Stability Assessment and Geomorphological Evolution of Sea Natural Arches by Geophysical Measurement: The Case Study of Wied Il-Mielah Window (Gozo, Malta)

The Wied il-Mielaħ Window (Gozo–Malta) is a limestone natural arch on the north-western coast of the island of Gozo in Malta. It is located at the end of the Wied il-Mielaħ valley north of the village of Għarb. This natural arch is less well known than the Azure Window, which collapsed in March 2017 following a heavy storm, but notwithstanding, it is an imposing and important natural monument too. In the past, the Wied il-Mielah valley was responsible for discharging wastewater from the surrounding localities to the Mediterranean directly at the Wied il-Mielah Window. The sewage flag was often clearly visible underneath the archway into the open sea. The natural features of the arch provide an outstanding touristic attraction. To avoid what happened to the Azure Window, a methodology for the evaluation of the collapse hazard, combining passive seismic, ground penetrating radar (GPR), geological/geomorphological surveys and mine engineering methods, is here proposed. In this study, a methodological approach was applied, based on the following: (i) passive seismic method to study the physical–mechanical characteristics of the rock mass that constitutes the window; (ii) GPR method in order to demonstrate the conservation state (i.e., the intensity of fracturing); (iii) geological/geomorphological surveys in order to obtain a crack pattern; and (iv) scaled span empirical analysis in order to evaluate the stability of the arch. The calculation of the safety factor, with a static method, gave a value equal to 3.75 with a probability of collapse of the marine arch within 50 and 100 years.

Prediction of marginal rock mass stability in clays of Quaternary deposits including the effect of filtration reservoir by the example of the Bachat coal mine

Introduction. During opencast mining, the development of mine workings in clays of Quaternary deposits is often accompanied by stability loss. As a rule, the reason for the development of geomechanical processes 46 "Izvestiya vysshikh uchebnykh zavedenii. Gornyi zhurnal". No. 3. 2021 ISSN 0536-1028 is the mismatch between the accepted geometry and physical-mechanical characteristics of soils. In case strength characteristics and moisture content are strongly related in the conditions of lacking water saturation sources, slopes remain stable for a long time. However, in the case of water saturation, soils change to plastic or liquid consistency, and the strength properties are reduced. In engineering practice, to prevent hazardous geomechanical processes, electrophysical methods are applied that make it possible to build the boundaries of soil moisture anomalies through electrical contrast. In the instances when the water-saturated strata occurrence depth and thickness is known from drilling and vertical electrical sounding data, electrical profiling makes it possible to promptly determine the variability of these characteristics of the anomaly. The authors proposed an algorithm of water-saturated strata thickness variability determination from the point of electrical profiling. Research aim is to predict the stability of the marginal rock mass in clays of Quaternary deposits, taking into account the effect of filtration reservoirs by combining engineering-geological databases and geophysical measurement results based on the 3D geological-geophysical model. Methodology includes the analysis of the engineering-geological databases and geophysical measurement results. Results. A 3D geological-geophysical model of the marginal rock mass with the boundaries of the watersaturated stratum was built from the results of the research to predict the stability of the marginal rock mass’s actual position. Slope stability analysis established the following: at design sections О3у3–О5у5 after deformation, retaining forces exceed shearing forces significantly (safety factor) because the weight of the base wedge grows due to landslide masses; potential landslide hazard area is associated with the design section О1у1, which is due to smooth fall of the stability factor from 1.38 (section О2у2) to 1.06, close to ultimate. Conclusions. For the extension of the marginal rock physical-mechanical properties variation it is appropriate to apply the method of electrical prospecting (sounding and profiling) determining watersaturated areas by local negative anomalies of effective electrical resistivity and establishing the function of loose Quaternary deposits thickness variation by the inversion of electrical profiling graphs with the use of the least square procedure and the golden section method. If landslide hazard areas are vast, the variation of the stability factor design values, which were determined based on 3D geological-geophysical models, may possess the non-monotonic character with several extreme values and significant gradients. For that reason, it is appropriate to carry out computerized analysis with a cyclic extension of the most hazardous areas by the coordinate of the profile and the direction of the design section in a plan.

Estimation of Depth to Bouguer Anomaly Sources Using Euler Deconvolution Techniques

The geophysical measurement of variations in gravitational field of the Earth for a particular location is carried out through a gravity survey method. These variations termed anomalies can help investigate the subsurface of interest. An investigation was carried out using the airborne satellite-based (EGM08) gravity dataset to reveal the geological information inherent in a location. Qualitative analysis of the gravity dataset by filtering techniques of two-dimensional fast Fourier transform (FFT2D) shows that the area is made up of basement and sedimentary Formations. Further enhancements on the residual anomaly after separation show the sedimentary intrusion into the study area and zones of possible rock minerals of high and low density contrasts. Quantitative interpretations of the study area by 3-D Euler deconvolution depth estimation technique described the depth and locations of gravity bodies that yielded the gravity field. The result of the depth to basement approach was found to be in the depth range of 930 m to 2,686 m (for Structural Index, SI = 0). The research location is a probable area for economic mineral deposits and hydrocarbon exploration.

