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- Volume 20, Issue 2, 2022
Near Surface Geophysics - Volume 20, Issue 2, 2022
Volume 20, Issue 2, 2022
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First application of a new seismo‐electric streamer for combined resistivity and seismic measurements along linearly extended earth structures
More LessABSTRACTLinearly extended earth structures (e.g., river embankments and earth dams) require specific characterization tools to efficiently evaluate stability conditions along their relevant extensions. For this aim, geophysical surveys, both seismic and electric, are commonly used and considered complementary to geotechnical investigations. They provide imaging of parameter variations along the whole structures and not just local information in correspondence of boreholes. Improvements in these survey methodologies can further increase the application of these geophysical methods, eventually also as fast‐screening characterization tools. In this paper, the first application of a new seismo‐electric streamer, specifically designed for these aims, is presented. The developed streamer allows the contemporary acquisition of seismic and electric data while being dragged along the investigated structure. The technical solutions adopted for its construction are briefly described, and its application to the characterization of an embankment is presented. The results obtained with this new system are comparable with usually adopted investigation tools with the advantage of reduced survey time and increased efficiency.
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Choosing optimal model parameterization for improving the accuracy of refraction seismic tomography
More LessABSTRACTSeismic ray tomography is a popular tool for reconstructing seismic velocity models from traveltime data. Here we study how the model parameterization affects the resolution and accuracy of the tomographic inversion for the near‐surface model building. In particular, we consider the weighting of the elements of the model perturbation vector based on the values of the initial velocity model. When the model parameters are defined in terms of velocities, then the tomographic‐inversion resolution is better for the shallow part but degrades for the deeper part of the model. The opposite is true when the model parameters are defined in terms of slowness values. This effect is associated with the method of forming the tomographic matrix. When linearizing the tomography problem for different model parameters, the matrix elements have different weight coefficients. This affects the inversion results and can lead to large errors. We suggest a new parameterization (in‐between the velocity and the slowness) that provides better quality of the tomographic inversion and balanced resolution between the shallow and deeper part of the model. The good performance of this new parameterization is confirmed by a series of synthetic tests and one real‐data example.
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An integrated long‐wavelength statics method applied to seismic processing of Tibetan permafrost
Authors Zhiwei Liu, Haixin Feng, Ying Lu, Yang Gao and Lei ZhouABSTRACTPermafrost in high‐altitude regions complicates near‐surface modelling and exacerbates static effects during seismic processing and interpretation, especially in the form of long‐wavelength issues in cases where it extends laterally beyond the length of the seismic wavelength. The long‐wavelength component of static corrections (statics) for permafrost when solved using a single conventional method is prone to seismic imaging artefacts due to limited field layouts and unmet preconditions. The nonlinearity between the velocities and the thickness from low‐velocity near surface to high‐velocity bedrocks ulteriorly complicates near‐surface modelling for long‐wavelength statics by first‐arrival inversion. Combining the advantages of as much long‐wavelength characteristics as possible associated with the permafrost and rugged topography, we estimate an integrated long‐wavelength statics method including tomographic inversion and refraction residual statics. The integrated method can approximate linearized near‐surface modelling through multi‐step processing. An example from the Qiangtang Basin (central Qinghai–Tibet Plateau) suggests that the integrated method is capable of gradual refining complex near‐surface modelling and long‐wavelength statics and eliminates upward‐stretching abnormalities caused by Tibetan permafrost.
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Stochastic electrical resistivity tomography with ensemble smoother and deep convolutional autoencoders
Authors Mattia Aleardi, Alessandro Vinciguerra, Eusebio Stucchi and Azadeh HojatABSTRACTTo reduce both the computational cost of probabilistic inversions and the ill‐posedness of geophysical problems, model and data spaces can be reparameterized into low‐dimensional domains where the inverse solution can be computed more efficiently. Among the many compression methods, deep learning algorithms based on deep generative models provide an efficient approach for model and data space reduction. We present a probabilistic electrical resistivity tomography inversion in which the data and model spaces are compressed through deep convolutional variational autoencoders, while the optimization procedure is driven by the ensemble smoother with multiple data assimilation, an iterative ensemble‐based algorithm. This method iteratively updates an initial ensemble of models that are generated according to a previously defined prior model. The inversion outcome consists of the most likely solution and a set of realizations of the variables of interest from which the posterior uncertainties can be numerically evaluated. We test the method on synthetic data computed over a schematic subsurface model, and then we apply the inversion to field measurements. The model predictions and the uncertainty assessments provided by the presented approach are also compared with the results of a Markov Chain Monte Carlo sampling working in the compressed domains, a gradient‐based algorithm and with the outcomes of an ensemble‐based inversion running in the uncompressed spaces. A finite‐element code constitutes the forward operator. Our experiments show that the implemented inversion provides most likely solutions and uncertainty quantifications comparable to those yielded by the ensemble‐based inversion running in the full model and data spaces, and the Markov Chain Monte Carlo sampling, but with a significant reduction of the computational cost.
