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- Volume 17, Issue 6, 2019
Near Surface Geophysics - 6 - Recent Developments in Induced Polarization, Thu Dec 05 00:00:00 UTC 2019
6 - Recent Developments in Induced Polarization, Thu Dec 05 00:00:00 UTC 2019
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Permeability estimation from induced polarization: an evaluation of geophysical length scales using an effective hydraulic radius concept
Authors Andreas Weller and Lee SlaterABSTRACTGeophysical length scales defined from induced‐polarization measurements can be used in models of permeability (k) prediction. We explore the relative merit of different induced‐polarization parameters as proxies of an effective hydraulic radius (reff) that can be used to predict permeability from a modified Hagen–Poiseuille equation. Whereas geometrical measures of the hydraulic radius are good proxies of reff, the induced‐polarization measures are not well correlated with reff. However, a new proxy of reff that considers both imaginary conductivity and formation factor shows an improved correlation with reff. The resulting model enables a better quality of permeability prediction compared with the other geophysical length scales, but does not reach the predictive quality of the models based on geometrical length scales. The specific polarizability defined when incorporating the effect of the formation factor on imaginary conductivity appears to be independent of pore geometry, indicating that it is the correct parameter representing the role of the surface electrochemistry on the induced‐polarization effect. However, the joint dependence of induced‐polarization measurements on both the pore radius and the tortuosity and porosity of the interconnected pore network is a limitation to the widely explored use of induced‐polarization measurements to isolate surface properties from volumetric properties of the interconnected pore network.
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Induced polarization applied to biogeophysics: recent advances and future prospects
ABSTRACTThis paper provides an update on the fast‐evolving field of the induced polarization method applied to biogeophysics. It emphasizes recent advances in the understanding of the induced polarization signals stemming from biological materials and their activity, points out new developments and applications, and identifies existing knowledge gaps. The focus of this review is on the application of induced polarization to study living organisms: soil microorganisms and plants (both roots and stems). We first discuss observed links between the induced polarization signal and microbial cell structure, activity and biofilm formation. We provide an up‐to‐date conceptual model of the electrical behaviour of the microbial cells and biofilms under the influence of an external electrical field. We also review the latest biogeophysical studies, including work on hydrocarbon biodegradation, contaminant sequestration, soil strengthening and peatland characterization. We then elaborate on the induced polarization signature of the plant‐root zone, relying on a conceptual model for the generation of biogeophysical signals from a plant‐root cell. First laboratory experiments show that single roots and root system are highly polarizable. They also present encouraging results for imaging root systems embedded in a medium, and gaining information on the mass density distribution, the structure or the physiological characteristics of root systems. In addition, we highlight the application of induced polarization to characterize wood and tree structures through tomography of the stem. Finally, we discuss up‐ and down‐scaling between laboratory and field studies, as well as joint interpretation of induced polarization and other environmental data. We emphasize the need for intermediate‐scale studies and the benefits of using induced polarization as a time‐lapse monitoring method. We conclude with the promising integration of induced polarization in interdisciplinary mechanistic models to better understand and quantify subsurface biogeochemical processes.
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Bacterial Stern layer diffusion: experimental determination with spectral induced polarization and sensitivity to nitrite toxicity
ABSTRACTSpectral induced polarization signatures have been used as proxies for microbial abundance in subsurface environments, by taking advantage of the charged properties of microbial cell membranes. The method's applicability, however, remains qualitative, and signal interpretation ambiguous. The adoption of spectral induced polarization as a robust geo‐microbiological tool for monitoring microbial dynamics in porous media requires the development of quantitative relationships between biogeochemical targets and spectral induced polarization parameters, such as biomass density and imaginary conductivity (σ″). Furthermore, deriving cell density information from electrical signals in porous media necessitates a detailed understanding of the nature of the cell membrane surface charge dynamics. We present results from a fully saturated sand‐filled column reactor experiment where Shewanella oneidensis growth during nitrate reduction to ammonium was monitored using spectral induced polarization. While our results further confirm the direct dependence of σ″ on changing cell density, Cole–Cole derived relaxation times also record the changing surface charging properties of the cells, ascribed to toxic stress due to nitrite accumulation. Concurrent estimates of cell size yield the first measurement‐derived estimation of the apparent surface ion diffusion coefficient for cells (Ds = 5.4 ±1.3 µm2 s−1), strengthening the link between spectral induced polarization and electrochemical cell polarization. Our analysis provides a theoretical framework on which to build σ″–cell density relations using bench‐scale experiments, leading to eventual robust non‐destructive monitoring of in situ microbial growth dynamics.
