Preliminary results of the upper mantle conductivity structure in Nigeria

Electromagnetic investigations of the Kapuskasing uplift show that the gross electrical conductivity structure of the present crust is subhorizontal (contrary to the lithology as defined by seismic experiments), with increasing conductivity with depth, a feature common to most continental crusts. The current upper crust of the Chapleau block includes zones of reduced resistivity; the near-surface expression of the Ivanhoe Lake cataclastic zone (< 1 km in depth and 600 m in width), with resistivities of a few hundred ohm metres, is typical of fluid infilling weathered rocks. At least two other zones are less resistive (ρ < 12 kΩ∙m) than the typical upper-crustal Chapleau block (> 40 kΩ∙m), these include a subhorizontal layer at ~ 5 km and a subhorizontal to dipping layer at ~ 2 km. The deeper layer is interpreted as imaging deep fluids (porosities > 0.5%) postdating the uplift. The shallower feature, possibly related to the seismically detected detachment zone dipping at ~ 15° could be imaging conductors such as recent fluids or remnants of solid films precipitated at grain boundaries by more ancient fluids.Auger spectrometry of high-grade rocks exposed near the extrapolated surface expression of the shallower conductor reveals that fragments of graphite films (3–30 nm thick) are commonly found at grain boundaries, whereas traces of sulphur and chlorine are relatively rare. The electrical resistivity of these rocks was measured in laboratory and is lower than normally observed for similar high-grade rocks from other parts of the Canadian shield (5–25 kΩ∙m as opposed to 30–100 kΩ∙m).The Kapuskasing Uplift has opened a new area of research on upper-mantle conductivity structure from surface electromagnetic field measurements, an endeavour believed impossible until now.

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Magneto-telluric determination of upper mantle conductivity structure at Victoria, British Columbia

Canadian Journal of Earth Sciences ◽

10.1139/e68-119 ◽

1968 ◽

Vol 5 (5) ◽

pp. 1209-1220 ◽

Cited By ~ 9

Author(s):

B. Caner ◽

D. R. Auld

Keyword(s):

Spectral Analysis ◽

Upper Mantle ◽

High Resistivity ◽

Conducting Layer ◽

Frequency Range ◽

Tidal Effects ◽

Conductivity Structure ◽

Wide Range ◽

Mantle Conductivity

Magneto-telluric data were obtained at Victoria over a very wide range of periods (2 s to 86 400 s). Only the data up to 15 000 s periods were used for interpretation of conductivity structure, since telluric data at longer periods were dominated by ocean-tidal effects; spectral analysis of one year's data was used to demonstrate the tidal effects. The telluric signals are strongly polarized in the whole frequency range, indicating an anisotropy in surface conductivity.The data indicate the existence of a finite conducting layer 10 ± 3 km thick and resistivity 100–125 ohm-meters, at a depth of 65 ± 5 km. A high resistivity zone (of the order of 4000–5000 ohm-meters) lies below this layer. There is no evidence for any further conducting zones down to a depth of at least 750 km.

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Inversion of the satellite observations of the tidally induced magnetic field in terms of 3-D upper-mantle electrical conductivity: Method and synthetic tests

10.5194/egusphere-egu21-7139 ◽

2021 ◽

Author(s):

Libor Šachl ◽

Jakub Velímský ◽

Javier Fullea

Keyword(s):

Magnetic Field ◽

Electrical Conductivity ◽

Upper Mantle ◽

Spherical Harmonic ◽

Model Space ◽

Conductivity Method ◽

Satellite Gravity ◽

Compositional Model ◽

Mantle Conductivity ◽

The Impact

We have developed and tested a new frequency-domain, spherical harmonic-finite element approach to the inverse problem of global electromagnetic (EM) induction. It is based on the quasi-Newton minimization of data misfit and regularization, and uses the adjoint approach for fast calculation of misfit gradients in the model space. Thus, it allows for an effective inversion of satellite-observed magnetic field induced by tidally driven flows in the Earth's oceans in terms of 3-D structure of the electrical conductivity in the upper mantle. Before proceeding to the inversion of Swarm-derived models of tidal magnetic signatures, we have performed a series of parametric studies, using a 3-D conductivity model WINTERC-e as a testbed.The WINTERC-e model has been derived using state-of-the-art laboratory conductivity measurements of mantle minerals, and thermal and compositional model of the lithosphere and upper mantle WINTERC-grav. The latter model is based on the inversion of global surface waveforms, satellite gravity and gradiometry measurements, surface elevation, and heat flow data in a thermodynamically self-consistent framework. Therefore, the WINTERC-e model, independent of any EM data, represents an ideal target for synthetic tests of the 3-D EM inversion. We tested the impact of the satellite altitude, the truncation degree of the spherical-harmonic expansion of the tidal signals, the random noise in data, and of the sub-continental conductivity on the ability to recover the sub-oceanic upper-mantle conductivity structure. We demonstrate that with suitable regularization we can successfully reconstruct the 3D upper-mantle conductivity below world oceans.

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