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- Volume 13, Issue 1, 2015
Near Surface Geophysics - Volume 13, Issue 1, 2015
Volume 13, Issue 1, 2015
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Is accuracy more important than precision in near‐surface refraction seismology?
More LessABSTRACTThe ubiquity of non‐uniqueness in near‐surface refraction seismology emphasises the importance of making a clear distinction between accuracy, that is, the validity of the model, and precision, that is, traveltime misfit errors. Non‐uniqueness can be resolved by identifying the most probable model within a global model space, based on the generalised reciprocal method (GRM), containing all geologically reasonable seismic velocities. All models within this model space have comparable misfit errors.
The strategy involves three elements. First, the XY parameter of the GRM facilitates a more effective differentiation of the various models of the seismic velocities in both the weathered and sub‐weathered regions than is possible with simplistic comparisons of traveltime misfit errors. Second, all valid models of the seismic velocities in the weathered and sub‐weathered regions are restricted to those which employ the same XY value in both regions. Finally, the most probable model employs the optimum XY value, which is that XY value(s) which results in the simplest model of the GRM‐derived seismic velocities in the sub‐weathered region.
Under suitable conditions where the refracting interface is sufficiently irregular, the optimum XY value can be measured to a precision of plus/minus half the station spacing. However, where that is either not possible or not undertaken, then a uniform velocity equivalent value can be computed from the cross‐over distance, with an inferred precision of plus/minus the optimum XY value.
The significance of non‐uniqueness in any investigation can be demonstrated with two models. The first employs the uniform velocity model equivalent of the optimum XY value, whereas the second employs an XY value equal to the cross‐over distance. These two models represent the end members of the set of models which are consistent with the traveltime data. They span the range of seismic velocities in the weathered layers from uniform or constant values to the hyperbolic cosine function. The latter represents the maximum vertical velocity gradient which is consistent with linear traveltime graphs and which approximates the vertical velocity gradients commonly used in many implementations of automatic refraction tomography.
Irrespective of whether the optimum XY value is determined with a precision of plus/minus half the station spacing or plus/minus the optimum XY value, the optimum XY value for uniform layers is commonly one quarter of the cross‐over distance. Therefore, the optimum XY value provides a more precise measure of the accuracy of the seismic velocities than is possible with traveltime misfit errors.
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Combining surface waves and common methods for shallow geophysical survey
Authors Renata Gaždová, Petr Kolínský, Jan Vilhelm and Jan ValentaABSTRACTGeophysical data were acquired during a survey of the Hluboká Fault in the Czech Republic, Central Europe. The recorded surface waves are studied in the frequency range 8‐200 Hz. Phase velocity dispersion curves of Rayleigh and Love waves are determined from pairs of three‐component seismograms with a 5 m receiver spacing by means of a frequency‐time analysis along the profile. Rayleigh waves are analysed on the vertical (Z) and radial (R) components and Love waves on the transversal (T) component. Dispersion curves from the vertical component are then inverted to 1‐D S‐wave velocity models using the isometric method. A set of 1‐D S‐wave velocity models representing a pseudo 2‐D S‐wave velocity distribution along the profile is obtained.
This velocity distribution is compared with the results of other geophysical methods and also with direct observation from a shallow paleoseismic trenching. A combination of the S‐wave velocities obtained from the surface wave analysis and P‐wave velocities from refraction tomography is used to estimate the Poisson ratio distribution. It is shown that the resolution capabilities of surface waves are comparable in this case with electric resistivity tomography in near surface medium and with P‐wave tomography in the depths exceeding approx. 15 metres.
