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- Volume 15, Issue 3, 2017
Near Surface Geophysics - Volume 15, Issue 3, 2017
Volume 15, Issue 3, 2017
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A shear‐wave seismic system using full‐waveform inversion to look ahead of a tunnel‐boring machine
ABSTRACTIn the near surface with unconsolidated soils, shear‐wave properties can often be characterised better and with a higher resolution than compressional‐wave properties. To enable imaging ahead of a tunnel‐boring machine, we developed a seismic prediction system with a few shear‐wave vibrators and horizontal receivers. The boring process is interrupted at regular intervals to carry out active surveys. The vibrators are then pushed against the rock or soil in front of the cutting wheel of the machine. The design of the vibrators is based on linear synchronous motor technology that can generate very low frequencies, starting at 5 Hz. These vibrators generate a force in a direction perpendicular to the tunnel axis. Horizontal receivers measure the particle velocity, mainly due to the horizontally polarised shear waves. Because imaging with conventional migration methods suffers from artefacts, caused by the incomplete aperture and inaccuracies in the assumed velocity model, we use two‐dimensional horizontally polarised shear full‐waveform inversion to resolve the subsurface shear properties. The classic cycle‐skipping problem, which can make the application of full‐waveform inversion cumbersome, is avoided by the capacity of the vibrators to generate low frequencies. In this paper, we demonstrate the capabilities of the proposed seismic system through a number of synthetic and field experiments.
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High‐resolution reflection seismic imaging for the planning of a double‐train‐track tunnel in the city of Varberg, southwest Sweden
Authors Mahdieh Dehghannejad, Alireza Malehmir, Mats Svensson, Mattias Lindén and Henrik MöllerABSTRACTA newly developed broadband digital‐based seismic landstreamer system was employed for the planning of a double‐train‐track tunnel in the city of Varberg, southwest Sweden. Twenty‐five seismic profiles, totalling more than 7.5 km of data, were acquired using a 2‐ to 4‐m receiver and source spacing. At places where it was not possible to move the streamer such as road crossings, wireless recorders connected to 28‐Hz geophones were used. In addition to the earlier refraction data analysis and first‐break traveltime tomographic modelling, reflection processing of the data was considered in this study, given the realisation of reflections in raw shot gathers and their good quality. Bedrock is strongly reflective in most cases but is not evident in the sections when it gets near the surface. Bedrock undulation is noticeable in most reflection sections, and at one occasion, strong diffraction is observed in the bedrock or near to it. The diffraction is originated, not known during the survey, from a 400‐m3 cylindrical (of about 3‐m‐height and 13‐m‐diameter) concrete‐made fire‐protection water tank situated in the bedrock and used in emergency situations. Reflection seismic data greatly complement the tomographic models and support deep bedrock where the excavation of the tunnel is planned in downtown Varberg. This interpretation implies different reinforcements and tunnel construction methods (e.g., roofed concrete) at this section of the tunnel. In addition, weakness zones associated with fracture systems are inferred from the reflection characteristics and in conjunction with the velocity models requiring verification by additional boreholes.
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Reverse time migration: introducing a new imaging technique for ultrasonic measurements in civil engineering
Authors Maria Grohmann, Sabine Müller, Ernst Niederleithinger and Sarah SieberABSTRACTUltrasonic echo testing is widely used in non‐destructive testing in civil engineering to investigate concrete structures, to measure thickness, and to locate and characterise built‐in components or inhomogeneities. Currently, synthetic aperture focusing techniques are mostly used for imaging. These algorithms are highly developed but have some limitations. For example, it is not possible to image the lower boundary of built‐in components like tendon ducts or vertical reflectors. We adopted reverse time migration for non‐destructive testing in civil engineering in order to improve the imaging of complicated structures in concrete. By using the entire wavefield, including waves reflected more than once, there are fewer limitations compared to synthetic aperture focusing technique algorithms. As a drawback, the required computation is significantly higher than that for the techniques currently used.
