Limitations and considerations for electrical resistivity and induced polarization imaging of riverbed sediments: Observations from laboratory, field, and synthetic experiments
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Petrophysics
Induced polarization
Electrical Resistivity Tomography
Conceptual models of aquifer systems can be refined and complemented with geophysical data, and they can assist in understanding hydrogeological properties such as groundwater storage capacity. This research attempts to use geoelectrical methods, Electrical Resistivity Tomography and Induced Polarization parameters, for mapping the subsurface in alluvial fans and to demonstrate its applicability; the Punata alluvial fan was used as a case study. The resistivity measurements proved to be a good tool for mapping the subsurface in the fan, especially when used in combination with Induced Polarization parameters (i.e., Normalized Chargeability). The Punata alluvial fan characterization indicated that the top part of the subsurface is composed of boulders in a matrix of finer particles and that the grain size decreases with depth; the electrical resistivity of these deposits ranged from 200 to 1000 Ωm, while the values of normalized chargeability were lower than 0.05 mS/m. The bottom of the aquifer system consisted of a layer with high clay content, and the resistivity ranged from 10 to 100 Ωm, while the normalized chargeability is higher than 0.07 mS/m. With the integration of these results and lithological information, a refined conceptual model is proposed; this model gives a more detailed description of the local aquifer system. It can be concluded that geoelectrical methods are useful for mapping aquifer systems in alluvial fans.
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An integrated geoelectrical study combining electrical resistivity tomography (ERT) and induced polarization (IP) have been used to delineate potential zones of Lead-Zinc mineralization in Umuobuna, Uburu, Ohaozara local government area, southeastern Nigeria. The wenner array configuration was used to acquire resistivity and induced polarization data sets along three traverses of approximate length of 1 km each and spacing of 5 to 25 m, using ABEM Terrameter SAS 1000. The profiles were inverted using RES2DINV software for 2D imaging to delineate possible zones of Pb-Zn mineralization and the geological structures at selected locations in the study area. The ERT surveys were carried out along the IP lines for comparison of anomalies across the profiles. The potential zones of Pb-Zn mineralization display contrasting values of resistivity and chargeability along the traverses. Resistivity values ranging from 7.53 to 7525 ?m and corresponding chargeability values of between -15.6 and 354 ms, respectively were encountered at some intervals along the traverses. These zones occur at width range 19.5 to 74.5 m and depths of between 0.9 and 15.5 m. Having a low to moderate resistivity and a corresponding high chargeability values at similar width range may be suggestive of Pb-Zn mineralization and test drilling is needed at the anomalous sites for confirmation. Key words: Pb-Zn, ERT, IP, resistivity, chargeability, mineralization.
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Electrical Resistivity Tomography
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Electrical Resistivity Tomography
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Electrical resistivity tomography (ERT) and time domain induced polarization (IP) techniques has been used to assess the spatial variability of the soil petrophysical properties in Covenant University Farm, Ota, southwestern Nigeria. Apparent resistivity and chargeability of the induced polarization effect were concurrently measured along six traverses using Wenner array. The observed data were inverted to produced 2D electrical resistivity and chargeability models of the soil. The inverse models were used to delineate the tilled layer from the untilled layer, and qualitatively assess the degree of compaction and lateral thickness of the soil. Other petrophysical properties such as amount of clay volume, moisture content and organic matter in the soil which are related to the electrical conductivity of the soil were also inferred. The study demonstrates the effectiveness of ERT and time domain induced polarization techniques for accessing the variations of soil conditions in large tracts of land for precision agriculture.
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The accurate characterization and mapping of low-grade ore deposits necessitate the utilization of a robust exploration technique. Induced polarization (IP) tomography is a powerful geophysical method for mineral exploration. An integrated survey using electrical resistivity tomography (ERT) and IP was employed in this study to characterize and map (Zn-Pb-Ag) ore deposits in NE New Brunswick, Canada. The survey encompassed twelve parallel lines across the study area. The 2D and 3D inversion of the results provided a detailed image of the resistivity and chargeability ranges of subsurface formations. The boundaries of sulfide mineralization were determined based on resistivity values of (700–2000 Ohm.m) and chargeability values of (3.5 mV/V) and were found to be located at an approximate depth of 80–150 m from the surface. The findings were validated through a comparison with data from borehole logs and mineralogy data analysis. The size and shape of sulfide deposits were successfully characterized and mapped in the study area using this cost-effective mapping approach.
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For both construction and exploration purposes, knowledge of the subsurface is important. Geophysical exploration methods can be used to acquire an overview of an area on which further investigations can be based. In this study direct current resistivity and induced polarization data, inverted with a Cole-Cole model in program AarhusInv, are used to characterize the bedrock at Onneslov in Scania, southwestern Sweden. Onneslov is situated at the Romele horst, which contain gneissic bedrock intruded by dykes. Resistivity ρ, with the unit Ωm, is a measurement of a materials' resistance to electric current. Induced Polarization, IP, which is measured as chargeability m with the unit mV/V, is the materials ability to charge up as a capacitor due to displacement of charges. Since both are physical properties of a material they can be used to investigate the material in the ground. The Cole-Cole model calculates the chargeability when the charging current was cut m0, as well as the frequency factor C and time constant τ. In contrast, commonly integral chargeability m is used, which uses a certain timeframe of the polarization decay. Magnetometry, percussion drillings, and borehole geophysics are also used to help refine and verify interpretations. The results show several NW trending linear elements in the ground. Many of them are interpreted as dykes or weakness zones. Weathering zones and weathered bedrock surfaces are also possible to identify, especially when adding normalised IP. The ground also contains numerous NE trending deformed and metamorphosed dykes that cannot be seen directly in the resistivity and IP data. However, these dykes most likely contribute to generally low resistivity values for bedrock in the area. C and τ follow the IP in terms of magnitude and are believed to be connected to textural differences in the ground. An additional finding is that the colour of the gneissic bedrock corresponds with changes in the gamma ray borehole logs, most likely due to the amount of potassium bearing minerals. (Less)
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Induced polarization
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Electrical Resistivity Tomography
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