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Denoising magnetic data using steering kernel regression
Authors Jens Tronicke and Urs BönigerABSTRACTGround‐based magnetic surveying is a common geophysical method to explore near‐surface environments in a non‐destructive manner. In many typical applications (such as archaeological prospection), the resulting anomaly maps are often characterized by low signal‐to‐noise ratios and, thus, the suppression of noise is a key step in data processing. Here, we propose the steering kernel regression (SKR) method to denoise magnetic data sets. SKR has been recently developed to suppress random noise in images and video sequences. The core of the method is the steering kernel function which represents a robust estimate of local image structure. Using such a kernel within an iterative regression based denoising framework, helps to minimize image blurring and to preserve the underlying structures such as edges and corners. Because such filter characteristics are desirable for random noise attenuation in potential field data sets, we apply the SKR method for processing high‐resolution ground‐based magnetic data as they are typically collected in archaeological applications. We test and evaluate the SKR method using synthetic and field data examples and also compare it to more commonly employed denoising strategies relying, for example, on fixed filter masks (e.g., Gaussian filters). Our results show that the SKR method is successful in removing random and acquisition related noise present in our data. Concurrently, it preserves the local image structure including the amplitudes of anomalies. As demonstrated by derivative based transformations, the mentioned filter characteristics significantly impact subsequent processing steps and, therefore, result in an improved analysis and interpretation of magnetic data. Thus, the method can be considered as a promising and novel approach for denoising ground‐based magnetic data.
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Applying airborne electromagnetics in 3D stochastic geohydrological modelling for determining groundwater protection
Authors Jan L. Gunnink and Bernhard SiemonABSTRACTAirborne electromagnetic (AEM) measurements provide information regarding the electrical properties of the subsurface for large spatial coverage in a limited time. In mapping and modelling for geological and geohydrological purposes, electrical properties (e.g. resistivity) need to be converted to relevant parameters, like lithology. Helicopter‐borne frequency‐domain EM measurements from an area in the Netherlands were combined with borehole data to create a 3D model of two contrasting lithologies (sand and clay) that served as proxy for assessing the vulnerability of the aquifer to surface contamination. By comparing the lithology found in boreholes with the resistivity derived from AEM at that location, a probabilistic relationship between these two variables was determined. This relationship was used to convert the AEM resistivity models into a 3D model of clay probability. Using geostatistical Monte Carlo simulations, the boreholes (hard data) and the probability of clay from the AEM resistivity models (soft data) were combined. AEM improved the 3D model substantially, compared to using only borehole data. An independent validation dataset verified the improvement of the 3D model using AEM data. Areas with a high probability of clay occurrence could be distinguished and a clay thickness map with uncertainty (standard deviation) was calculated. Using a simple groundwater model, the capability of the clay to protect the underlying aquifer from contamination was quantified. This resulted in the delineation of distinct areas that are well protected due to the large travel time for infiltrating water from the surface to the aquifer.
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Joining multiple AEM datasets to improve accuracy, cross calibration and derived products: The Spiritwood VTEM and AeroTEM case study
Authors Vincenzo Sapia, Andrea Viezzoli and Greg OldenborgerABSTRACTAirborne time‐domain electromagnetic methods (AEM) are useful for hydrogeological mapping due to their rapid and extensive spatial coverage and high correlation between measured magnetic fields, electrical conductivity, and relevant hydrogeological parameters. However, AEM data, preprocessing and modelling procedures can suffer from inaccuracies that may dramatically affect the final interpretation. We demonstrate the importance and the benefits of advanced data processing for two AEM datasets (AeroTEM III and VTEM) collected over the Spiritwood buried valley aquifer in southern Manitoba, Canada. Early‐time data gates are identified as having significant flight‐dependent signal bias that reflects survey flights and flight lines. These data are removed from inversions along with late time data gates contaminated by apparently random noise. In conjunction with supporting information, the less‐extensive, but broader‐band VTEM data are used to construct an electrical reference model. The reference model is subsequently used to calibrate the AeroTEM dataset via forward modelling for coincident soundings. The procedure produces calibration factors that we apply to AeroTEM data over the entire survey domain. Inversion of the calibrated data results in improved data fits, particularly at early times, but some flight‐line artefacts remain. Residual striping between adjacent flights is corrected by including a mean empirical amplitude correction factor within the spatially constrained inversion scheme. Finally, the AeroTEM and VTEM data are combined in a joint inversion. Results confirm consistency between the two different AEM datasets and the recovered models. On the contrary, joint inversion of unprocessed or uncalibrated AEM datasets results in erroneous resistivity models which, in turn, can result in an inappropriate hydrogeological interpretation of the study area.
