Practical Magnetotellurics: References

Fluid-Deposited Graphitic Inclusions in Quartz: Comparison Between KTB (German Continental Deep-Drilling) Core Samples and Artificially Reequilibrated Natural Inclusions

Changes in electrical resistance during rock fracture in the presence of a carbonaceous atmosphere have been investigated using Nugget sandstone and Westerly granite. The experiments were performed in an internally heated, gas-pressure vessel with a load train that produced strain rates between 10−6 and 10−5 s−1. Samples were deformed at temperatures of 354° to 502°C and pressures of 100 to 170 MPa in atmospheres of Ar or mixtures of 95% CO2 with 5% CO or 5% CH4, compositions that are well within the field of graphite stability at the run conditions. In experiments using Nugget sandstone, resistance reached a minimum value when the maximum temperature was achieved and good electrode contact was made. The resistance then increased as the experiment continued, probably due to dry out of the sample, a change in the oxidation state of the Fe-oxide associated with the cement, or destruction of current-bearing pathways. At approximately 200-MPa end load, the rock sample failed. Plots of load and resistance versus time show several interesting features. In one experiment, for example, as the end load reached about 175 MPa, resistance stopped increasing and remained fairly constant for a period of approximately 0.5 hour. During loading, the end load displayed small decreases that were simultaneous with small decreases in resistance; when the end load (and the displacement) indicated rock failure, resistance decreased dramatically, from ∼150 MΩ to 100 MΩ. In a single experiment, the Westerly granite also showed a decrease in resistance during dilatancy. The nature and distribution of carbon in the run products were studied by electron microprobe and time-of-flight secondary-ion mass spectroscopy (TOP-SIMS). Carbon observed by mapping with the former is clearly observed on micro-cracks that, based on the microtexture, are interpreted to have formed during the deformation. The TOF-SIMS data confirm the electron-probe observations that carbon is present on fracture surfaces. These observations and experimental results lead to the hypothesis that as microfractures open in the time leading up to failure along a fracture, carbon is deposited as a continuous film on the new, reactive mineral surfaces, and this produces a decrease in resistance. Subsequent changes in resistance occur as connectivity of the initial fracture network is altered by continued deformation. Such a process may explain some electromagnetic effects associated with earthquakes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1998JB900032

Carbon‐enhanced electrical conductivity during fracture of rocks

The lower crust is generally considered to be an aseismic, weak zone where fluid distribution might be governed by textural equilibrium geometries. Saline fluids below the transition from brittle to ductile rheology have been advanced as a joint explanation for deep crustal conductivity and seismic reflectivity, the depth of onset of both phenomena being apparently bounded by isotherms in the 300-450 °C temperature range. Some petrologists, meanwhile, contest that the deep crust should be devoid of extensive fluid networks. This review exposes some geophysical exceptions to the statistical norm suggested by global geophysical data compilations and presents counter-arguments that the lower crust in places may be both dry and strong, that fluids if at all present at such depths may not necessarily be connected and that fluid mobility in the lower crust may be more limited and heterogeneous than commonly assumed. Laboratory data on crustal rocks implies that the transition from brittle to ductile rheology actually occurs over a much broader range of temperatures than 300-450 °C, and the apparent association of deep crustal conductive horizons with a temperature field of 300-450 °C may be interpretable in terms of formation temperatures of graphite, rather than fluids and brittle-ductile transition rheology. High v P /v S ratios from a 6 km thick, seismically layered zone below the Weardale granite, NE England can be explained by underplated mafic material. They are unlikely to be explained by fluids in an area where deep crustal conductance has been shown to be relatively low, unless conventional assumptions regarding deep crustal fluid distribution are inadequate or false. Perusal of the literature reveals that lower crustal seismicity is less seldom than generally appreciated. Interpretation of earthquakes nucleating at lower crustal depths is ambiguous, but in some tectonic regimes may indicate preservation of brittle rheology to the Moho and a lower crust that is predominantly mafic and dry. A better understanding of lower crustal deformation mechanisms and history may provide better insight into deep crustal conductivity mechanisms. Recent rock mechanical experiments suggest that permeability (and thus fluid connectivity) may be decreased by ductile shearing, whereas ductile shearing may aid graphitisation at lower crustal temperatures. If the lower crust in some regions is strong, this may explain the apparent preservation of both extant- and palaeostress orientations in interpretations involving electrical anisotropy.