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Improving the galvanic contact resistance for geoelectrical measurements in debris areas: A case study
Authors Mirko Pavoni, Alberto Carrera and Jacopo BoagaABSTRACTThis paper aims to test, in a quantitative way, the different approaches that can be applied to improve the contact resistance problem in a debris environment for the acquisition of electrical resistivity tomography. We collected various datasets on the same investigation line in a blocky ground surface of a landslide deposit, using different coupling systems: single electrodes placed between the boulders, adding extra electrodes in parallel and drilled single electrodes inside the blocks. We performed the measurements in natural dry conditions, then we added salt water nearby the electrodes hammered among the boulders and we filled the drilled holes with a conductive carbomer‐based gel. The results clearly demonstrate that using salt water significantly reduces the contact resistances, but also that, if salt water is not available, we can collect a good quality dataset in dry conditions by connecting more electrodes in parallel. Drilling the electrodes directly inside the boulders decreases the data quality but, if necessary, we demonstrate that the use of a commercial carbon polymer gel can provide a marked improvement in contact resistances.
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Investigating soil conditions around buried water pipelines using very‐low‐frequency band alternating current electrical resistivity survey
Authors Motoharu Jinguuji and Toshiyuki YokotaABSTRACTUnderground pipeline infrastructures, such as water supply and industrial water pipes, were rapidly constructed in Japan during the 1970–1980s economic boom and have been ageing quickly. In general, corrosion of buried metal water pipes depends on the physicochemical properties of the soil around them. Conventionally, when conducting such investigations, the soil is excavated and sampled to analyse these properties in laboratories. As this damages the paved road surface, an alternative method is required. Resistivity is a significant physical property measured when investigating the corrosion risk of underground pipelines. Therefore, if geophysical exploration can facilitate the investigation of soil resistivity from the ground surface, it will play a key role in the renewal planning of water pipes. Traditionally, geophysical methods such as electrical and electromagnetic exploration have been used for measuring subsurface resistivity. Electrical exploration is a robust and noise‐tolerant method; however, it requires electrode installation. Most roads over the pipelines are paved, making conventional electrical exploration using metal electrodes a challenging task. Therefore, the Geological Survey of Japan, the National Institute of Advanced Industrial Science and Technology, has developed a very‐low‐frequency band alternating current electrical surveying technique that uses water‐saturated polyvinyl alcohol sponge electrodes. In this technique, electrodes are placed on an asphalt or concrete paved surface and the electrode measures the soil resistivity profile and detects the corrosive soil distribution without damaging the paved surface. Moreover, we improved this equipment to enable vertical electrical survey of numerous points and acquired data for 740 m of the survey line in 1 day at Maborikaigan coast in Yokosuka City. We found that the resistivity values varied with the depths and locations of the buried water pipes. These variations indicated that surveys and evaluations on long survey lines along water pipe burial routes are essential for risk management in water pipelines.
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Optimizing GPR time‐zero adjustment and two‐way travel time wavelet measurements using a realistic three‐dimensional numerical model
Authors Hossain Zadhoush and Antonios GiannopoulosABSTRACTTime‐zero adjustment or the true ground surface for ground penetrating radar (GPR) applications is a very important aspect and an essential factor in order to position subsurface targets, especially those located at shallow depths, at their true position in depth. As the transmitted and received signals from GPR antennas are affected by the presence of different materials with varying electromagnetic properties, adjusting the time zero appropriately is important, but often not straightforward to position accurately. This paper uses a realistic three‐dimensional numerical model of a GPR transducer in order to examine where is the best location for time zero on a GPR trace. It is shown that, in order to establish a robust and consistent time‐zero position, careful consideration is also needed around how the two‐way travel time of the reflected GPR wavelet is estimated. Starting with a simple homogeneous model with a set of different targets, a better process of time‐zero adjustment and time picking of the GPR wavelets is put forward, that is verified using further more complex and realistic heterogeneous models. Further verification is obtained by using experimental data.
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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)