- Home
- A-Z Publications
- Near Surface Geophysics
- Previous Issues
- Volume 15, Issue 5, 2017
Near Surface Geophysics - Volume 15, Issue 5, 2017
Volume 15, Issue 5, 2017
-
-
Interpolating GPR data using anti‐alias singular spectrum analysis (SSA) method
Authors Saulo S. Martins, Jandyr M. Travassos and Mauricio D. SacchiABSTRACTGround Penetrating Radar data are often acquired along profiles employing bistatic equipment with a fixed distance between the transmitter (Tx) and receiver (Rx) antennae. Even in cases where more than two antennae are used, the number of channels tends to be relatively small, resulting in either a limited number of offsets or gathers with inadequate far offsets. Estimating stacking velocity and performing migration from this type of datasets are difficult. In this paper, we present techniques to interpolate both aliased and non‐aliased datasets in the offset domain and the common‐midpoint domain. The latter permits us to increase the fold of the survey and consequently improve the process of velocity analysis and migration. We assess the reconstruction efficiency of the interpolator using both synthetic and real data to different degrees of decimating. In both cases, the unaliased version of both datasets provides an accurate solution for a careful comparative analysis. At the end of this work, we make a further comparison between the resulting migrated and stacked sections for both the original and reconstructed datasets in order to highlight the efficiency of the interpolation algorithms.
-
-
-
Characterization of the sedimentary fabrics in ornamental rocks by using GPR
Authors Javier Rey, Julián Martínez, Violeta Montiel, Francisco Cañadas and Nicolás RuizABSTRACTIn this study, the use of ground‐penetrating radar for characterising ornamental sedimentary rocks was tested. Specifically, the ability of this non‐invasive geophysical prospecting method to identify fabrics and textures in rocks was investigated. Blocks mined from quarries were analysed, and ornamental rocks with the same lithologies as other widely utilised ornamental rocks with a variety of sedimentary fabrics and textures were selected. Rocks with clastic brechoid, cross‐laminated sparitic, massive or layered micritic, and laminar bindstone textures were analysed. Antennas that provided the maximum detail and a sufficient depth of penetration were used. The low electrical conductivity of carbonates permitted the use of high‐frequency antennas (800 MHz and 1.6 GHz), which were useful in studying the entire thickness of a boulder (up to 2 m).
The cross‐laminations in the oolitic limestones, the laminar bindstones of travertines, the differentiation between massive and brecciated fabrics, and the massive and slaty fabrics of the mic‐ritic limestones were examined using these two frequencies. In micritic textures without discontinuities (neither sedimentary nor diagenetic), the radargrams could detect facies with few reflections (Crema Marfil). In addition to analysing fabrics and textures, the ground‐penetrating radar measurements could identify anisotropy in these rocks, which makes ground‐penetrating radar an effective tool for evaluating the mechanical state of a boulder prior to its cutting.
-
-
-
Airborne mapping of sensitive clay—stretching the limits of AEM resolution and accuracy
More LessABSTRACTDue to postglacial uplift, lowlands in Canada, Norway, Sweden and Russia are prone to formation of highly unstable, sensitive, and leached marine clay (quick clay). Quick‐clay failures are dramatic due to its high water content, resulting in liquefaction. It thus poses a major hazard for society and construction projects in particular, and knowledge of its extent is of vital importance. Quick‐clay assessment is usually undertaken in geotechnical boreholes having the disadvantage of giving only information at the borehole location. To overcome this limitation, geophysical ground‐based methods like electrical resistivity tomography have been used successfully. However, when a larger area has to be investigated, electrical resistivity tomography surveys become costly and time consuming. We show results from an airborne electromagnetic survey aiming at detection of different clay units for a road project in southeastern Norway. Airborne electromagnetic data clearly show structures within the sediment layer that correspond well with results from geotechnical boreholes. While a clear distinction between clay and quick clay cannot be derived from airborne electromagnetic alone, our study shows that this method has high‐enough resolution and accuracy to map differences in clay units, which can subsequently be probed at specified locations. Thus, by using airborne electromagnetics to target borehole locations, the costs for the geotechnical drilling program can be reduced significantly.
-
-
-
Self‐potential for monitoring soil remediation by smouldering: a proof of concept
Authors Mehrnoosh Ebrahimzadeh, Panagiotis Tsourlos and Jason I. GerhardABSTRACTNear‐surface soils contaminated with non‐aqueous phase liquids, such as coal tar, crude oil, and chlorinated solvents, remain a serious problem. Smouldering remediation is a technique now being applied in the field for in situ destruction of non‐aqueous phase liquids. Based on a self‐sustaining exothermic reaction, smouldering remediation generates a hot region (>400 °C) that propagates through the subsurface. Self‐potential is here considered for the first time as a non‐destructive means for monitoring the smouldering remediation process. First, a series of sandbox experiments were conducted to investigate the magnitude of the thermoelectric coupling coefficient () for different sand sizes, water contents, and heat sources. Measured values ranged from ‐0.47 mV/°C for coarse, water‐saturated sand to ‐0.05 mV/°C for fine sand with a saturation of 30%. Next, self‐potential measurements were conducted during several laboratory smouldering remediation experiments, examining the response as a function of both space and time. A significant self‐potential anomaly was observed on the surface during the smouldering period. Moreover, the magnitude of the self‐potential anomaly was demonstrated to be highly correlated to the separation distance between the (moving) reaction front and the (stationary) self‐potential electrode positions. Overall, this research suggests that the self‐potential method has a significant promise as a non‐invasive monitoring tool for in situ smouldering remediation of contaminated sites.
