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- Volume 20, Issue 3, 2022
Near Surface Geophysics - Volume 20, Issue 3, 2022
Volume 20, Issue 3, 2022
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Enhancing potential field data using its dip angle
More LessABSTRACTThe dip angle of a seismic horizon is a commonly used attribute of seismic data. It has the useful property that it is almost independent of the amplitude of the features being analysed. When appropriately rescaled, it can also be a useful edge detector for potential field data. Rescaling is necessary because the gradients of potential field data are not dimensionless. The rescaling also allows the amplitude balancing effect to be controlled, which is useful when poor quality data are being processed. The dip angle of synthetic data and aeromagnetic data from South Africa are analysed and the filter is found to be effective.
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Characterization of a river embankment using a non‐destructive direct current electrical survey
Authors Ryosuke Umezawa, Motoharu Jinguuji and Toshiyuki YokotaABSTRACTRiver embankments are earthen structures constructed to protect areas from flooding. If a river embankment collapses, it can cause major damage to human lives; therefore, proper inspection and evaluation are essential to determine the problem areas in river embankments, and repair them if necessary, before any damage occurs. Electrical surveys are a reliable method that has been used widely. Recently, electrical surveys have been used to inspect long structures such as river embankments, highlighting the need for developing non‐destructive and less fieldwork electrical survey methods. In this study, we examined a direct current electrical exploration system using polyvinyl alcohol sponge roller electrodes, which are non‐destructive and can be moved smoothly without damaging the ground surface. We conducted initial experiments on a test ground in which this new system was towed by an unmanned ground vehicle to perform an electrical survey and compared the results with those obtained using conventional methods that utilize stainless steel electrodes driven into the ground. The results indicate that the direct current electrical survey using the polyvinyl alcohol electrodes yielded almost the same results as those obtained using the stainless steel electrodes. In addition, both methods were tested on a river embankment to investigate the effectiveness of the towed direct current electrical survey. We confirmed that the towed survey method was effective by comparing the resistivity cross‐sections and fieldwork required for both survey types.
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Effects of grain size, solution salinity and pH on the electrical response of oil‐bearing carbonate sands
Authors Yaping Deng, Jiazhong Qian, Qiankun Luo, Haichun Ma, Lei Ma and Kaijian XuABSTRACTResearchers around the world have carried out numerous studies on the electrical response of oil‐contaminated silica sands. However, few studies consider the oil contamination in carbonate sands that exhibit quite different characteristics to silica sands. The purpose of this work is to study the influence of grain size (0.5–1.0, 1–3 and 3–5 mm), solution salinity (100, 10 and 1 Ω·m) and pH (4.58, 6.5 and 9.25) on the resistivity and saturation exponent of oil‐bearing carbonate sands. Experimental columns were packed with crushed limestone grains, solution and oil. Electrical resistivity measurements were subsequently performed on the packed columns. Grain size exhibits a complex effect on the electrical response of partially saturated limestone grains. Resistivity decreases slightly with increasing grain size when water saturation is greater than 0.4, while it shows an inconsistent trend when water saturation is lower than 0.4. The resulting saturation exponent n decreases slightly when grain size increases from 0.5–1 mm to 1–3 mm, while it shows a positive change in n values for grain size of 3–5 mm. Changes in solution salinity significantly influence the resistivity and saturation exponent of limestone grains, with higher solution salinity resulting in lower resistivity and higher saturation exponent. However, with solution pH increasing from 4.58 to 9.25, it shows very limited effect on the resistivity and saturation exponent of limestone grains. This can be attributed to the strong buffering ability of limestone. Effects of grain size, solution salinity and pH on the resistivity and saturation exponent of oil‐bearing carbonate sands were investigated. Resistivity is found to show a decreasing trend with increasing grain size from 0.5–1 mm to 3–5 mm above a certain saturation (e.g. Sw > 0.4), while it shows a rapid increase below a certain saturation (e.g., Sw0.4) for a larger grain size of 3–5 mm. Resistivity decreases and the saturation exponent increases with the increase of solution salinity for oil‐bearing carbonate sands. Resistivity and saturation exponent do not vary significantly with solution pH ranging from 4.58 to 9.25 for oil‐bearing carbonate sands.
