performed crossing a water passage near Stockholm during the pre‐investigation phase of a tunnel building project. The objective was to determine the bedrock interface and qualitatively assess the rock quality. The scope of this study is to present a field case in an urban environment and show improvements of geophysical results due to additional model constraints by a joint inversion. Results of individual inversions show a large transition zone below the seabed from electric resistivity tomography. Some parts of the seismic refraction tomography have a low model resolution, due to gas‐bearing sediments with a low velocity together with a high noise level, which leads to insufficient investigation depth that makes it difficult to determine the bedrock interface. However, the bedrock interface could be reconstructed in the resistivity model by performing a joint inversion, using the seismic velocity model to constrain the electric resistivity tomography model and vice versa. Adjacent geotechnical soundings support the joint inversion results. A vertical low resistive zone could be identified as a dominant fracture zone in the southern part of the investigated area. In general, the joint inversion approach significantly improved the electric resistivity tomography results and provided a more reliable bedrock estimation.
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Karst‐induced sinkhole detection using an integrated geophysical survey: a case study along the Riyadh Metro Line 3 (Saudi Arabia)
More LessABSTRACTNowadays, most planned large infrastructure projects increasingly use multi‐technique geophysical methods integrated with geotechnical surveys to assess detection and exploration of karst‐related systems. This article focuses on the case of a large sinkhole found during the geophysical surveys that were carried out along Riyadh’s new Metro Line 3 (Saudi Arabia). This line is the longest (41 km) of the six that are currently under construction in the framework of the largest public rail infrastructure project, the Riyadh Metro Project (176 km).
A multi‐technique geophysical survey combining seismic (1040 m of seismic refraction tomography and four downhole tests), electrical (1035 m of electrical resistivity tomography), and electromagnetic methods (1040 m of ground penetration radar) was conducted to shed light on the subsurface geology along the section of this case study. The combination of the geophysical methods led to early identification of a subsurface area of lower resistivity and seismic velocity than the background values of the carbonate bedrock. It also revealed smaller fractures that could lead to future sinkhole formation. A subsequent dense investigatory borehole grid (14 boreholes, five probeholes, 38 standard penetration tests, nine TV‐logging, and five pressuremeter tests) further confirmed the presence of a buried sinkhole.
This paper shows the results of each individual geophysical method as well as the final geotechnical interpretation based on the combination of geophysical methods with borehole drilling. It concludes that the use of a single method for karst assessment, whether a geophysical method or borehole drilling, does not allow a sufficiently detailed geotechnical profile of the ground. This case study provides basic guidance on the most suitable and accurate techniques to detect similar karstic features across Riyadh.
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Assessing subsoil void hazards along a road system using H/V measurements, ERTs and IPTs to support local decision makers
More LessABSTRACTBetween 2008 and 2014, nine sinkholes occurred in northeastern Elba Island (Tuscany, Italy), an area with mostly flat terrain (called “Il Piano”) separating the municipalities of Rio nell’Elba and Rio Marina. The last sinkhole damaged the only road (SP26) between the harbour of Rio Marina and the northwestern part of the island. A bypass was immediately built, but the SP26 remains closed. Considering that sinkholes could be densely clustered in sinkhole prone areas, their detection and forecasting are key aspects of local administrative policies. In this paper, we present the results of an integrated geophysical survey aimed at (i) characterizing the geology of the area surrounding the SP26, and (ii) assessing the subsoil void hazard around the road system to support the decision to replace or restore the SP26. Therefore, for the purposes of this research, 120 singlestation seismic noise measurements were taken following the horizontal‐to‐vertical spectral ratio (H/V) or Nakamura technique, while eight 2D electrical resistivity tomography (ERT) and 17 3D‐ERT/induced polarisation measurements were also carried out in the study area. The H/V method allowed the estimation of the mean thickness of the alluvium, whereas the 2D/3D‐ERTs and IPTs permitted the characterisation of the electrical behaviours of the materials and the localisation of the lenticular sand and gravel bodies within a sandy silt layer. The large amount of collected data made the zonation of the subsoil void hazards possible.
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Site characterisation in Barreiro urban area (Portugal) using H/V and ReMi techniques
Authors Henrique Vicêncio, Paula Teves‐Costa and Paulo Sá CaetanoABSTRACTBarreiro is an urban county located in the Lisbon metropolitan area (Portugal) with several hazardous industrial facilities. This area shows a moderate seismicity, but due to its geotectonic location, it has been subject in the past to earthquakes responsible for high social and economic losses. Earthquakes that occurred in 1531 (Mw ≈ 6.6), 1755 (M ≥ 8) and 1909 (Mw ≈ 6) are examples of destructive events that hit Barreiro. In this study, we present an analysis of ambient vibration measurements together with local geology and geotechnical properties of the shallower units. Horizontal‐to‐vertical spectral ratio (H/V) computed from ambient vibrations, shear wave velocity profiles, and value distributions were compared with the geological and geotechnical properties of the shallower units, looking for potential site effects.
