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- Volume 18, Issue 3, 2020
Near Surface Geophysics - Volume 18, Issue 3, 2020
Volume 18, Issue 3, 2020
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Acoustic global–local full‐waveform inversion for P‐wave velocity estimation of near‐surface seismic data acquired in Luni, Italy
Authors Eusebio Stucchi, Silvio Pierini, Andrea Tognarelli and Jimi BrunodABSTRACTIn this work, we describe an experiment concerning global–local full‐waveform inversion, carried out on a P‐wave seismic reflection profile that was acquired at Luni, an archaeological site in Italy. The global full‐waveform inversion makes use of a two‐grid genetic algorithm scheme and recorded refraction and diving waves, to build an initial velocity model of the subsurface. Two important pieces of a priori information which help to better constrain the inversion results are the refraction velocity model and the Dix‐converted semblance velocity field obtained from time processing. A good match between observed and predicted data allows us to use the estimated velocity field as the starting point for a local, gradient‐based full‐waveform inversion that inverts the recorded data (except the surface waves). The final estimated velocity field shows two main discontinuities: one is very shallow and related to the refractor velocity model used and the other corresponds to the strongest reflection event observed in the pre‐stack depth‐migrated section, at a depth of 100 m. The pre‐stack depth‐migrated common image gathers provide evidence of a good horizontal alignment of this reflection, indicating an accurate velocity estimation down to 100 m depth that corresponds to the maximum offset used in the acquisition.
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Characterization of silty to fine‐sandy sediments with SH waves: full waveform inversion in comparison with other geophysical methods
ABSTRACTWe apply seismic full waveform inversion to SH‐ and Love‐wave data for investigating the near‐surface lithology at an archaeological site. We evaluate the resolution of the applied full waveform inversion algorithm through ground truthing in the form of an excavation and sediment core studies. Thereby, we investigate the benefits of full waveform inversion in comparison with other established methods of near‐surface prospecting in terms of resolution capabilities and interpretation security. The study is performed in a presumed harbour area of the ancient Thracian city of Ainos. The exemplary target is the source of a linear magnetic anomaly oriented perpendicular to the coast, which was found in a previous magnetic gradiometry survey, suggesting a mole. The SH‐wave full waveform inversion recovered a subsurface SH‐wave velocity model with submeter resolution showing lateral and vertical velocity variation between 40 and 150 m/s. To tame the non‐linearity of the full waveform inversion, a sequential inversion of frequency bands has to be combined with time‐windowing in order to separate the Love wave from the reflected SH wavefield. We compare the full waveform inversion results with multichannel analysis of surface waves, standard seismic reflection imaging, electrical resistivity tomography and electromagnetic induction. It turns out that the respective depth sections are correlated to a certain degree with the full waveform inversion results. However, the structural resolution of the other geophysical methods is significantly lower than for the full waveform inversion. An exception is the reflection seismic imaging, which shows the same resolution as full waveform inversion but can only be interpreted together with the full waveform inversion–based velocity model. An archaeological excavation as well as coring data allows ground truthing and a direct understanding of the geophysical structures. The results show that the target was a sort of near‐surface trench of about 3–4 m width and 0.8 m to 1.0 m depth, filled with silty sediment, which differs from the layered surrounding in colour and composition. The ground truthing revealed that only SH‐wave full waveform inversion and seismic reflection imaging could image the trench and sediment structure with satisfying lateral and depth resolution. We emphasize that the velocity distribution from SH‐wave full waveform inversion agrees closely with the excavated subsurface structures, and that the discovered changes in seismic velocity correlate with changes in the sand content in the respective sediment facies sequences. The study demonstrated that SH‐wave full waveform inversion is capable to image structural and lithological changes in the near subsurface at scales as low as 0.5 m, thus providing the high resolution needed for archaeological and geoarchaeological prospection.
