Application of Combined Geophysical Methods for the Examination of a Water Dam Subsoil

The paper presents the results of geophysical measurements that were carried out in the vicinity of the water dam/water reservoir supplying the city of Bielsko-Biala with drinking water. The measurements were performed in order to non-invasively detect faults, fractured zones and areas filled with breccia, which may be, at the same time, a preferential path of groundwater flow. The aforementioned factors influence the stability of the dam. The general identification of the examined media was realized by the electrical resistivity tomography method. The ERT surveys were supplemented by capacitively-coupled resistivity. The electrical methods allowed them to recognize geological settings, indicate possible fault locations, and point out the fault plane as a path of water flow. The ground penetrating radar method detected fractured and filled water areas and underground water paths in the dam’s forefield as a result of the method’s very high resolution. The high resolution seismic reflection method provided a clear and high resolution image of the relatively deep geological structure and verified a location and the run of the faults. In general, the complex geophysical-geological interpretation enabled classification of the unconsolidated/fractured zones associated with faults as a place where the erosion process is the most intense and can bring danger on the dam. Finally, it was confirmed that the area should be the subject of geophysical monitoring.

Crustal structure and upper mantle anisotropy of the Afar triple junction

The Afar region is a tectonically distinct area useful for studying continental break-up and rifting. Various conflicting models have been suggested to explain the lateral variations of the anisotropy in this region. To address this issue, we investigated the tectonics of the Afar region using receiver function and shear-wave splitting measurements based on broadband seismic data from 227 stations in the region. Further, the receiver function results were inverted to obtain the crustal thickness and Vp/Vs ratio of the region. Our results reveal a thick African crust (thicker than 40 km) with typical Vp/Vs values for the continental crust, elongated down to 21 km along the rift system with very high Vp/Vs values near the fractured zones, suggesting crustal thinning near the fractured zones. Our shear-wave splitting measurements indicate a general fast axis orientation of N030E. However, substantial disparities in the fast anisotropy direction exist in the triple junction region, with some stations displaying a direction of N120E, which is perpendicular to the fast directions measured at the surrounding stations. In addition, many stations located close to the rifts and within the Arabian Plate provide mostly null measurements, indicating the presence of fluids or isotropic media. This study uses several methodologies to unravel the structure and evolution of the Afar region, providing valuable insight into the Afar, a tectonically distinct region, which will be useful for elucidating the mechanisms and characteristics of a continental break-up and the rifting process.

Data mining based investigation of the impact of imbalanced dataset over fractured zone detection

Several studies have been conducted in recent years to discriminate between fractured (FZs) and non-fractured zones (NFZs) in oil wells. These studies have applied data mining techniques to petrophysical logs (PLs) with generally valuable results; however, identifying fractured and non-fractured zones is difficult because imbalanced data is not treated as balanced data during analysis. We studied the importance of using balanced data to detect fractured zones using PLs. We used Random-Forest and Support Vector Machine classifiers on eight oil wells drilled into a fractured carbonite reservoir to study PLs with imbalanced and balanced datasets, then validated our results with image logs. A significant difference between accuracy and precision indicates imbalanced data with fractured zones categorized as the minor class. The results indicated that the accuracy of imbalanced and balanced datasets is similar, but precision is significantly improved by balancing, regardless of how low or high the calculated indices might be.