Simulations for polyamide and concrete structures showed the potential for non‐destructive testing. The simulations were followed by experiments at a polyamide specimen. Here, having acquired almost noise‐free measurement data to test the algorithm, we were able to determine the shape and size of boreholes with sufficient accuracy. After these successful tests, we performed experiments at a reinforced concrete foundation slab. We obtained information from the data by reverse time migration, which was not accessible by traditional imaging. The imaging of the location and structure of the lower boundary of the concrete foundation slab was improved. Furthermore, vertical reflectors inside the slab were imaged clearly, and more flaws were found. It has been shown that reverse time migration is a step forward in ultrasonic testing in civil engineering.
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Higher‐order correlative stacking for seismic data denoising based on the multiple‐domain combination
Authors Jinghe Li, Rui Qi, Yujie Zhang and Bin XiongABSTRACTTo improve the denoising performance of seismic data contaminated with random and coherent noises, a hybrid denoising scheme is proposed in this paper. It aims to whiten the random noise and identify the coherent noise for the preserved or prominent seismic features. Using the wavelet and curvelet basis functions in curvelets alternately, the hybrid denoising scheme utilises the representation of edges and singularities along curves. Then it adapts the wavelet‐based higherorder correlative stacking denoising method from seismic exploration sequentially. With regard to seismic records for bedrock surface detection after the artificial backfill, the noisy data are significantly improved both in terms of denoising and improving fidelity. To illustrate the advantage of the hybrid denoising scheme, a comparison of the performances between the different individual denoising methods, including the higher‐order correlative stacking method and curvelets with wavelet and curvelet basis functions, has been presented for the complex seismic records contaminated with different noises. Numerical case studies and a field data analysis have been used to show that the proposed hybrid denoising scheme is more effective for seismic data containing complex features than the individual denoising methods.
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A controlled monitoring study of simulated clandestine graves using 3D ground penetrating radar
More LessABSTRACTA 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.
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Probe rotation effects on direction of arrival estimation in array‐type directional borehole radar
Authors Satoshi Ebihara, Kazushige Wada, Shinsuke Karasawa and Kentaro KawataABSTRACTThis paper discusses the relationship between feeding line delay compensation and direction of arrival estimation in an array‐type directional borehole radar. In this radar, since the space available for an array antenna is limited by the borehole diameter, direction of arrival estimation is based on small travel time differences among array elements, and an accurate compensation of feeding line delays is important. Computer simulation confirmed that the radar probe rotation in a borehole leads to errors in the direction of arrival estimation of greater than 15° if the delays associated with the feeding array antenna elements are not compensated to within 0.1 ns. This may be caused by a failure to measure the time delays of electrical circuits around the feeding points of the antenna elements. In this case, we suggest that the lengths of system components other than the coaxial cables should be kept to less than 3 cm. Based on these investigations, we developed an array‐type directional borehole radar for a geotechnical project to locate foundation piles. In a field experiment, we confirmed that direction of arrival estimation errors were below about 15°, although the radar probe rotated through more than 180° during the measurement, thanks to correct compensation of the cables. With the correct compensation, we demonstrated three‐dimensional location of a buried cylindrical conducting object, which was located at 2 m from the radar in wet soil. We were able to estimate the reflection point position with an accuracy of 34 cm, which is the averaged error of the three‐dimensional location, while allowing the radar probe to rotate.