Geotechnical characterisation was based on the analysis of 289 logs and 1109 Standard Penetration Tests. A total of 214 refraction microtremor measurements were performed, obtaining profiles and values for 34 different sites. The results from ambient vibration records carried out at 136 sites (Vicêncio, Teves‐Costa and Sá Caetano 2015) were also used. Spatial distributions of profiles, values, the thickness of the shallow formations, the number of blows from Standard Penetration Tests (NSPT values), and H/V peak frequencies are presented and discussed. Correlations between these parameters were established. Three areas prone to seismic amplification, for frequencies between 2.5 Hz to 8 Hz, were identified. Ground classification was performed according to Eurocode 8 using the estimated values. The results will be made available to the Barreiro municipality for support of land and emergency planning.
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Site characterisation in Kangra Valley (NW Himalaya, India) by inversion of H/V spectral ratio from ambient noise measurements and its validation by multichannel analysis of surface waves technique
Authors Mahajan Ambrish Kumar and Praveen KumarABSTRACTThe ambient noise measurements were performed at 200 sites in and around the upcoming urban centre of Kangra Valley to derive the predominant frequency of soil. The shear‐wave velocity of the soft soil cover is obtained by joint‐fit inversion modelling of the horizontal‐to‐vertical spectral ratio and the dispersion curves derived from multichannel simulation with one receiver survey. Simultaneously, shear‐wave velocity investigations were also performed employing an active 24‐channel engineering seismograph using multichannel analysis of surface waves. Finally, the derived one‐dimensional shear‐wave velocity profiles were compared between these two different approaches, which were found to be in good agreement. The shear‐wave velocity investigations of the study area have indicated that the majority of the sites either fall in soil class D (, stiff soil) or class C (, very stiff soil) as per NEHRP classification. The microtremor data also suggest high fundamental frequency (4 to >20 Hz) within and on the fringes of the basin, thus covering 80% of the study area, which is in agreement with the known shear‐wave velocity variation in the Kangra Valley. The large variation in high frequencies cannot be attributed to the presence of thick loose alluvial sediments (gravels and sand) but can be related to the presence of moraine deposits or bedrock (upper conglomerates) underneath the basin. However, a few isolated locations in the northern and southeastern parts of the basin are characterised by a low predominant frequency (2–3 Hz) or frequency less than even 2 Hz. The results further suggest that both multichannel analysis of surface waves and horizontal‐to‐vertical spectral ratio methods, in combination with multichannel simulation with one receiver, are complementary to each other and are suitable for estimating the shear‐wave velocity structure for hilly urban regions, where exploring a large area is a major challenge. The analysis further reveals that Kangra Valley can have a major devastation from a near‐source earthquake rather than from a far‐source event.
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S‐wave velocity structure of Chlef City, Algeria, by inversion of Rayleigh wave ellipticity
Authors Khalissa Layadi, Fethi Semmane and Abdelkarim Yelles‐ChaoucheABSTRACTChlef City, Algeria, which is located in the Lower Cheliff Basin, is vulnerable to seismic hazards. Since there is no constrained velocity model for the Lower Cheliff Basin, particularly at greater depths, we conducted an ambient vibration investigation to map the shear‐wave velocity structure beneath the city, with the primary goal of supplementing the existing microzonation studies. Here, we inverted the Rayleigh wave ellipticity measurement curve of ambient vibrations measurements from 120 sites in Chlef City to estimate the shear‐wave velocity structure. The study area was subdivided into six zones based on similarities between the observed horizontal‐to‐vertical spectral ratio of the ambient vibrations curves. Our resultant shear‐wave velocity models show that the observed fundamental frequencies (0.3–1.6 Hz) are related to a thick layer (~800 m) of upper Miocene deposits, where on average. Generally, the Mesozoic basement shows . Moreover, the clear peaks observed at higher frequencies and only in the northwest part of the city are related to a thin layer of Quaternary deposits ().
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Geophysical measurements for site effects characterisation in the urban area of Girona, Spain
Authors Albert Macau, Beatriz Benjumea, Anna Gabàs, Fabian Bellmunt and Sara FiguerasABSTRACTAmplitude level, duration, and spectral content of earthquake ground motions are strongly influenced by local soil conditions. Reliable estimation of site effects is, therefore, crucial in order to avoid damage to infrastructures and mitigate other losses. Traditional geophysical exploration techniques are restricted in urban environments due to the presence of anthropogenic noises causing low seismic signal‐to‐noise ratio and other logistical issues. These problems are even more critical when the maximum investigation depth extends to hundreds of metres.
