Ivan Dostál

Determination of cavities using electrical resistivity tomography

Abstract Geophysical surveys for cavity detection are one of the most common nearsurface applications. The usage of resistivity methods is also very straightforward for the air-filled underground voids, which should have theoretically infinite resistivity in the ERT image. In the first part of the paper, we deal with the comparison of detectability of the cavity by several types of the electrode arrays, the second part discusses the effect of a thin layer around the cavity itself, by means of 2D modelling. The presence of this layer deforms the resistivity image significantly as the resistive anomaly could be turned into a conductive one, in the case when the thin layer is more conductive than the background environment. From the electrical array analysis for the model situation a dipole-dipole and combined pole-dipole shows the best results among the other involved electrical arrays.

Determination of shear surface of landslides using electrical resistivity tomography

Abstract Geophysical methods offer a broad spectrum of information by dealing with slope deformations. The electrical resistivity tomography (ERT) method is mainly applied for spatial localization of the landslide body and depicting the shear zone position. This article presents the application of the ERT method for the landslide hazardous areas by means of numerical modelling. Four different synthetic models with very small resistivity contrast (30 Ohm.m/50 Ohm.m), where each model represents a different type of slope deformation, were tested by several factors affecting the final inverse model: measurement point density, L1 and L2 norm and L-norm roughness filter components. The higher measurement points density helps mainly to detect the boundaries at greater depths. Inverse models computed using the L1 norm bring satisfactory results for compact anomalous bodies, i.e. water saturated landslide body. In the case of subtle conductive zones, i.e. shear planes, the L2 norm based inversion is recommended. For enhanced reconstruction of skewed anomalous objects, roughness filter including a diagonal component produces more accurate inverse image. The article also demonstrates the ability of the ERT method to detect and describe the shape of the slope deformation even by a relative subtle resistivity contrast

Integrated Geophysical and Geological Investiga-tions of Karst Structures in Komberek, Slovakia

A complex of geophysical methods were used to investigate a small karst area aimed at the production of detailed geological mapping, to confirm geological localization of known sinkholes, and to find possible continuations of caves and voids below the surface. The dipole electromagnetic profiling and radiometric mapping (the gamma-ray spectrometry method) were applied to determine the spatial distribution of hard carbonate rocks and weathered valley-fill sediments. Detailed high-definition magnetometry was carried out at selected sites in the studied region with the aim of distinguishing between sinkholes and man-made lime-kilns, pits where limestone was heated and transformed into lime. The microgravity and the electrical-resistivity tomography (ERT) methods were used to create high-resolution images of the underground cave. The results of ERT and the geological survey were used as an initial model for gravity modeling. Subsurface cavities of various sizes are contrasting geophysical objects, and the electrical resistivity can range from very conductive to relatively resistive depending on the composition of the filling materials. The interpretation of resistivity properties is not always straightforward. We must distinguish air-filled (high-resistivity) and loamy water-filled (low-resistivity) cavities and fractures. The combined geophysical methodology permits us to determine a more accurate near-surface geological model, in our case the parallel interpretation of a strong conductive anomaly in the ERT inversion and a predominant density decrease in the gravity modelling yield the presence of cavities at depths approximately of 50 to 60 m below the surface.

The resistivity image of the Muráň fault zone (Central Western Carpathians) obtained by electrical resistivity tomography

The resistivity image of the Muráň fault zone (Central Western Carpathians) obtained by electrical resistivity tomography The paper describes the application of geophysical prospecting techniques for estimation of the faults inclination. The field survey was carried out across the Muráň fault structure in the Slovenské rudohorie Mts (central Slovakia). Three different geophysical methods were used to map the fault zone: Electrical Resistivity Tomography (ERT), induced polarization (IP) and radon emanometry. All these methods have been used to locate the fault zone area, but the principal aims of this research are to test the efficiency of the 2D ERT technique to recognize the geometrical characterization of the fault and to improve our tectonic knowledge of the investigated area. For the synthetic cases, three geometric contexts were modelled at 60, 90 and 120 degrees and computed with the l2 norm inversion method, the l1 norm with standard horizontal and vertical roughness filter and the l1 norm with diagonal roughness filter. In the second phase this geophysical methodology was applied to fieldwork data. Our results confirm that the ERT technique is a valuable tool to image the fault zone and to characterize the general geometry, but also the importance of setting up the right inversion parameters. The main contribution of the geophysical investigations in this case was the determination of the location and confirmation of the inclination of the Muráň fault. The result of this study is the ability to make a visual estimation of the direction and dip of the fault. Pursuant to this work the dipole-dipole electrode configuration produces the best resolution, particularly for the location of vertical and dipping structures. The advantage of this array is that it shows the ability to assess the trend of the dip and therefore it can be strongly recommended. The result is also a case study of a small scale tectonic survey involving geophysical methods.