Using uncrewed aerial vehicles for wider coverage for geophysical observations

<p>Sodankylä Geophysical Observatory (SGO) has conducted research and provided high quality data series for numerous geophysical disciplines for more than a hundred years. As the next step in developing SGO’s measurement network, we are developing capabilities to operate uncrewed aerial vehicles (UAV’s), which can be equipped with a wide variety of geophysical measurement instruments, such as magnetometers, radars or imaging devices. During the first phase we will build a fleet of multirotor drones with variable characteristics. Any individual aircraft can be optimized for e.g. covering wide areas, covering high altitudes, or lifting heavy instrument payloads. In the next phase the coverage of the measurements will be further expanded by use of fixed-wing aircraft, helium balloons and rockets. In this presentation we will give an overview of the current status of the aircraft and supporting instrumentation. Also, future plans and objectives are discussed.</p>

The Influence of Geostatistical Prior Modeling on the Solution of DCT-Based Bayesian Inversion: A Case Study from Chicken Creek Catchment

Low frequency loop-loop electromagnetic induction (EMI) is a widely-used geophysical measurement method to rapidly measure in situ the apparent electrical conductivity (ECa) of variably-saturated soils. Here, we couple Bayesian inversion of a quasi-two-dimensional electromagnetic (EM) model with image compression via the discrete cosine transform (DCT) for subsurface electrical conductivity (EC) imaging. The subsurface EC distributions are obtained from multi-configuration EMI data measured with a CMD-Explorer sensor along two transects in the Chicken Creek catchment (Brandenburg, Germany). Dipole-dipole electrical resistivity tomography (ERT) data are used to benchmark the inferred EC fields of both transects. We are especially concerned with the impact of the DCT truncation method on the accuracy and reliability of the inversely-estimated EC images. We contrast the results of two different truncation approaches for model parametrization. The first scenario considers an arbitrary selection of the dominant DCT coefficients and their prior distributions (a commonly-used approach), while the second methodology benefits from geostatistical simulation of the EMI data pseudosection. This study demonstrates that DCT truncation based on geostatistical simulations facilitates a robust selection of the dominant DCT coefficients and their prior ranges, resulting in more accurate subsurface EC imaging from multi-configuration EMI data. Results based on geostatistical prior modeling present an excellent agreement between the EMI- and ERT-derived EC fields of the Chicken Creek catchment.

The Estimation of Hydraulic Properties from Geophysical Measurement of Subsoil Depend on Regression Equation

This paper illustrated an establishment relationship between electrical resistivity by using electrical resistivity imaging (ERI) technique and hydraulic conductivity. The test conducted in two locations (Kampung Semerbok, Rembau and Felda Bukit Rokan Utara, Gemencheh) in Malaysia. Schlumberger array configuration was adopted by using ABEM SAS 4000 with eighty-one (81) electrodes for first site and forty-one (14) electrodes for second site. The total length of resistivity survey line was 400 m and 200 m for site one and two respectively. Statistical analysis based on regression equation was involved to find the relationship between hydraulic conductivity and resistivity. This result was compared with the hydraulic conductivity obtained from pumping tests for the well which is located within the resistivity survey line range. This study showed a good relationship between resistivity and hydraulic conductivity and can be used as preliminary tool to assess subsurface zone with non- invasive non-destructive for the soil with reducing time and cost.

The hydrogeological mapping of the southwestern part of Serang Regency, Banten, Indonesia

The development of Serang City as the central government of Banten Province leads to persistently increasing demand for clean water in the area and its surroundings. This study aimed to perform the hydrogeological mapping of the southwestern part of Serang Regency. The study area covered four districts, namely Baros, Padarincang, Paburuan, and Ciomas. These four districts are the clean water source of Serang City that also functions as a buffer zone designed for the development of freshwater fishery and wetland agriculture. Hydrogeological mapping in the study area is expected to be one of the inputs in planning the use of water resources. The method used in this research included analyses of drill data, geophysical measurement results, the findings of previous studies, hydrological map scale 1:250000, and geological map scale 1:100000, as well as geomorphological mapping and field survey. This research produced six classes of hydrogeological units that covered areas with scarce groundwater productivity up to those with high groundwater productivity.

Application of geophysical methods for the subsurface treatment of the Piekary highway junction in shallow mining terrain

The study presents the application of geophysical methods in the subsurface treatment of the A-1 highway in the area of the Piekary junction in the terrain of historic zinc and lead ore mining, in the northern part of the Upper Silesian Coal Basin in Poland (USCB). The study area was under threat of discontinuous deformations, mainly sinkholes. This threat resulted from the shallow exploitation of zinc and lead ores, as well as contemporary exploitation of coal seams at greater depths. An original methodology of subsurface treatment has been developed as an adaptation to the geophysical measurement results. The methodology uses the gravimetric method at the initial and final control stages, as well as the seismic and borehole georadar methods during the detailed evaluation stage. The results of the study allowed different categories of sinkhole hazard to be determined in the area of the Piekary junction. The scope of the liquidation works was selected to match the hazard levels. Blasting of varying intensities and the borehole injection of the filling material were performed. The combination of substrate treatment methodologies and highway construction protection systems have proved to be effective.