Application of Electrical Resistivity Method to Site Characterisation for Construction Purposes at Institute of Agriculture Research and Training Moor Plantation Ibadan

This research was carried out within Institute of Agriculture Research and Training Moor Plantation Ibadan, Southwestern Nigeria, with the aim to ascertain suitability of the proposed site for building construction and usage. The geophysical investigation involved three electrical resistivity techniques; Vertical Electrical Sounding (VES) using the Schlumberger configuration, 2D ERT and 2-D electrical imaging using Dipole-dipole electrode configuration. Two traverses were established E–W direction cutting across geologic strike with a distance of 80 m and of varying inter-traverse spacing. Eight (8) VES stations were occupied covering the entire study area for layer stratification and geoelectric parameters. The results were qualitatively and quantitatively interpreted and are presented as sounding curves and geoelectric sections. The 2-D imaging gave information on the subsurface characteristic in the area with generally low apparent resistivity indicating low competence material. The results obtained from the VES delineate three geoelectric units which comprise of the topsoil, weathered layer and fresh basement. The results from the VES were used to determine the second order parameters. The entire results correlate well with one another showing that all the techniques used were complemented. This study has further justified the need for geophysical site investigation as pre-condition before any construction to avoid problems of differential settlement. In determining of foundation material, topography elevation, clay content and the depth of weak zones should be put into consideration, since the depth of the weak zone is appreciably high.

Electrical Imaging of a Shallow Free-Phase Stray Gas Plume from an Abandoned Exploration Well

Geophysical imaging of free-phase gas (FPG) within aquifers is an emerging method for understanding the mechanisms controlling stray gas migration from oil and gas wells. Crystal Geyser is an unsealed and partially cased well that transports stray CO2 gas to the shallow subsurface. Accumulations of subsurface CO2 FPG near Crystal Geyser have been inferred, but the actual location and dimensions remained unclear. Here, the subsurface FPG distribution surrounding Crystal Geyser was characterized by interpreting 2D electrical resistivity images with previous drilling records and field mapping. An approximately 70-metre-wide FPG plume was located laterally between Crystal Geyser’s conduit and the Little Grand Wash Fault. The FPG plume spanned the vertical extent of approximately 20 to 55 metres below the ground surface, located within the Slick Rock Member sandstone with the relatively low permeability Earthy Member silty sandstone acting as a caprock. The FPG plume was identified from an anomalously high resistivity zone within the Slick Rock Member that was not caused by lateral lithofacies changes or fault displacement. The conceptual FPG migration pathways beneath Crystal Geyser are presented, based on the interpreted FPG distribution from the electrical resistivity images combined with previous site characterization and the principles of buoyant FPG migration. FPG accumulates within the Slick Rock Member by buoyant up-dip migration beneath siltstone capillary barriers of the Earthy Member. FPG leaks to the ground surface within high permeability preferential pathways along the Little Grand Wash Fault and the conduit of Crystal Geyser.

Visualization of Charge-Transfer Complex for the Detection of 2, 4, 6-Trinitrotoluene Using Terahertz Chemical Microscope

This study focuses on the visualization of a charge-transfer complex, namely a Meisenheimer complex, for the detection of uncharged 2, 4, 6-trinitrotoluene (TNT) explosives by developing a terahertz chemical microscope (TCM) imaging system. The organic amine 3-aminopropyltriethoxysilane (APTES) was immobilized on an SiO2-film-coated TCM sensing plate, where it interacted with TNT molecules. The surface electrical potential distribution of TNT, APTES, and the charge-transfer complex was mapped. An electrical potential shift occurred due to the formation of a charge-transfer complex between the electron-rich amino-silane APTES and electron-deficient TNT molecules on the surface of the sensing plate. The electrical imaging and detection of TNT explosives by using the TCM imaging system were demonstrated by measuring the amplitude of the terahertz pulse caused by this electrical potential shift. N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane and N1-(3-trimethoxysilylpropyl)diethylenetriamine were used for further evaluation and comparison of color changes arising from the amine-TNT interactions. The results have shown that TCM imaging is a promising method for the detection of uncharged TNT explosives at a low (sub-parts-per-million) concentration.

Characterization of Pliocene Biogenic Gas Reservoirs from the Western Black Sea Shelf (Romanian Offshore) by Integration of Well Logs and Core Data

The successful interpretation of open-hole well logging data relies on jointly using all available petrophysical and geological information. This paper presents relevant case studies related to the integration of well logs with core measurements for exploration wells drilled in the Romanian continental shelf area of the Western Black Sea basin. The analyzed wells targeted gas-bearing sands and silts complexes of Early Pliocene (Dacian) age, developed in a deltaic to shallow marine sedimentary environment in two distinct fields. The wireline logging programs included conventional formation evaluation logs, pressure surveys, nuclear magnetic resonance, and borehole electrical imaging logs. The core dataset comprised routine and special measurements (porosity, grain density, permeability, water saturation, and Archie parameters) carried out at quasi-reservoir confining pressure. The wireline logging suites were interpreted via a deterministic workflow, including core-derived interpretation parameters. Other core-derived parameters were used for constraining and validating the log interpretations. The results show that a problem related to the ambiguity of formation water resistivity can be overcome through resistivity–porosity dependencies constructed to include potential aquifer zones in the proximity of the Dacian gas-bearing reservoirs. This study also revealed and quantified uncertainties regarding the estimation of gas–water contacts from formation pressure surveys, which can be mitigated by the confirmation or correction of pressure-derived fluid contacts via the well log interpretation results. Lastly, we identified a probable resistivity logs suppression effect related both to high contents of capillary-bound water and also to the limited resolution of electrical logging tools in the presence of sand-shale thin bedding or laminations.