Determination of dipping contacts using electrical resistivity tomography

Determination of dipping contacts using electrical resistivity tomography Generally, all electrode arrays are able to delineate the contact of two lithostratigraphic units especially with very high resistivity contrast. However, the image resolution for the location of vertical and dipping structures is different. The responses of dipole-dipole (DD), Wenner alpha (WA), Schlumberger (SCH) and combined pole-dipole (PD) arrays have been computed using the finite difference method. Comparison of the responses indicates that: (1) The dipole-dipole array usually gives the best resolution and is the most detailed method especially for the detection of vertical structures. This array has shown the best resolution to recognize the geometrical characterisation of the fault. (2) The pole-dipole has shown the second best result in our test. The PD is an effective method for detection of vertical structures with a high depth range, but the deepest parts are deformed. (3) Wenner alpha shows a low resolution, inconvenient for detailed investigation of dip structures. (4) The Schlumberger array gives a good and sharp resolution to assess the contact between two lithological units but gives poor result for imaging geometry of dipping contact.

Complex geophysical investigation of the Kapušany landslide (Eastern Slovakia)

Abstract Geophysical survey is a very useful and popular tool used by engineering geologists to examine landslides. We present a case study from the Kapušany landslide, Eastern Slovakia, where a broad spectrum of geophysical methods were applied along two perpendicular profiles in order to compare the ability of the methods to detect as many structural features of the landslide as possible. The 2D Electrical Resistivity Tomography inverse model was capable of defining the geological structure of the landslide and defining the shear zone, however the resolution of the inverse model does not allow us to identify cracks or other minor features of the landslide. These, however, were well recorded in the results of Dipole Electromagnetic Imaging and the Self Potential method. In addition microgravimetry, Gamma-Ray Spectrometry and Soil Radon Emanometry were experimentally employed to validate the results obtained from electrical methods and afterwards final geological models, based on the integrated interpretation of all involved methods were constructed.

The resistivity image of the Upper Cretaceous Horné Belice Group: a case study from the Hranty section (Považský Inovec Mts., Western Carpathians)

Abstract The Tatricum crystalline basement in the northern Považský Inovec Mts. contains several narrow tectonic slices with different rock composition. Some of them composed of the Upper Cretaceous mass flow deposits (the Horné Belice Group) are considered unique within the framework of the Internal Western Carpathians and particularly within the Tatricum. Tectonic interpretation of their structural position is longer a matter of debate. Contrasting resistivity properties of the Hercynian mica schists and the Upper Cretaceous sandstones and shales were confirmed by the parametric geophysical measurements. The Hranty section, the structurally highest and most internal Upper Cretaceous tectonic slice was investigated by the electric resistivity tomography. Two longitudinal and two transverse resistivity profiles were measured and combined into a 3D image which suggests that the low resistivity Upper Cretaceous rocks form relatively shallow and flat lying structures folded and deformed between the crystalline basement slices.

Landfill Geophysical Imaging - Babica Landfill Case Study

One of major environmental challenges in Slovakia is to deal with numerous contaminated sites polluted by improper waste management in the past. We present a case study from Babica landfill located in western part of Slovakia near the village Bosany. The landfill was established in 1970, in a very inappropriate environment of abandoned meander. Waste composition of the Babica landfill is mainly formed by waste from the leather and footwear production. Broad spectrum of geophysical methods were applied on several profiles in order to describe the spatial limitation of landfill materials and to monitor the spread of contamination from the landfill in the surrounding environment. Combination of several geoelectrical methods (EMI, ERT, VES and SP) allows us to define the resistivity model of the area, where the contaminated area was clearly outlined by low resistivity values. Moreover VES measurements were sorted in four elementary classes defined by the shape of the VES curve, which allows us to create schematic model of the area. Self-Potential method proved the active connection of the abandoned meander with the river. Additionally, soil vapour method survey revealed three outbreaks of methane gas generation that are connected to the area near the old meander.

Landslide Geophysical Imaging - Kapušany Landslide Case Study

Geophysical survey is very useful and popular tool used by engineering geologists to examine landslides. We present a case study from the Kapusany landslide, eastern Slovakia, where a broad spectrum of geophysical methods were applied on several profiles in order to compare the ability of the methods to detect as many landslide’s structural features as possible. The 2D Electrical Resistivity Tomography inverse model was capable to define the geological structure of the landslide and to define the shear zone, however the resolution of the inverse model does not allows us to identify cracks or other minor landslide’s features. These, however, were nicely recorded in the results of Dipole Electromagnetic Profiling and Self Potential method. In addition microgravity was employed to validate the results obtained from electrical methods and afterwards a final geological model, based on the integrated interpretation of all involved methods was constructed. This model will serve as a reference model for long term landslide monitoring using modern geophysical and engineering geological approaches.

Investigation in Sinkhole Terrains Using Complex of Geophysical Methods – Case Study in the Karst Area, Slovakia

Complex of geophysical methods was used to investigate small karst area for purposes to make of detailed geological mapping survey, to confirm geological localization of known sinkholes and pits and find possible continuation of caves and voids below the surface. The electromagnetic (the Electromagnetic Conductivity method – CMD) and radiometric (the Gamma-Ray Spectrometry method) mapping was applied to determine the spatial distribution of hard carbonate rocks and weathered valley-filled sediments. Detailed high-definition magnetometry was realized on selected sites of the studied region with the aim to distinguish between real sinkholes and man-made lime-kilns (unearthed pits, where limestone was heated and transformed into lime), which have been built at the site during the history. The microgravity and the electrical resistivity tomography (ERT) methods were used to create high-resolution images of underground cave. The result of ERT and geological survey was used as an entry model for the gravity modelling. The most important results from ERT and microgravity are two interpreted low density (low resistivity) structures - these can be important from the point of view of karst structures investigation.