René Putiška

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.

Case history: integrated geophysical survey at Katarínka Monastery (Slovakia)

Katarinka (St. Catherine) is the ruin of an abandoned Franciscan monastery from the early 17th century located in the western Small Carpathians in Slovakia. Historical sources and paintings suggest that, beside the remains of the monastery that are still visible, a circle of eight chapels, a pilgrim’s hospice, a cemetery, and garden terraces originally surrounded the main building of the monastery. From 2009 to 2012, geophysical campaigns were performed to find evidences and positions of remains of these buildings of the monastery campus. An initial magnetic overview survey revealed multiple local accumulations of disordered dipole anomalies. Since these accumulations did not allow a structural interpretation, ground penetrating radar measurements were conducted. The ground penetrating radar results clearly showed wall structures beneath almost all magnetic anomaly accumulations. In between the remains of the monastery main building, ground penetrating radar and electrical resistivity tomography were performed at different areas that were difficult to access because of a strong cover of vegetation and steep topography.

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.

Umbrisols at Lower Altitudes, Case Study from Borská lowland (Slovakia)

Abstract Umbrisols generally develop in a cool and humid climate. Therefore, occurrence of these soils in the Borská lowland of southwestern Slovakia is very uncommon, and this inspired the aim of this paper: Analysis of the natural conditions suitable for Umbrisol development. Umbrisols in the Borská lowland developed from aeolian quartz sands accumulated on Neogenne marine clay sediments. Their occurrence is connected with the groundwater table relatively close to the ground surface and this particularly determines Umbrisol genesis in this area. Sufficient input via organic matter is an important factor for formation of the umbric horizon, and only the rich herbaceous undergrowth of the prevailing planted pine and mixed pine-oak forests is capable of providing it. A growth of diep-rooted grass is closely connected with higher soil moisture content, and quite moist areas occur in the deeper inter-dunes depressions. Constant soil moisture in these sites is facilitated by water capillary elevation. While Umbrisols are transformed to Arenosols at increased altitude, they can be transformed to Gleysols in deep depressions. Herein, induced polarization provided suitable geophysical method for detection of arenic Umbrisol inclusions. Sharp transformation of the humus layer to dry non-polarized aeolian quartz sands enabled the surface horizon to be distinguished by induced polarization

Response of channel dynamics to recent meander neck cut-off in a lowland meandering river with artificial training history: the Morava River, Czech Republic

ABSTRACT One-decade-long observations of post-cut-off channel development and oxbow lake formation are presented to offer insight into the morphological response of the Morava River to neck cut-off. The main objectives were: calculation of the amount of sediment released to the channel by collapse of the meander neck; comparison of floodplain erosion and deposition rates before and after cut-off; and description of the rate and pattern of oxbow lake sedimentation. The amount of sediment released due to cut-off (~35 000 m3) equalled 2–5 years of sediment input by lateral erosion averaged for the past six decades. Bank erosion rates substantially exceeded long-term averages (0.63–6.21 m year−1, post cut-off). Despite a low entrance angle, alluvial plugs in a newly formed oxbow lake were established within a few months. Between 2006 and 2016, sedimentation of the alluvial plugs proceeded at an average rate of 0.55 m year−1, reflecting the time since cut-off and the frequency of overbank flows.

Applications of Shallow Seismic Refraction Measurements in the Western Carpathians (Slovakia): Case Studies

Abstract Shallow seismic measurement, specifically seismic refraction tomography, is an effective geophysical method that has applications in various sectors. It enables the search for and determination of the course of the interfaces, thus helping to resolve geological, environmental, hydrogeological, engineering, geotechnical and other problems. The paper demonstrates the possibilities of using these methods through examples of shallow seismic measurements that have been performed at various four locations in the Western Carpathian Mountains. The first case study describes Monastery Pond at Katarínka. It was found that, the basement of the Monastery pond is at a depth of 2-3 m below the surface and the results were also confirmed by electrical resistivity tomography (ERT). The next measurement through the thermal power station waste storage showed that the storage area base runs at a depth of about 20 m under the measured profile. The third case study addresses the depth of groundwater depth in the area of Borská nízina. The measurement confirmed the assumed depth of ground water level at 3.35 m below the surface. In the last case study, border fault between the Turiec Basin and the Malá Fatra Mts. was mapped by application of shallow refraction methods. The results show that shallow seismic methods shed light on the problem and in combination with other geophysical methods are an effective tool with great potential. They provide very useful data for shallow mapping applications.