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High resolution 3D subsurface mapping using a towed transient electromagnetic system ‐ tTEM: case studies
More LessABSTRACTGeophysical methods are routinely applied for investigation of near surface in areas of infrastructure, water supply, artificial infiltration, farming, waste deposits, construction, etc. A new towed transient electromagnetic tool, called ‘tTEM’, provides rapid, efficient, high‐resolution imaging of subsurface hydrogeology, and can deliver densely spaced profiles of resistivity. These profiles can be used to map a three‐dimensional subsurface in high resolution. In this paper, we present three case studies where the towed transient electromagnetic system was used to map the subsurface at the hectare scale. In the first case, we used towed transient electromagnetic to map raw materials in the northern Jutland, Denmark. The survey was carried out to map possible sand and gravel deposits. In the towed transient electromagnetic models, the potential sand/gravel targets are identified as high resistive bodies in the top 30 m. These bodies can have significant lateral variation at scales much less than one hundred metres. In the second case study, towed transient electromagnetic was used to map the thickness of a protecting clay layer above an aquifer in Vildbjerg, a town in the central part of Denmark. Results show that the overlying clay layer has sufficient thickness (>15 m) to protect the underlying aquifer from pesticide pollution in the area. Finally, in the third case study, we used towed transient electromagnetic for mapping geology in the vicinity of a landfill. The inversion results reveal a hitherto unknown buried valley‐like feature within the top 30 m of the subsurface that was not identified by older, regional TEM surveys – a feature that can have significant impact on the water flow.
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Imaging of groundwater contamination using 3D joint inversion of electrical resistivity tomography and radio magnetotelluric data: A case study from Northern India
Authors A. Devi, M. Israil, A. Singh, Pravin K. Gupta, P. Yogeshwar and B. TezkanABSTRACTThe impact of untreated sewage irrigation and waste disposal practices on groundwater is investigated by 3D joint inversion of radio magnetotelluric and electrical resistivity tomography data. In this case study, electrical resistivity tomography and radio magnetotelluric field measurements were carried out on several profiles near a waste disposal site which was irrigated with untreated sewage water for agriculture purpose. In addition, radio magnetotelluric and electrical resistivity tomography measurements were carried out, far away from the waste site, to derive the uncontaminated geology. The data were analysed earlier using 2D inversion techniques. However, for the 2D inversion of the electrical resistivity tomography and radio magnetotelluric data, assumptions about the strike direction are required. As no clear strike direction is evident for the contamination, we considered the problem as 3D and interpreted the present data set using the 3D inversion algorithm ‘AP3DMT‐DC’. The inverted 3D resistivity model shows an unconfined aquifer of low resistivity which is overlain by an unsaturated slightly resistive near surface formation. With increasing distance from the waste sites, an increase in the resistivity of the shallow unconfined aquifer is observed. Furthest away from the waste site undisturbed geology is expected. We derived consistent and meaningful 3D resistivity models. The uncontaminated reference site indicates an increased resistivity for the aquifer layer. A synthetic 3D study was carried out to demonstrate and validate algorithm performance as well as convergence capabilities. The study demonstrates that the two methods, electrical resistivity tomography and radio magnetotelluric, complement each other. Besides, a better resolved inverted model is obtained through a 3D joint inversion, in comparison to individual 2D and 3D inversions.
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Measuring vertical soil water content profiles by combining horizontal borehole and dispersive surface ground penetrating radar data
ABSTRACTTo investigate transient dynamics of soil water redistribution during infiltration, we conducted horizontal borehole and surface ground penetrating radar measurements during a 4‐day infiltration experiment at the rhizontron facility in Selhausen, Germany. Zero‐offset ground penetrating radar profiling in horizontal boreholes was used to obtain soil water content information at specific depths (0.2, 0.4, 0.6, 0.8 and 1.2 m). However, horizontal borehole ground penetrating radar measurements do not provide accurate soil water content estimates of the top soil (0–0.1 m depth) because of interference between direct and critically refracted waves. Therefore, surface ground penetrating radar data were additionally acquired to estimate soil water content of the top soil. Due to the generation of electromagnetic waveguides in the top soil caused by infiltration, a strong dispersion in the ground penetrating radar data was observed in 500 MHz surface ground penetrating radar data. A dispersion inversion was thus performed with these surface ground penetrating radar data to obtain soil water content information for the top 0.1 m of the soil. By combining the complementary borehole and surface ground penetrating radar data, vertical soil water content profiles were obtained, which were used to investigate vertical soil water redistribution. Reasonable consistency was found between the ground penetrating radar results and independent soil water content data derived from time domain reflectometry measurements. Because of the improved spatial representativeness of the ground penetrating radar measurements, the soil water content profiles obtained by ground penetrating radar better matched the known water storage changes during the infiltration experiment. It was concluded that the combined use of borehole and surface ground penetrating radar data convincingly revealed spatiotemporal soil water content variation during infiltration. In addition, this setup allowed a better quantification of water storage, which is a prerequisite for future applications, where, for example, the soil hydraulic properties will be estimated from ground penetrating radar data.
