Reliable characterization of the karst system is essential for risk assessment where many associated hazards (e.g., cover-collapse dolines and groundwater pollution) can affect natural and built environments, threatening public safety. The use of multiple geophysical approaches may offer an improved way to investigate such cover-collapse sinkholes and aid in geohazard risk assessments. In this paper, covered karst, which has two types of shallow caves (vadose and fluvial) located in Tarimba (Goias, Brazil), was investigated using various geophysical methods to evaluate their efficiency in the delineation of the geometry of sediments filled sinkhole. The methods used for the investigation were Electrical Resistivity Tomography (ERT), Seismic Refraction Survey (SRS), Seismic Refraction Tomography (SRT) and the Very Low Frequency Electromagnetic (VLF-EM) method. The study developed several (2D) sections of the measured physical properties, including P-wave velocity and electrical resistivity, as well as the induced current (because of local bodies). For the analysis and processing of the data obtained from these methods, the following approaches were adopted: ERT inversion using a least-square scheme, Karous-Hjelt filter for VLF-EM data and time-distance curves and Vp cross-sections for the SRS. The refraction data analysis showed three-layered stratigraphy topsoil, claystone and carbonate bedrock, respectively. The findings obtained from ERT (three-layered stratigraphy and sediment-filled doline), as well as VLF-EM (fractured or filled caves as a positive anomaly), were found to be consistent with the actual field conditions. However, the SRS and SRT methods did not show the collapsed material and reached the limited the depth because of shorter profile lengths. The study provides a reasonable basis for the development of an integrated geophysical approach for site characterization of karst systems, particularly the perched tank and collapse doline.

Detection of Cover Collapse Doline and Other Epikarst Features by Multiple Geophysical Techniques, Case Study of Tarimba Cave, Brazil

https://www.tandfonline.com/doi/pdf/10.1080/20909977.2020.1734999

Geotechnical assessment of fractured limestone bedrock using DC resistivity method: a case study at New Minia City, Egypt

Geotechnical hazards are the main factors that control decisions about construction in structurally complex zones. In the Mokattam area, Cairo, Egypt, different techniques, such as geotechnical techniques, remote sensing, and geophysical tools, are used and integrated to study the subsurface lithology, caves and structures to evaluate the possible geotechnical problems in these zones for expected future development in this area. Remote sensing applications indicate that the main trends of surface lineaments are E-W to NW–SE, whereas the minor trends are N-S to NE-SW. Ten vertical electrical sounding stations (VESs) were acquired and interpreted to delineate the subsurface stratigraphy. The interpretation results of these data show that the subsurface stratigraphy consists of 6 geoelectric units arranged from the surface to the bottom: fractured limestone, clay, marly limestone, dolomitic limestone, marl and marly limestone, and limestone. Nine electrical resistivity tomography (ERT) profiles, twelve shallow seismic refraction (SSR) profiles, and ten ground-penetrating radar (GPR) profiles were acquired to study the shallow subsurface stratigraphy. The integration results of ERT and SSR measurements indicate that the shallow subsurface stratigraphy is composed of different rock units. The first layer is fractured limestone, which represents the surface layer of the upper plateau and contains caves in some areas. Then, layers of clay and marl, which are important geotechnical hazard sources, are present. Finally, a layer of limestone appears in the bottoms of some sections, and this limestone layer also contains caves. The GPR profiles have the highest resolution and can discriminate the top horizon into three to four detailed stratigraphic units, each of which has different amplitude values and caves and fractures in some profiles.

Integration of geophysical techniques to detect geotechnical hazards: a case study in Mokattam, Cairo, Egypt