Moisture–Conductivity Calibration for Electrical Imaging of Horticultural Substrate

Electrical imaging studies of laboratory- and small-scale plant root zones are gaining increasing attention. However, for essential moisture–conductivity calibrations on numerous substrate columns with variability in dry density, the use of one conventional strategy is relatively laborious or complicated. Thus, in this work, a relatively convenient calibration method is presented, and the objective is to identify its feasibility and potential to assess the effects of factors (e.g., volumetric water content (VWC), and dry density) on conductivity and establish necessary moisture–conductivity curves for porous materials (e.g., soils and substrates). In the method, with a specially designed fixture, variable VWCs, dry densities and related complex conductivities of the samples can be easily acquired through static compaction. The results show that the in-phase conductivity (or magnitude of conductivity) increases with the increasing VWC or dry density, primarily owing to the increase in the dominant pore water connectivity. Moreover, the effect of dry density on conductivity is relatively smaller than that of VWC. Thus, for the substrates at dry densities with certain variability, good power law relations (R2 ≥ 0.99) between in-phase conductivity (or magnitude of conductivity) and VWC at different frequencies can be established. Overall, the proposed approach is practical, promising, and relatively time- and labor-saving.

Integrating Geophysical, Geochemical and Geotechnical Characteristics to Assess Soil and Groundwater Resources in Ovade, South-South Nigeria

Integrated methods were employed to assess soil and groundwater resources in an oil and gas producing area. Earth resistivity meter was used for the electrical imaging of the subsurface. Physico-chemical, geochemical and geotechnical parameters of soil and groundwater samples were determined. The results of the resistivity data revealed that a high resistive plume with resistivity > 2,000 Ohm-m has penetrated the soil beyond 20 m beneath the surface in some parts. The water samples show mild acidity with pH values in the range 4.3-5.5. Total dissolved solids (TDS) is low 26.55- 38.05 mg/L and conductivity ranges from 55.38-65.60 µS/cm. Iron content in water is fairly high (0.878-0.994 mg/L). Groundwater samples analysis using ultra violet (UV) detector and gas chromatography (GC) show that total petroleum hydrocarbons (TPH) is very low (< 0.01 mg/L and <0.031mg/L) respectively. It can be attributed to the sealing of the confined aquifer by impermeable clay in the case of the groundwater. The average permeability of the soil is 8.098 ´ 10-3cm/s indicating good drainage. The results have shown that shallow boreholes are prone to pollution. Groundwater of the area needs treatment before it can be consumed and integrated methods of investigation should be carried out periodically.

Vertical multi-frequency FDEM loop-loop soundings for sub-surface conductivity imaging: comparison of HCP and PERP configurations for different environments

&lt;p&gt;Frequency domain loop-loop electromagnetic induction (FDEM) soundings using decametric coil-separations and multi-frequency sources have been used for decades to investigate the electrical conductivity of top 100 m of the subsurface. The most common coil configurations include horizontal and vertical co-planar (HCP and VCP) setups, and the data recorded with a rather large station spacing are typically processed assuming 1D layered media. In many geological situations, the subsurface shows significant lateral contrasts in the electrical material properties, especially, in regoliths close to earth&amp;#8217;s surface. Here, the HCP and VCP 2D/3D sensitivity functions show complex and rather extended lateral sensitivity patterns. Therefore, in presence of high lateral variations in the uppermost layers, assuming 1D layered media for interpreting HCP and VCP profiles is often not valid. Furthermore, using rather large lateral station spacings often hinders the identification (and removal) of 2D/3D effects. In consequence, the overall 1D FDEM profiling procedure is often considered to be less robust than other electrical imaging techniques (e.g., DC tomography) to depict near-surface horizontal variations of the subsurface.&lt;/p&gt;&lt;p&gt;In shallower FDEM applications focusing on the characterization of the uppermost soil layers, portable loop-loop FDEM sensors (e.g. rigid boom systems with coil separations &lt; 6 m) are used to explore the subsurface electrical properties. Here, it is commonly known that the PERP configuration shows better lateral resolution and apparent conductivity maps closer to the actual conductivity distribution. The latter feature is in fact crucial for the validity and applicability of the 1D approximation. The robustness of the PERP configuration regarding the 1D assumption can be explained by its sensitivity pattern showing a preponderant sign and a rather focused pattern, centered approximately below the receiver.&lt;/p&gt;&lt;p&gt;In order to evaluate the benefit of the PERP configuration for systems with decametric coil separation, we present two case studies, where densely sampled profiles of 1D inversions of multi-frequency FDEM HCP and PERP data are compared to 2D ERT inverted models and additional independent borehole and rigid-boom FDEM sensor data. In the first case study, we explore a coastal environment near Bourbourg, France, where only minor lateral variations in the subsurface are expected. Here, our results demonstrate that a 1D inversion of HCP and PERP data result in similar models. In the second case study, we explore debris flow deposits close to Braunsbach, Germany, which are characterized by significant near-surface lateral variability. In this case, only the 1D inversion of our PERP data results in a pseudo 2D model being in agreement with the inverted 2D ERT data. These two case studies confirm that the 1D inversion of PERP data (only) yields results that are more robust regarding 2D/3D artifacts than the 1D inversion of HCP data, or a joint inversion of HCP/PERP data. In conclusion, we propose that the 1D inversion of spatially densely sampled multi-frequency PERP data should be further evaluated in view of characterizing the lateral variations within the first 20 m of the subsurface because it could represent an efficient alternative to ERT methods in selected applications.&amp;#160;&amp;#160;&lt;/p&gt;