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- Volume 14, Issue 5, 2016
Near Surface Geophysics - Volume 14, Issue 5, 2016
Volume 14, Issue 5, 2016
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The downward continuation of the tilt angle
More LessABSTRACTThe tilt angle filter can be used both to enhance images of aeromagnetic data and as the basis of semi‐automatic interpretation methods. For vertically magnetised and vertically dipping contacts, rescaling by factor is equivalent to computing from sources with all depths decreased by a factor . This downward continuation is much less sensitive to noise than Fourier transform‐based downward continuation techniques, and because it reduces interference, it allows more accurate source depth determination from methods such as the tilt‐depth method. The method can be applied irrespective of the source and magnetisation type if the tilt angle of the analytic signal amplitude is used.
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Cosmic muon imaging: a challenging application to investigate deeper volcanic structures in Canary Islands, Spain
ABSTRACTMuon radiography of volcanoes has become a promising technique to investigate density contrasts in active volcanoes with a spatial resolution unreachable for conventional geophysical techniques. Experiments performed at volcanoes such as Asama, Showa‐Shinzan and Satsuma‐Iwojima, all in Japan, have provided new insights for volcanologists and scientists in general. In this paper, we discuss the very challenging application of muon radiography to investigate deeper volcanic structures such as Teide volcano in Tenerife and Cumbre Vieja in La Palma, Canary Islands, due to the strong social interest emerging from the potential danger which they represent. Applications to other volcanoes can be envisaged. The emulsion films exposed at Canary Islands, as described in this paper, are under analysis and results will soon be available.
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Strictly horizontal lateral parameter correlation for 1D inverse modelling of large datasets
More LessABSTRACTThis paper presents new developments to the lateral parameter correlation method, a method that can be used to invoke lateral smoothness in model sections of one‐dimensional inversion models. The lateral parameter correlation method has three steps. First, all datasets are inverted individually. Next, a laterally smooth version of each model parameter is found by solving a simple constrained inversion problem by postulating identity between the uncorrelated and correlated parameters and solving the equations including a model covariance matrix that ensures lateral smoothness. As a final step, all sounding data are reinverted with the correlated model parameter values as a priori values to produce models that better fit the data. Because the method separates inversion and correlation, it is much faster than methods where the inversion and correlation are solved simultaneously. The new development to the lateral parameter correlation method presented in this paper is an option to perform a strictly horizontal correlation, thereby avoiding the model artefacts sometimes seen when correlating along layers, namely that formations tend to follow the topography. Furthermore, a solution to the intractable computation times arising with large datasets is formulated, employing a tessellation of the plane and an averaging scheme within the subareas that reduces the size of the numerical lateral parameter correlation inversion problem while maintaining correct correlation within a very large area. In a field example, correlation along layers and the new strictly horizontal correlation are compared, and it is demonstrated that the horizontal correlation removes the above mentioned artefacts that can appear when correlating along layers. The lateral parameter correlation method is very flexible and is capable of correlating models from inversion of different data types, including information from boreholes, and it lends itself easily to “embarrassingly parallel” computation.
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A comparison between 2D azimuthal and 3D resistivity imaging techniques in determining the subsurface fracture zones within Abu‐Jir Fault Zone, Southwest Karbala, Central Iraq
Authors Ahmed Srdah Al‐Zubedi and Jassim M. ThabitABSTRACTThis experimental study presents a comparison between two‐dimensional azimuthal and three‐dimensional electrical resistivity imaging techniques at a given location within the Abu‐Jir Fault Zone, Central Iraq. The aim is to examine which one is a better technique in one order to study the subsurface fracture zones and to use it within and outside the fault zone. Our results show that the two‐dimensional azimuthal imaging technique is quite successful in imaging the structural geology, particularly for effective identification and delineation of the subsurface fracture zones in all directions, whereas the three‐dimensional imaging technique, carried out using data acquired in parallel two‐dimensional imaging lines, gives limited and distorted images about these zones because the Y‐spacing between the survey lines is quite small compared to the length of these lines. The two‐dimensional azimuthal technique can be useful in deep investigations to determine the extent of the structures, particularly in areas with unknown geological and structural settings.
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1D sequential inversion of portable multi‐configuration electromagnetic induction data
Authors Julien Guillemoteau, François‐Xavier Simon, Erika Lück and Jens TronickeABSTRACTWe present an algorithm that performs sequentially one‐dimensional inversion of subsurface magnetic permeability and electrical conductivity by using multi‐configuration electromagnetic induction sensor data. The presented method is based on the conversion of the in‐phase and out‐of‐phase data into effective magnetic permeability and electrical conductivity of the equivalent homogeneous half‐space. In the case of small‐offset systems, such as portable electromagnetic induction sensors, for which in‐phase and out‐of‐phase data are moderately coupled, the effective half‐space magnetic permeability and electrical conductivity can be inverted sequentially within an iterative scheme. We test and evaluate the proposed inversion strategy using synthetic and field examples. First, we apply it to synthetic data for some highly magnetic environments. Then, the method is tested on real field data acquired in a basaltic environment to image a formation of archaeological interest. These examples demonstrate that a joint interpretation of in‐phase and out‐of‐phase data leads to a better characterisation of the subsurface in magnetic environments such as volcanic areas.
