The upper mantle conductivity analysis method using observatory records of the geomagnetic field

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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Upper-mantle conductivity determined by the geomagnetic lunar daily variation

Physics of The Earth and Planetary Interiors ◽

10.1016/0031-9201(75)90065-5 ◽

1975 ◽

Vol 10 (4) ◽

pp. 377-381

Author(s):

R.J. Jady

Keyword(s):

Upper Mantle ◽

Daily Variation ◽

Mantle Conductivity

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Upper mantle conductivity structure of the back-arc region beneath northeastern China

Geophysical Research Letters ◽

10.1029/2001gl012983 ◽

2001 ◽

Vol 28 (19) ◽

pp. 3773-3776 ◽

Cited By ~ 43

Author(s):

Masahiro Ichiki ◽

Makoto Uyeshima ◽

Hisashi Utada ◽

Zhao Guoze ◽

Tang Ji ◽

...

Keyword(s):

Upper Mantle ◽

Northeastern China ◽

Conductivity Structure ◽

Mantle Conductivity ◽

Back Arc

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Deep array electromagnetic sounding on the Baltic Shield: External excitation model and implications for upper mantle conductivity studies

Tectonophysics ◽

10.1016/j.tecto.2007.07.006 ◽

2007 ◽

Vol 445 (1-2) ◽

pp. 3-25 ◽

Cited By ~ 15

Author(s):

Elena Yu. Sokolova ◽

Ivan M. Varentsov

Keyword(s):

Baltic Shield ◽

Upper Mantle ◽

External Excitation ◽

Electromagnetic Sounding ◽

Mantle Conductivity ◽

The Baltic ◽

Excitation Model

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A review of electromagnetic investigations in the Kapuskasing uplift and surrounding regions: electrical properties of key rocks

Canadian Journal of Earth Sciences ◽

10.1139/e94-094 ◽

1994 ◽

Vol 31 (7) ◽

pp. 1042-1051 ◽

Cited By ~ 9

Author(s):

Marianne Mareschal ◽

Ron D. Kurtz ◽

Richard C. Bailey

Keyword(s):

Grain Boundaries ◽

Upper Mantle ◽

Field Measurements ◽

Surface Expression ◽

High Grade ◽

Near Surface ◽

Solid Films ◽

Conductivity Structure ◽

Mantle Conductivity ◽

Detachment Zone

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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MTS studies on the upper mantle conductivity in China

Pure and Applied Geophysics ◽

10.1007/bf00874507 ◽

1987 ◽

Vol 125 (2-3) ◽

pp. 465-482 ◽

Cited By ~ 2

Author(s):

Liu Guodong

Keyword(s):

Upper Mantle ◽

Mantle Conductivity

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Decomposition analysis of the BEAR magnetotelluric data: implications for the upper mantle conductivity in the Fennoscandian Shield

Geophysical Journal International ◽

10.1111/j.1365-246x.2005.02744.x ◽

2005 ◽

Vol 163 (3) ◽

pp. 900-914 ◽

Cited By ~ 29

Author(s):

I. Lahti ◽

T. Korja ◽

P. Kaikkonen ◽

K. Vaittinen ◽

Keyword(s):

Upper Mantle ◽

Decomposition Analysis ◽

Fennoscandian Shield ◽

Magnetotelluric Data ◽

Mantle Conductivity

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The response of the local geomagnetic field to geodynamic processes during the preparation of the 1988 Spitak earthquake

Геология и геофизика Юга России ◽

10.23671/vnc.2020.1.59064 ◽

2020 ◽

Author(s):

А.Г. Григорян ◽

Д.А. Лиходеев

Keyword(s):

Electrical Conductivity ◽

Physical Properties ◽

Upper Mantle ◽

Geomagnetic Field ◽

Power Plants ◽

External Source ◽

Crust And Upper Mantle ◽

Strong Earthquakes ◽

The Earth ◽

Geodynamic Processes

Актуальность работы. Изучение изменений локального геомагнитного поля с целью выявления предвестников сильных землетрясений, особенно в сейсмоактивных регионах, где расположены большие города и объекты особо важного значения (АЭС, водохранилище и т.п.) остается одной из главных задач современной науки. В разных странах мира, используя магнитометрические методы, проводятся исследования по поиску предвестников сильных землетрясений. Цель. Однако, за первую половину XX века, несмотря на отдельные попытки ученых Японии и других стран, серьезных результатов достичь не удалось. Установлено, что с развитием геодинамических процессов в земной коре, особенно при подготовке сильных землетрясений, происходят изменения в магнитных свойствах горных пород (электропроводности, диэлектрической и магнитной проницаемости). Геомагнитные вариации, создаваемые внешним источником, несут в себе важную информацию об изменениях в физических свойствах в земной коры и верхней мантии, а так же позволяют оценить эти изменения. Методы. Представлена методика, которая позволяет с помощью изучения вариаций локального геомагнитного поля, создаваемых внешним источником, выявить изменения в электропроводности на разных глубинах земной коры и верхней мантии, связанные с развитием геодинамических процессов. С этой целью использован расчетный параметр N(A), который является отношением амплитуд вариаций геомагнитного поля внешнего происхождения, измеренных синхронно на разных парах станций. Изучены вариации с периодами 1025, 3060 минут и Sq-вариации. Метод применяется в низкоширотных областях Земли, где вариации переменного геомагнитного поля хорошо выделяются. Результаты. Используя предлагаемую методику, на территории Армении были выявлены аномальные изменения локального отклика геомагнитного поля перед Парванийским 1986 г. (М5,4) и Спитакским 1988 г. (М7,0) землетрясениями. Предполагается, что причинами изменений в физических свойств геологической среды в частности электропроводности, являются дегазация Земли и вертикальная фильтрация флюидов в верхние слои земной коры Relevance. The study of local geomagnetic field changes in order to identify harbingers of strong earthquakes, especially in seismically active regions where large cities and especially important objects (nuclear power plants, a storage reservoir, etc.) are located remains one of the main tasks of modern science. In different countries studies are being conducted to search for precursors of strong earthquakes, using magnetometric methods. Aim. However, for the first half of the 20th century, despite some attempts by scientists from Japan and other countries, no serious results were obtained. It has been established that with the progress of geodynamic processes in the earths crust, especially during the preparation of strong earthquakes, changes in the magnetic properties of rocks (electrical conductivity, dielectric and magnetic permeability) occur. However, geomagnetic variations created by an external source carry important information about changes in physical properties, in particular, electrical conductivity in the earths crust to the upper mantle, and make it possible to evaluate these changes. Methods. A technique that allows to identify changes in electrical conductivity at different depths of the earths crust and upper mantle associated with the development of the geodynamic process, using the study of local geomagnetic field variations created by an external source, is presented. For this purpose, parameter N(A), which is the ratio of the amplitudes of variations of the geomagnetic field of external origin, measured synchronously at different pairs of stations, was used. Variations with periods of 10-25, 30-60 minutes and Sq-variations were studied. The method is used in low latitude areas of the Earth, where variations of the variable geomagnetic field stand out well. Results. Anomalous changes in the local geomagnetic field were revealed in Armenia before the Parvania 1986 (M 5.4) and Spitak 1988 (M 7.0) earthquakes, using the proposed methodology. It is assumed that the causes of changes in the physical properties of the geological environment, in particular, electrical conductivity, are most likely to be the degassing of the Earth and the vertical filtration of fluids into the upper layers of the earths crust

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