-
-
-
Forward and inverse modelling of electrical properties of some sandstone rocks using renormalisation group method
Authors Mohamed M. Gomaa and Mohamed A. KassabABSTRACTElectrical properties of some sandstone rocks (conductivity and dielectric constant) were investigated as a function of frequency (100 Hz to 100 kHz). Such heterogeneous specimens contain semiconducting constituents (silt and clay) and insulating constituents (sand, air, and carbonates). A theoretical pseudo‐random renormalisation group method was used to predict the electrical properties of such random mixture. Four phases were used in the model to consider the detailed texture of the constituent materials. The measured electrical properties (conductivity and dielectric constant) versus frequency of these samples were analysed to predict the texture or connections of these four phases. From the analysis, these four phases were suggested to be (1) carbonates, (2) sand coated with clay and coated with air, (3) carbonates coated with clay and coated with air, and (4) pure sand. The forward problem predicts the electrical properties versus frequency using the pseudo‐random renormalisation group method for certain concentrations of the constituent materials distributed among the random four phases. The forward model is used to fit the measured response to obtain the expected concentrations of rock constituents in the proposed four phases using a parameter search method (inverse problem). The obtained accordance between the experimental and modelled characteristics in much of the frequency band used suggests that the pseudo‐random renormalisation group method with the supposed four phases reasonably represents the main mineralogical composition of the rock.
-
-
-
Grey wolf optimisation for inversion of layered earth geophysical datasets
Authors Akash Chandra, Aayush Agarwal, Shalivahan and Roshan K. SinghABSTRACTGeophysical observables using optimisation algorithms are interpreted in terms of physical properties defining the Earth. The field of optimisation is a dynamic field as no single algorithm can solve all optimisation problems. We implement the grey wolf algorithm, an apex predator‐based method, in optimizing geophysical datasets over a layered earth. The grey wolf optimiser is a swarm‐based meta‐heuristic algorithm and has two extremely interesting social practices, viz., social leadership hierarchy and hunting behaviour. The leadership hierarchy is simulated by employing different types of grey wolves where hunting strategies are implemented as optimisation methods. Global minimum from the grey wolf optimiser has been obtained with a pack of 7 wolves and 1500 iterations. To evaluate the efficacy of the grey wolf optimiser, we performed inversion of noise‐contaminated vertical electrical sounding (apparent resistivity), induced polarisation sounding (apparent chargeability), and magneto‐telluric apparent resistivity data. Subsequently, we implemented grey wolf optimisation on field apparent resistivity, apparent chargeability, and magnetotelluric apparent resistivity data adapted from published literature. Both noise‐contaminated synthetic and field data have been compared with popular population‐based algorithms, i.e., particle swarm optimisation and ant colony optimisation and with a local optimisation algorithm—ridge regression. The sensitivity analysis was performed by inverting the noise‐contaminated datasets using the grey wolf optimiser, particle swarm optimisation and ant colony optimisation with six different search space. It is observed that the grey wolf optimiser is least sensitive to the varied search space. The results obtained from the grey wolf optimiser as compared with other techniques are relatively more stable and the obtained normalised RMS deviation is either less or equal with ridge regression, particle swarm optimisation, or ant colony optimisation. This is due to the fact that the grey wolf optimiser does not converge prematurely and avoids getting trapped in a local minimum, as a balance between exploration and exploitation is maintained. We also compared the grey wolf optimiser using L1‐norm and L2‐norm as misfit functions for field data examples. The grey wolf optimiser using L1‐norm resulted in a more stable solution. The execution time of the grey wolf optimiser is least as compared with other population optimisation techniques. Grey wolf optimisation could be applied for routine interpretation of geophysical datasets.