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Estimation of sediment texture from spectral induced polarisation data using cluster and principal component analysis
Authors Silvia Inzoli, Mauro Giudici and Johan Alexander HuismanABSTRACTSpectral induced polarisation data are usually interpreted with simple models in order to derive petrophysical relationships between electrical and sedimentological properties, such as texture, clay content, and permeability. The aim of this work is to explore the value of spectral induced polarisation in addition to conventional direct‐current resistivity measurements for determining textural properties of saturated samples collected from alluvial deposits. For this, an advanced data processing approach that combines cluster and principal component analysis was developed and applied to integral parameters derived from Debye decomposition of spectral induced polarisation data. This data processing procedure allowed identifying groups of samples with a similar spectral induced polarisation response and to derive a characteristic grain‐size distribution for each group of samples. The method to estimate the grain‐size distribution from spectral induced polarisation data was successfully validated using independent sediment samples. The remaining uncertainty in the estimation of sediment texture from spectral induced polarisation data was attributed to the effect of pore size distribution and mineralogy, which were not considered in the present work but can be added in the future within the same conceptual workflow.
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Characterisation of shallow marine sediments using high‐resolution velocity analysis and genetic‐algorithm‐driven 1D elastic full‐waveform inversion
Authors M. Aleardi, A. Tognarelli and A. MazzottiABSTRACTWe estimate the elastic properties of marine sediments beneath the seabed by means of high‐resolution velocity analysis and one‐dimensional elastic full‐waveform inversion performed on two‐dimensional broad‐band seismic data of a well‐site survey. A high‐resolution velocity function is employed to exploit the broad frequency band of the data and to derive the P‐wave velocity field with a high degree of accuracy. To derive a complete elastic characterisation in terms of P‐wave and S‐wave velocities and density of the subsurface, and to increase the resolution of the estimates, we apply a one‐dimensional elastic full‐waveform inversion in which the outcomes derived from the velocity analysis are used as a priori information to define the search range. The one‐dimensional inversion is done using genetic algorithm as the optimisation method. It is performed by considering two misfit functions: the first uses the entire waveform to compute the misfit between modelled and observed seismograms, and the second considers the envelope of the seismograms, thus relaxing the requirement of an exact estimation of the wavelet phase. The full‐waveform inversion and the high‐resolution velocity analysis yield comparable profiles, but the full‐waveform inversion reconstruction is much more detailed. Regarding the full‐waveform inversion results, the final depth models of P‐ and S‐wave velocities and density show a fine‐layered structure with a significant increase in velocities and density at shallow depth, which may indicate the presence of a consolidated layer. The very similar velocities and density‐depth trends obtained by employing the two different misfit functions increase our confidence in the reliability of the predicted subsurface models.
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Application of 2D elastic Rayleigh waveform inversion to ultrasonic laboratory and field data
More LessABSTRACTIn addition to geophysical applications from the near surface to a global scale, seismic full‐waveform inversion can be applied to ultrasonic data on the centimetre and decimetre scales for nondestructive testing of pavements, facades, plaster, sculptures, and load‐bearing structures such as pillars or core samples from boreholes, which can consist of geo‐materials and non‐geomaterials. Classical non‐destructive testing approaches are based on the inversion of body‐wave travel times to deduce P‐wave velocity models. In contrast, surface waves (especially Rayleigh waves) are well suited to quantify surficial alterations of material properties, e.g., due to weathering. Furthermore, ultrasonic measurements of test samples with known material parameters close the gap between synthetic tests or analytical solutions and field data applications to estimate the accuracy of seismic modelling and inversion codes. Due to the scale invariance of the problem, the full‐waveform‐inversion approaches developed on the ultrasonic scale are also applicable to larger scale geophysical problems. In this paper, we demonstrate the potential of two‐dimensional Rayleigh waveform inversion on the ultrasonic scale using two data examples acquired with a single‐fold, low‐coverage acquisition geometry. For a simple homogeneous Plexiglas block in a controlled laboratory environment, we discuss the accuracy of visco‐elastic Rayleigh wave modelling, as well as the sensitivity of two‐dimensional elastic full‐waveform inversion with respect to small data residuals. While the inversion is restricted to the recovery of the S‐wave velocity model, a passive visco‐elastic modelling approach with a homogeneous average ‐model is required in order to describe amplitude loss and dispersion of the Rayleigh wave. The applicability of this approach under field conditions is illustrated for an ultrasonic data example from the weathered sandstone facade of the Porta Nigra in Trier (Germany). In addition to a random medium resolution analysis of the full‐waveform‐inversion result, we particularly emphasise the importance of lateral model smoothing to mitigate small‐scale inversion artefacts and to avoid erroneous interpretations. The estimated two‐dimensional S‐wave velocity anomalies correlate well with prominent surficial weathering effects in the upper about 3 cm.
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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)