The city of Girona, located in northeastern Spain, has seismic hazard represented by a peak ground acceleration value of 0.08 g for a return period of 500 years. The city was built at the confluence of four rivers, generating a complex surface geology with bedrock outcrops and the presence of stiff soils, soft soils, and also a volcanic basalt layer. This paper presents the results of the geophysical work, which was carried out in order to characterise the soil layers present in the urban area of Girona. All information obtained in this research will be useful in computing the amplification of ground motion and to perform microzonation studies. We have obtained the shear‐wave velocity profile in the study area using a combination of seismic noise array and multichannel analysis of surface waves techniques. Using the horizontal‐to‐vertical spectral ratio method, we have obtained the soil fundamental frequency. The combination of shear‐wave velocity and values of soil fundamental frequency provides a complete map of the bedrock topography. The expected velocity inversion due to the presence of shallow basalt flow has been targeted. Electrical resistivity tomography is found to be suitable to define the volcanic basalt thickness. This thickness value is used to constrain the inversion of surface wave dispersion curves and reduce shear‐wave velocity uncertainty. The new methodology overcomes the limitations that are typical to urban conditions and other geological complexities.
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Shear‐wave velocity structure from MASW and SPAC methods: The case of Adra town, SE Spain
ABSTRACTThe damage distribution in the town of Adra (south‐eastern Spain) during the 1993 and 1994 Adra earthquakes of magnitude Mw~5.0 and maximum intensity degree of VII (European Macroseismic Scale) was mainly concentrated in the southeast sector, where the low‐diagenetic (soft) sediments outcrop. As new urbanizations are being planned in this sector, a soil classification based on the shallow shear‐wave velocity () structure is needed. For the purpose of earthquake disaster mitigation, the Spatial Autocorrelation (SPAC) and the Multichannel Analysis of Surface Waves (MASW) methods were used to propose integrated 2D models for the seismic response characterization of the shallow geology. Joint inversion of H/V spectral ratios of ambient noise, interpreted under the Diffuse Field Approach and the dispersion curves derived from the SPAC method allowed us to obtain more constrained models. Both SPAC and MASW methods provided similar results for the surveyed geological formations. From these models, a classification of the geological formations was carried out in terms of values and Eurocode 8 (European Committee for Standardization 1998) classes. Lower values in the 180‐360 m/s range were found in the southeastern sector of the town, where soft sediments outcrop and some building damage was reported during the 1993‐1994 earthquakes. The highest values exceeding 800 m/s appear in the northern sector, where the hardest rocks outcrop and no building damage was reported. The combination of the well‐suited database prepared for different geological formations with the 1:5,000 scale geological mapping was an important step to obtain the detailed soil microzonation map of Adra. This approach offered a new predictive insight into the building damage distribution, which would contribute to the appropriate urban planning for the future growth of the town.
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Seismic interferometry facilitating the imaging of shallow shear‐wave reflections hidden beneath surface waves
Authors Jianhuan Liu, Deyan Draganov and Ranajit GhoseABSTRACTHigh‐resolution reflection seismics is a powerful tool that can provide the required resolution for subsurface imaging and monitoring in urban settings. Shallow seismic reflection data acquired in soil‐covered sites are often contaminated by source‐coherent surface waves and other linear moveout noises (LMON) that might be caused by, e.g., anthropogenic sources or harmonic distortion in vibroseis data. In the case of shear‐wave seismic reflection data, such noises are particularly problematic as they overlap the useful shallow reflections. We have developed new schemes for suppressing such surface‐wave noise and LMON while still preserving shallow reflections, which are of great interest to high‐resolution near‐surface imaging. We do this by making use of two techniques. First, we make use of seismic interferometry to retrieve predominantly source‐coherent surface waves and LMON. We then adaptively subtract these dominant source‐coherent surface waves and LMON from the seismic data in a separate step. We illustrate our proposed method using synthetic and field data. We compare results from our method with results from frequency–wave‐number (f‐k) filtering. Using synthetic data, we show that our schemes are robust in separating shallow reflections from source‐coherent surface waves and LMON even when they share very similar velocity and frequency contents, whereas f‐k filtering might cause undesirable artefacts. Using a field shear‐wave reflection dataset characterised by overwhelming LMON, we show that the reflectors at a very shallow depth can be imaged because of significant suppression of the LMON due to the application of the scheme that we have developed.
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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)