-
-
-
Unique insight into the seasonal variability of geophysical properties of field soils: practical implications for near‐surface investigations
Authors Daniel Boddice, Nicole Metje and David ChapmanABSTRACTElectromagnetic wave propagation methods are extensively used in geophysical prospecting, such as in archaeological and utility surveys. The signal penetration and attenuation of electromagnetic waves depend strongly on the apparent permittivity and electrical conductivity of the soil, which vary on a seasonal basis, affecting the detection of buried features, especially their detected depth. Nevertheless, there is a significant lack of high‐quality long‐term seasonal field monitoring data of electromagnetic properties in different soil conditions to aid the understanding of how these properties vary in field conditions. The results reported in this paper will contribute to addressing this scarcity of data. Long‐term data are presented and analysed from bespoke time‐domain reflectometry monitoring stations designed to enable collection of apparent relative dielectric permittivity, bulk electrical conductivity, and temperature data at a high temporal resolution (hourly) from three remote sites with different soils over an extended period of time (16–23 months). In addition to providing an extensive dataset, the data highlight the importance of using accurate electromagnetic soil data for geophysical prospecting. The greatest changes in geophysical properties for all sites are detected in the near‐surface soils (< 0.5 m), where many buried utilities are generally found, with rapid wetting events and slower drying events greatly affecting both the apparent relative dielectric permittivity and the bulk electrical conductivity. However, the most critical factor for determining these properties is the soil water holding capacity, which, in turn, is a function of the clay mineralogy and content. An analysis of the ratio of energy loss to energy storage shows that the optimum time for ground penetrating radar surveying is during the dry periods and when the soil temperature is low, displaying the significance of soil temperature on survey outcomes, due to its significant effect on the bulk electrical conductivity. The results from this paper will aid survey planning, thereby ensuring a better underground target detection rate.
-
-
-
Magnetic imaging of the Kurungnakh Island ice complex upper layer structure, Lena Delta, Russia
Authors Leonid Tsibizov and Olga RusalimovaABSTRACTGround‐based detailed magnetic surveys have so far been minimally used in permafrost research but they have a great potential to explore near‐surface structures and to reveal their hidden features. This study presents the results of a two‐elevation magnetic survey on a Pleistocene ice complex covered by Holocene deposits and penetrated by ice wedges. A polygonal pattern, which is not distinctly visible on the surface, is successfully identified in the anomalous magnetic field. Obtained data allow to allocate confidently the polygon distribution and to discriminate the ice wedges of different thicknesses. Typical values of total magnetic field anomalies related to ice wedges and frozen sediments in between are about several nanoteslas measured at the elevations of 0.4 and 1.15 m above the surface. Magnetic susceptibility of the permafrost samples is on the order of 10‐4. Observed data were modelled using a numerical magnetic model to estimate quantitatively geometrical and magnetic parameters of the ice complex structure. The best‐fitting model provides the following results: the upper part of the Pleistocene and Holocene ice wedges shows a thickness of 8 and 3 m, respectively, and the upper boundary of the Pleistocene ice wedges where they contact with Holocene wedges rests at 1 m depth. The magnetic susceptibility of frozen sediments between ice wedges is about 0.14 × 10‐3. An anomalously high vertical gradient within the study area has been observed. The presumable sources for this anomaly were considered. Our results suggest that detailed magnetometry could be efficient as it is a rapid and non‐invasive reconnaissance permafrost research.
-
-
-
Comparison of stabiliser functions for surface NMR inversions
Authors Denys Grombacher, Gianluca Fiandaca, Ahmad A. Behroozmand and Esben AukenABSTRACTSurface nuclear magnetic resonance is a geophysical technique providing non‐invasive aquifer characterization. Two approaches are commonly used to invert surface nuclear magnetic resonance data: (1) inversions involving many depth layers of fixed thickness and (2) few‐layer inversions without predetermined layer thicknesses. The advantage of the many‐layer approach is that it requires little a priori knowledge. However, the many‐layer inversion is extremely ill‐posed and regularisation must be used to produce a reliable result. For optimal performance, the selected regularisation scheme must reflect all available a priori information. The standard regularisation scheme for many‐layer surface nuclear magnetic resonance inversions employs an L2 smoothness stabiliser, which results in subsurface models with smoothly varying parameters. Such a stabiliser struggles to reproduce sharp contrasts in subsurface properties, like those present in a layered subsurface (a common near‐surface hydrogeological environment). To investigate if alternative stabilisers can be used to improve the performance of the many‐layer inversion in layered environments, the performance of the standard smoothness stabiliser is compared against two alternative stabilisers: (1) a stabiliser employing the L1‐norm and (2) a minimum gradient support stabiliser. Synthetic results are presented to compare the performance of the many‐layer inversion for different stabiliser functions. The minimum gradient support stabiliser is observed to improve the performance of the many‐layer inversion for a layered subsurface, being able to reproduce both smooth and sharp vertical variations of the model parameters. Implementation of the alternative stabilisers into existing surface nuclear magnetic resonance inversion software is straightforward and requires little modification to existing codes.
-
Volumes & issues
-
Volume 22 (2024)
-
Volume 21 (2023)
-
Volume 20 (2022)
-
Volume 19 (2021)
-
Volume 18 (2020)
-
Volume 17 (2019)
-
Volume 16 (2018)
-
Volume 15 (2017)
-
Volume 14 (2015 - 2016)
-
Volume 13 (2015)
-
Volume 12 (2013 - 2014)
-
Volume 11 (2013)
-
Volume 10 (2012)
-
Volume 9 (2011)
-
Volume 8 (2010)
-
Volume 7 (2009)
-
Volume 6 (2008)
-
Volume 5 (2007)
-
Volume 4 (2006)
-
Volume 3 (2005)
-
Volume 2 (2004)
-
Volume 1 (2003)