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Investigating the applicability of induced polarization method in ore modelling and drilling optimization: a case study from Abassabad, Iran
Authors Kamran Mostafaei and Hamidreza RamaziABSTRACTThe main issue in mineral exploration is to obtain as much information as possible in a time‐ and cost‐efficient manner. This may be achieved through a combination of geophysical methods including the induced polarization method. The induced polarization is the most applicable method in case of metal exploration through drilling. We argue that the relationship between induced polarization, electrical resistivity and ore grade data may be used in resource estimation. This argument is tested in the active copper mine of Madan Bozorg, Abassabad, which is located in the Miami‐Sabzevar mineralization belt in NW Iran. Within the borehole locations, geophysical profiles of induced polarization and electrical resistivity were designed and surveyed using combined resistivity sounding and profiling array. Two‐dimensional models of induced polarization and electrical resistivity were prepared through inversion of geophysical data, after the primary processing. The three‐dimensional block models of induced polarization and electrical resistivity were also compiled using geostatistical methods. Obtained two‐dimensional and three‐dimensional models of induced polarization and electrical resistivity were checked using exploratory boreholes to investigate the relationship among induced polarization, electrical resistivity and copper grade. The three‐dimensional block model of the copper ore was prepared using cokriging and artificial neural network, using a combination of induced polarization and drilling data. In the cases where borehole data were unavailable, the presence of copper ore was predicted by artificial neural network and cokriging methods, and the three‐dimensional models of ore body were constructed in all of the study area. Obtained models based on the predicted copper ore distribution were checked and compared with ore distribution model compiled using drilling data. The results indicate a good agreement between predicted and real results. This led to reduction of borehole number to about 45% and the optimization of boreholes locations. Finally, an optimized drilling plan has been prepared and presented for the Abassabad copper mine.
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Influence of smectite and salinity on the imaginary and surface conductivity of volcanic rocks
Authors Léa Lévy, Andreas Weller and Benoit GibertABSTRACTWe investigate the complex conductivity behaviour of natural volcanic rocks containing variable amounts of smectite in multi‐salinity experiments. We compare the results with relationships established for sandstones. Considering only samples with little volume of metallic particles, we observe similar and small phase‐angles at low frequency for all samples at all salinities (less than 25 mrad at 1 Hz). Yet, a wide range of cation exchange capacity, porosity and formation factor is covered by the sample set: 0.5–50 meq/100 g, 4–40% and 18–780, respectively. Our results show that, in the absence of metallic particles, the ratio between imaginary conductivity and surface conductivity is significantly lower for altered volcanic rocks than for sandstones and decreases with the smectite content. These observations indicate that an increased smectite content causes more conduction and less polarization, which could be explained by the onset of a continuous conduction pathway throughout connected interfoliar spaces of smectite. Due to this pathway, cations from the pore fluid may penetrate the solid lattice, for example through connected smectite aggregates clogging the fracture network, thus preventing polarization. We also observe that the relationship between imaginary conductivity and surface conductivity, at one salinity or over the whole salinity range, is not more significant than the relationship between the imaginary conductivity and the total real conductivity. Therefore, we suggest that the imaginary conductivity cannot be used to discriminate the contributions from smectite and pore water to the total conductivity of altered volcanic rocks.
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Anisotropic complex electrical conductivity of black shale and mudstone from the Moffat Shale Group (Ireland)
Authors L. Römhild, M. Sonntag, D. Kiyan, R. Rogers, V. Rath and J.H. BörnerABSTRACTThe geological setting in the north of Ireland, especially concerning the origin of the Moffat Shale Group, has long been under discussion. Within the Tellus Programme of the Geological Survey Ireland, airborne electromagnetic measurements revealed high‐conductivity anomalies that have been interpreted as the response of a black shale. In order to petrophysically characterize the Moffat Shale, a laboratory study using material from two shallow boreholes was carried out. The study focuses on spectral induced polarization measurements on 23 oriented samples in the frequency range from 10−4 to 105 Hz.
The sample material can be categorized into two groups. A mudstone‐like rock type shows weakly frequency‐dependent, porosity‐driven conductivities with a strong anisotropy. On the other hand, black shale samples are characterized by moderately anisotropic but strong polarization effects especially at low frequencies and a strong conductivity increase towards higher frequencies. The polarization in the black shale is controlled by the texture and volume fraction of the polarizable components. The spectral induced polarization data are processed by means of a Debye decomposition approach. The anisotropy of the complex electrical conductivity is determined by utilizing the foliation dip angle and assuming tilted transverse isotropic conditions. The relevance of the laboratory findings for airborne electromagnetic surveys is addressed with a synthetic one‐dimensional modelling study.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)