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Inversion of borehole magnetic data for prospecting deep‐buried minerals in areas with near‐surface magnetic distortions: a case study from the Daye iron‐ore deposit in Hubei, central China
Authors Shuang Liu, Xiangyun Hu, Jianchao Cai, Jianhui Li, Chunling Shan, Wei Wei, Qi Han and Yajun LiuABSTRACTSignificant near‐surface magnetic distortions and weakly magnetic responses complicate the interpretation of surface magnetic data and compound difficulties associated with the prospecting of deep‐buried orebodies in old mines, such as the Daye iron‐ore deposit. To address this problem, we used three‐component borehole magnetic measurements from multiple drillholes to evaluate a magnetisation inversion approach that incorporates these data to obtain magnetisation intensity distributions. The magnetisation direction was estimated from rock and ore sample magnetic property measurements. Linear optimisation problem equations were solved using the pre‐conditioned conjugate gradient algorithm. Tests on two‐dimensional synthetic data returned favourable results. In the case study of the Daye iron‐ore deposit, several concealed magnetite orebodies were discovered using three‐component borehole magnetic measurements. Two‐dimensional magnetisation inversion for borehole magnetic data indicated that magnetic sources were located at the contact zones between the diorite and marble rocks, as verified by other drillings. A three‐component borehole magnetic survey shows advantages of moving away from shallow interferences and containing important additional information compared with a surface total field and provides a feasible approach to explore deep‐buried and concealed minerals in areas with strong near‐surface distortions.
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Mapping of hydrocarbon‐ and scrap‐metal‐contaminated soil using volatile organic compounds and electromagnetic profiling methods
More LessABSTRACTThe electromagnetic profiling method detects lateral changes in soil conductivity up to a certain depth. Volatile organic compounds are most commonly found in sites contaminated by hydrocarbons. This work presents the results of the application of electromagnetic profiling and volatile organic compound methods for detecting hydrocarbon and buried scrap metal contamination zones at a 48‐ha site that was used as an industrial waste dump for many years. Potentially contaminated zones were configured based on a joint analysis of low resistivity anomalies (mature pollution) and high concentrations of volatile organic compounds, which reduced the amount of soil sampling through optimal location of the drilling points. Chemical analysis in 54 soil samples showed the presence of three hydrocarbon‐contaminated zones that produced total petroleum hydrocarbon values higher than 10,000 ppm. Buried scrap metal was detected by anomalous in‐phase measurements obtained by an electromagnetic profiling survey, facilitating future removal works.
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A multi‐disciplinary characterisation of Heath Steele belt deposits, Bathurst Mining Camp: a basis for identifying new targets for the exploration
Authors Vanessa Biondo Ribeiro and Marta Silvia Maria MantovaniABSTRACTAlthough the Bathurst Mining Camp has been explored for more than 40 years, high‐resolution airborne geophysical survey has led to the detection of several new volcanogenic massive sulphide occurrences in the region. The discovery of these concealed exploration targets depended directly on the knowledge of the multi‐parameter signature of the major deposits and the search for the repeatability of these signatures in other regions. The Heath Steele B deposit, within the southern part of the Bathurst Mining Camp, together with an associated magnetite‐rich iron formation and adjacent tuffaceous sedimentary rocks, produced strong, co‐extensive magnetic and conductivity anomalies. However, the strong anomalies observed in the geophysical data can be related to non‐mineralised lithology such as mafic dykes or graphite. So a direct identification of exploration targets using geophysical data only can be difficult. For this reason, the joint interpretation of geophysical and geochemical signature can considerably improve the identification of new deposits. Using these techniques, we identify three new possible targets, characterised by local magnetic and apparent conductive highs. Two of these areas are located in the Heath Steele belt, and their geochemistry has been studied in detail. The proximity of these targets (less than 1.5 km) from known occurrences of hydrothermal sediments (magnetite, siderite, and stilpnomelane) confirms their exploration potential. The fourth identified area is most likely associated with a regional anomaly related to an unmapped ferromagnetic structure and was earlier disregarded as a possible target. The joint interpretation of geophysical and geochemical data allowed us to identify two new targets for exploration and to exclude one area interpreted as associated with a regional anomaly. Therefore, the search parameters (physical proprieties and hydrothermal sediment geochemical pattern distribution) are established with a success index of 75%. The index was limited by the absence of geochemistry data over the third target. The procedure presented in this paper can be used to search for new targets elsewhere in the Bathurst area and even in other base metal 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)