Stress And Seismicity In The Lower Continental Crust: A Challenge To Simple Ductility And Implications For Electrical Conductivity Mechanisms

A three-dimensional (3-D) electrical resistivity model of Mono Basin in eastern California, unveils a complex subsurface filled with zones of partial melt, fluid-filled fracture networks, cold plutons, and regional faults. In 2013, 62 broadband magnetotelluric stations were collected in an array around southeastern Mono Basin from which a 3-D electrical resistivity model was created with a resolvable depth of 35 km. Multiple robust electrical resistivity features were found that correlate with existing geophysical observations. The most robust features are two 300 ± 50 km3 near-vertical conductive bodies (3–10 Ω m) that underlie the southeast and northeastern margin of Mono Craters below 10 km depth. These features are interpreted as magmatic crystal-melt mush zones of 15 ± 5% interstitial melt surrounded by hydrothermal fluids and are likely sources for Holocene eruptions. Two conductive east dipping structures appear to connect each magma source region to the surface. A conductive arc-like structure (< 0.9 Ω m) links the northernmost mush column at 10 km depth to just below vents near Panum Crater, where the high conductivity suggests the presence of hydrothermal fluids. The connection from the southernmost mush column at 10 km depth to below South Coulee is less obvious with higher resistivity (200 Ω m) suggestive of a cooled connection. A third, less constrained conductive feature (4–10 Ω m) 15 km deep, extending to 35 km is located west of Mono Craters near the eastern front of the Sierra Nevada escarpment and is coincident with a zone of sporadic, long-period earthquakes that are characteristic of a fluid-filled (magmatic or metamorphic) fracture network. A resistive feature (103–105 Ω m) located under Aeolian Buttes contains a deep root down to 25 km. The eastern edge of this resistor appears to structurally control the arcuate shape of Mono Craters. These observations have been combined to form a new conceptual model of the magmatic system beneath Mono Craters to a depth of 30 km.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015JB012071

Imaging the magmatic system of Mono Basin, California, with magnetotellurics in three dimensions

The relations among electrical conductivity and graphite content, metamorphic grade, and fluid:rock interaction are investigated for a suite of regionally metamorphosed graphitic carbonate rocks from the Waits River Formation, NE Vermont. Graphitization was complete by the lowest grade of metamorphism attained in the area (∼450°C and 450 MPa). In low-grade rocks, graphite occurs as inclusions within calcite and along grain boundaries, and in medium-grade rocks, it concentrates near porphyroblasts. In neither suite does graphite form interconnected networks. Average reduced carbon abundances are 4400 and 2800 ppm for the low- and medium-grade rocks, respectively. High-grade rocks are almost void of graphite, except for one specimen. The δ13C values were determined for graphite and carbonates and δ18O values were determined for the carbonates and silicate residues. There is no change in isotopic values between low- and medium-grade rocks. In contrast, high-grade rocks show marked decreases in δ13C and δ18O values of ∼4‰ compared to low- and medium-grade rocks. Both isotopic analyses and graphite depletion in high-grade rocks were caused by influx of large quantities of magmatic water during peak metamorphism. Electrical conductivity was measured in the laboratory on representative specimens. Rocks with high carbon content (>7000 ppm) from both the low- and high-grade zones display almost an order of magnitude higher electrical conductivity than expected from their fluid content alone. Graphite does not form an interconnected network in these rocks, yet it combines with the saline fluids to significantly increase electrical conductivities.

Carbonaceous material in metamorphosed carbonate rocks from the Waits River Formation, NE Vermont, and its effect on electrical conductivity

Handle everyday research tasks with reliable, citation-backed results

Your personal Research Agent to handle research tasks with citation-backed results

Popular Tasks used by Researchers

How can I help with your research?

Meet SciSpace

Get more enhanced response by uploading the PDFs you want me to reference.

No relevant PDFs in your library

SciSpace is the AI research assistant for academics. Run systematic literature reviews on 280M+ papers, and write papers with cited sources. Trusted by 1M+ students, PhDs & researchers.

SciSpace | AI for Research

Analyze PDFs

Code & Manuscripts

Funding & Grants

Literature & Patents

Medical & Clinical Data

Systematic Review

Visualize & Present

Web & Data

Build a Google Scholar-like website for your research.

Build a website

Create charts and images for your research

Create a Chart

Write a paper for submission to a journal

Draft a manuscript

Patent Search

Design eye-catching scientific posters in minutes.

Scientific Poster Generation

Systematic Literature Review

One task is running at the moment. Your messages will be shown right after.

Drag and drop or click here to browse

Loved by <highlight>1 million+</highlight> researchers

Extract a list of specific topics and their sources from unstructured text

Topics

Compare and analyze relevant papers that matches with your search

Papers

Get insights from PDFs and bookmarked papers from your library

My library

Recent searches

Try searching for:

Catch AI-generated content in scholarly and non-scholarly content

{ai} Detector

Ai Writer

Get PDF Summaries, highlighted text explanations 

Chat with PDF

Effortlessly create in-text citations and bibliographies in APA and 2,500 other formats

Citation generator

Get explanations, summaries, and answers on academic papers

Ease up your research workflow with {scispace}'s cohort of exciting AI tools

Elevate your academic writing skills and convey your ideas the way you want

Paraphraser

Explore our range of reading and writing tools

Your file is being prepared and should be ready in a few minutes. If it's a large file, it might take a bit longer. You can close this window, and we'll email you the file when it's done.

You have reached a maximum limit of <strong>{limit}</strong> columns in the table. Remove at least <strong>1</strong> column to add or create another one.

Françoise Pineau

Author Tools

Chat about Author

Papers

Carbonaceous material in metamorphosed carbonate rocks from the Waits River Formation, NE Vermont, and its effect on electrical conductivity