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Direct‐wave denoising of low‐frequency ground‐penetrating radar in open pits based on empirical curvelet transform
Authors Tao He and Haili ShangABSTRACTLow‐frequency ground‐penetrating radar is known to be effective in the detection of geological disasters in open pits owing to its good detection depth, high resolution and portability. However, because of the lack of a shield layer and the poor anti‐interference capability, low‐frequency ground‐penetrating radar has certain limitations. A strong linear interference due to direct waves can result in a complex mining environment, hindering the detection of geological disasters in open pits. Conventional methods, such as wavelet transform and curvelet transform, are ineffective, as they cannot adaptively remove noise in accordance with signal characteristics. In this paper, considering the high apparent velocity and energy of direct‐wave noise in open‐pit ground‐penetrating radar data, an empirical curvelet transform method is proposed to suppress the interference signals. The empirical curvelet transform not only is multi‐scale and multi‐directional, but can also adaptively perform filtering based on the characteristics of the ground‐penetrating radar interference signals. This method is applied to the simulation and processing of measured ground‐penetrating radar data and is compared with the conventional curvelet transform. The results confirm the effectiveness of this method.
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Basement faults deduction at a dumpsite using advanced analysis of gravity and magnetic anomalies
ABSTRACTA study of the fracture systems in the basement of the Matatlan urban waste dump of Guadalajara City, near the western edge of the Rio Grande de Santiago canyon (southwestern Mexico), is reported. The Matatlan dumpsite was developed on top of andesitic and rhyolitic rocks. Measurements at outcrops indicated that N–S (0°–15° and 165°–180°), N45–90E and N105–135E fracture systems affect the basement. Statistical analysis (micromagnetic method) of magnetic lineament azimuths indicated the presence of two lineament groups. The major one (N60–120E) has a maximum at N90–105E, and a secondary group (N120–165E). The gravity anomalies are featured by a major group (N60–120E, with maximums at N60–75E, and N105–120E) and a secondary group (N120–180E). Thus, a good to fair correlation exists between the measured fracture orientations and the lineaments featuring the gravity and magnetic anomalies. Horizontal derivative and tilt angle depicted horizontal limits of anomaly‐causative bodies. Results of the micromagnetic method are consistent with fracture orientations and with the limits established through edge‐detection techniques. Lineaments established by means of the gravity and magnetic Euler deconvolution are also consistent. Gravity and magnetic Euler deconvolution helped to estimate the depth extent of the limits of gravity and magnetic common sources. Euler deconvolution indicated the fault or contact nature of these lineaments. Furthermore, Euler deconvolution indicated slopes of the limits of gravity and magnetic sources. The fault or fracture nature of the major lineaments was finally confirmed by forward modelling (forward 2D cooperative modelling of gravity and magnetic profiles). For example, lineaments delimiting the northern depression could be characterized as normal faults. In general, this study indicates that it is possible, based on gravity and magnetic data, to characterize fractures affecting the basement of a dumpsite, a key parameter in hydrogeophysical studies. In particular, through this study, the inferred structures could be associated with regional tectonic faulting, which might provide migration paths to unhealthy lixiviate outside the dumpsite, giving rise to the contamination of the neighbouring environment. In particular, the study explains the presence of an unhealthy leachate waterfall to the northeast of the dumping site. Heavy metal contents in samples from this waterfall support that lixiviates originate at the dumpsite.
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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)