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Detection of karst caves during tunnel construction using ground‐penetrating radar and advanced drilling: A case study in Guangxi Province, China
Authors Riyan Lan, Zonghui Liu, Maomao Liu, Qiyu Guan, Yuanfang Yan, Huaifeng Sun and Dong ZhouABSTRACTIn karst topography, hidden caves or underground rivers may imperil the safety of tunnels. It is critical to accurately detect the location and geometric shape of hidden karst caves for assessing the safety of tunnels. This paper reports a case where a large karst cave was exposed during the construction of the Nongmo tunnel in Duan County, Guangxi, China. A 2D ground‐penetrating radar survey was employed to preliminarily detect the distribution of hidden karst caves along the tunnel axis. Advanced drilling was used to obtain the velocity of electromagnetic waves and verify the 2D ground‐penetrating radar detection. Ground‐penetrating radar 3D grid detection was carried out for further detecting the geometric shape of the hidden karst caves. Rotary thrust power ratio, which is a function of drilling speed, rotating speed, propulsion pressure and torque, was proposed to reflect the integrity of the surrounding rocks. It was found that the depth of the peak value in the rotary thrust power ratio curve is consistent with that of the amplitude jump in ground‐penetrating radar A‐scans and in time–frequency distributions. The results of 3D attributes analysis indicate that due to the variation of highlights in different ground‐penetrating radar signal attributes, the obtained karst cave shapes using different attributes would be various. A multi‐attributes fusion method was proposed to generate a data volume considering various attributes. The 2D geometric shapes at different depths were obtained using the anomalous body lineament in the generated date volume slices. The 3D geometric shapes were reproduced by combining the obtained 2D geometric shapes at various depths. Based on different detection and analysis methods, the outline shape of karst caves at YK364 + 199 ∼ YK364 + 184 of the Nongmo tunnel was revealed in detail.
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The effect of near‐surface azimuthal anisotropy on a joint interpretation of seismic and electrical resistivity data
ABSTRACTIn this paper, we study the anisotropic near surface with seismic refraction surveying and electrical resistivity tomography. We present a field data processing case study and show a significant dependence of the determined P‐ wave velocity and electrical resistivity on the acquisition line direction. We show that the anisotropy influences not only the determined velocities themselves, but also the estimated depths of the determined boundaries and can cause misinterpretation of the arrival time curves and lead to hidden layer issues. We justify our field data‐processing results with ray modelling and synthetic data processing. As a result of our case study, we suggest a consistent direction of anisotropy for electrical resistivity and seismic surveying for the particular site in the Novosibirsk region.
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Modelling 3D elastodynamic wave scattering due to density and Lamé parameter contrasts of near‐surface scatterers
Authors Utku Harmankaya and Ayse KaslilarABSTRACTInvestigating the near‐surface structure of the Earth through geophysical methods is a crucial aspect in many areas of study in geotechnical engineering, environmental science and exploration. Among several geophysical methods, seismic‐based ones are widely used for characterizing near‐surface features that are distinguished by contrasts in elastic parameters. In this paper, we model 3D elastodynamic wave propagation and scattering using a method based on domain‐type integral representation with Born approximation. We calculate the scattered wavefield by considering the first‐order perturbations in density and Lamé parameter contrasts of scatterers. Contrasts in Lamé parameters can be useful for determining the material properties of subsurface structures in cases of weak contrasts in velocities accompanying considerable Lamé parameter variations. We examine the effects of each parameter contrast on a series of models involving a subsurface scatterer. We also compare the seismograms obtained from our method with those from a 3D finite‐difference wavefield modelling program, where we observe good agreement between the modelling results. Sensitivity of the wavefield to the perturbation in each model parameter is also examined by calculating and analysing the Fréchet derivatives. In general, the method discussed here can provide a solid foundation for prospective imaging studies involving density and Lamé parameters simultaneously.
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A rapid imaging method of the seismic back‐scattered wavefield for urban road near‐surface anomalous structures
Authors Xinglin Lu, Xuquan Hu, Chaopeng Luo, Zhengyu Xu, Xian Liao, Longhuan Liu and Zhihong FuABSTRACTUrban road near‐surface anomalous structures are buried at shallow depths (≦20 m) and small scale. Current geophysical methods, such as the transient electromagnetic, ground‐penetrating radar and shallow seismic methods with reflection wave, refraction wave and Rayleigh surface wave, do not provide sufficient detection accuracy for near‐surface anomalous structures. Although the back‐scattered method is promising in this task, it still needs improvement. This paper proposes an efficient near‐surface seismic back‐scattered method to realize the rapid lateral imaging of urban road anomalous structures. The proposed method uses a principal component analysis algorithm to directly extract the anomalous structure information. It does not require the knowledge of parameters such as velocity or the seismic wavelet so that the solutions are not affected by these parameters. Moreover, the proposed method retains the high transverse resolution of the back‐scattered wavefield and does not lose the wavefield information. Simulation and experimental operation show that the proposed method is very efficient. Combined with the hardware equipment provided by the towed seismograph, the proposed method can realize rapid detection and data imaging, which has a high engineering application value.
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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)