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Utilities detection through the sum of orthogonal polarization in 3D georadar surveys
Authors Maurizio Lualdi and Federico LombardiABSTRACTGround penetrating radar (GPR) is widely used in subsurface investigations for extracting the position and the route followed by the utility, an issue that gains more and more importance when considering the cost related to trench damage and disruptions. However, it has been noted that various targets of GPR surveys, especially linear and elongated targets, have polarization‐dependent scattering characteristics. This implies that the visibility of a subsurface scatterer in the acquired data depends on the used antenna configuration and its orientation with respect to the feature to be imaged.
Furthermore, wave attributes could be modified by the surrounding soil anisotropy and heterogeneity degree. As the GPR antennas are composed of directional dipoles, any changes in the propagation plane of the returning wave affects the recording of GPR data.
This work presents an approach based on a combination of mutually orthogonal GPR 3D data volumes through which polarization issues can be overcome, ensuring target detection even when the position and material are adverse. The strategy is evaluated through two field examples: in homogeneous soil this technique fully recovers the polarization mismatch, providing results that are closely similar to the ones that would be obtained with the optimal configuration; in heterogeneous environments it overcomes the wavelet alteration, depolarization included, strongly enhancing the signal to noise ratio and improving target reconstruction.
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Ultra‐shallow seismic and GPR methods applied on sand‐covered limestone in northern Jutland, Denmark
More LessABSTRACTThis report gives an analysis and a geophysical interpretation of an ultra‐shallow high‐resolution seismic profile recorded on sand‐covered limestone. The interpretations include velocity and structural elements characterising the limestone surface. The seismic profile consists of short offset gathers with a very limited number of traces and with a shot‐point distance along the profile giving presumed equidistant reflection points on the subsurface interfaces. By subsequent band‐pass filtering and NMO‐correction, this field set‐up is found to be sufficient to create a detailed image of the immediate subsurface down to 20 m depth on the investigation site. This recording procedure facilitates a considerable increase in profiling speed compared to the denser shot‐point recordings normally required to obtain CDP gathers. A small number of additional larger gathers is recorded in order to verify that target reflectors are within the data window and source generated noise is outside. Interval velocities are inferred from a subset of the shot gathers by means of normalised semblance analyses demonstrating that critically refracted signals are not present in the gathers. Two major reflecting interfaces are detected, one in the sand above the limestone and the other on the top of the limestone, and there is a fault in the limestone. In addition, a small depression and possible cracks/karst formations in the limestone surface are visible. Anomalies in the amplitude of a reflector in the sands along the profile can be linked with the cracks/karst formations. Multiples are formed in the sand layers above the limestone, but they can be attenuated by predictive decon‐volution. The seismic velocity field indicates progressively higher water content in the sand layers above the limestone along the profile. Georadar facies descriptions provide additional insight into the more recent geological development on the investigation site. A georadar profile recorded near the limestone fault has penetration depth and resolution high enough to yield a facies model supporting geological interpretations which include the identification of aeolian sands on top of marine beach sand deposits. Furthermore, beach foreland and buried remains of sand dunes can be identified by their structural facets as derived from the georadar interpretations. The top of the marine beach sand is identified on the seismic and the georadar profiles and may be associated with the Littorina Sea bottom in the sand layers above the limestone.
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Geophysical evidence for the hydro‐tectonic origin of the Sabkha El Sheikh Zwayed Lake and the shallow fresh water supplies, Northern Sinai, Egypt
Authors Mohamed A. Khalil and Fernando M. SantosABSTRACTThe El Sheikh Zwayed Lake is a saline lake located in northern Sinai Peninsula. The origin of the lake as well as the shallow fresh water supplies between the lake and the Mediterranean are the subject of many geological, hydrogeological, and geophysical studies. Two main hypotheses have been sug gested to explain the phenomenon of the lake (i.e. hydrogeological and/or tectonic). Using direct current resistivity (DCR) and gravity data, we investigated which of these processes explain the jux taposition and the contradiction between the location of freshwater between the sea and a saline lake to the south which is supplied by a saline aquifer. We infer from the 1D and 2D resistivity inversions that a shallow grabenal structure exists between the lake and the coast. The gravity measurements enable a subsurface NE‐SW doubly plunging anticline to be suggested (and related to Syrian arc system) to the south‐west side of the lake as well as a NE‐SW associated deep inferred fault‐line crossing the lake. The results show that the hydrogeological regime in the study area is tectonically con trolled; whereby the graben structure works as a hydrogeological barrier between the lake and the sea.
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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)