Application scenarios and experimental methods in the field of modern electrical exploration are becoming increasingly complicated, expanding requirements on instrumentations. Currently, the commonly used resistivity method instrument systems are mostly single-channel transmitting and receiving, which lowers the measurement efficiency. The multichannel array resistivity method instrument system developed in this article adopted an improved automatic measurement method to realize array synchronous transmitting and receiving, which effectively improve the measurement efficiency. The instrument mainly consists of two parts: a transmitter and a receiver subsystem. The transmitter subsystem realizes array transmitting through centralized electrode units or serial electrode management nodes. The former can support up to 256 electrodes with a maximum transmit power of 600 W; the latter can support up to 65 535 electrodes with a maximum transmit power of 5 kW. The receiver subsystem consists of multiple eight-channel receivers. The root-mean-square (rms) noise of the receivers is lower than $9.38 ~\mu \text{V}$ , the dynamic range is better than 120 dB, and the isolation between channels is better than 119 dB. Combined with a built-in 4G and Wi-Fi module, it can achieve real-time monitoring of data quality and cloud storage functions. The results of the water tank comparison experiment between the developed instrument and GRx8-32 showed well consistency. By applying the developed system to the monitoring of the perfusion process of structured cementitious materials, the relative changes in resistivity of the perfusion process can be visually monitored, which lays a foundation for the application of the instrument in 2-D and 3-D detection and time-shift monitoring in other fields.

Development of a Multi-Channel Array Resistivity Method Instrument System

The aim of this paper is to determine the capability of the ultrasonic technique to predicate and evaluate some elastic and geotechnical properties within sand stone layers. For this purpose, 15 rock samples were collected from Tanjero Formation that is exposed in Dokan area, northeastern Iraq. Elastic wave velocities (Vp and Vs)  and density (P) values for rock samples were determined by applying laboratory testing. From Vp, Vs, and density values, the porosity, elastic modulus, and some geotechnical parameters were calculated. The relationship between P wave velocity and elastic properties of samples was derived. Empirical equations were obtained by applying the regression analyses between Vp and the measured elastic properties. Good correlations were found, which may supply a good estimation of elastic properties from velocity information related to the geological setting of the area. This work confirmed the capability of the ultrasonic technique for predicating the elastic properties of rocks.

Geophysical and Geotechnical Evaluation of Tanjero Sandstone Layers at Dokan Area Using Ultrasonic Wave Method

Utilizing shallow seismic refraction in defining the geotechnical properties of the foundation materials: A case study at New Minia City, Nile Valley, Egypt

Two-dimensional electrical resistivity imaging and seismic refraction, in the form of down-hole survey, were applied to delineate the subsurface section and elastic moduli and identify geotechnical characteristics of subsurface materials in the 10th of Ramadan industrial area, Cairo, Egypt. The results of four 2-D profiles of electrical resistivity, in the form of dipole–dipole and Wenner configurations, revealed that the subsurface section contains two main geo-electrical layers; the first is made of sand, some silt, and gravels, reflecting low resistivity values ranging from 25 to 65.5 ohm m. This layer is overlying a high resistivity layer (65.5 to135 ohm m), corresponding to medium to coarse sand, with gravel and calcareous materials. It is worth noting that that the down-hole technique was used to measure velocities of P and S waves in order to derive the low strain dynamic elastic properties, such as Poisson’s ratio, Shear modulus, stress ratio, concentration index, N-value, and the ultimate and allowable bearing capacities of the subsurface soil, down to an approximate depth of 30 m in the borehole at the site. In addition, the Vs30 value was calculated and revealed that the soil is categorized as a NEHRP class (D). Furthermore, the results of geotechnical parameters and elastic moduli were found to be realistic and sensible for the purposes of engineering constructions and imply that the soil in the study area is characterized by fairly to moderately competent quality. A new empirical correlation is proposed between the obtained Vs and resistivity values, where Vs = 1.0302 ρ + 172.74.

Preliminary Assessment of the Soil Foundation Characteristics Utilizing the 2D Resistivity Imaging and Down-Hole Seismic Refraction Techniques: A Case Study in Tenth of Ramadan City, Egypt

M.H. Farag

Papers

Utilizing shallow seismic refraction in defining the geotechnical properties of the foundation materials: A case study at New Minia City, Nile Valley, Egypt

Geotechnical assessment of fractured limestone bedrock using DC resistivity method: a case study at New Minia City, Egypt

Preliminary Assessment of the Soil Foundation Characteristics Utilizing the 2D Resistivity Imaging and Down-Hole Seismic Refraction Techniques: A Case Study in Tenth of Ramadan City, Egypt