Effects of near‐surface conditions on the reflectivity imaging of magnetotelluric data

1986 ◽  
Author(s):  
Kerry Stinson ◽  
Shlomo Levy ◽  
Doug Oldenburg
Keyword(s):  
Magnetotelluric Data ◽  
Near Surface ◽  
Surface Conditions ◽  
Reflectivity Imaging

A hybrid regularization scheme for the inversion of magnetotelluric data from natural and controlled sources to layer and distortion parameters

Geophysics ◽  
2012 ◽  
Vol 77 (4) ◽  
pp. E301-E315 ◽  
Author(s):  
Thomas Kalscheuer ◽  
Juliane Hübert ◽  
Alexey Kuvshinov ◽  
Tobias Lochbühler ◽  
Laust B. Pedersen
Keyword(s):  
Transfer Functions ◽  
Homogeneous Distribution ◽  
Stable Configuration ◽  
Inversion Algorithm ◽  
Magnetotelluric Data ◽  
Data Set ◽  
Near Surface ◽  
Minimum Solution ◽  
Magnetic Transfer ◽  
Distortion Parameters

Magnetotelluric (MT), radiomagnetotelluric (RMT), and, in particular, controlled-source audiomagnetotelluric (CSAMT) data are often heavily distorted by near-surface inhomogeneities. We developed a novel scheme to invert MT, RMT, and CSAMT data in the form of scalar or tensorial impedances and vertical magnetic transfer functions simultaneously for layer resistivities and electric and magnetic galvanic distortion parameters. The inversion scheme uses smoothness constraints to regularize layer resistivities and either Marquardt-Levenberg damping or the minimum-solution length criterion to regularize distortion parameters. A depth of investigation range is estimated by comparing layered model sections derived from first- and second-order smoothness constraints. Synthetic examples demonstrate that earth models are reconstructed properly for distorted and undistorted tensorial CSAMT data. In the inversion of scalar CSAMT data, such as the determinant impedance or individual tensor elements, the reduced number of transfer functions inevitably leads to increased ambiguity for distortion parameters. As a consequence of this ambiguity for scalar data, distortion parameters often grow over the iterations to unrealistic absolute values when regularized with the Marquardt-Levenberg scheme. Essentially, compensating relationships between terms containing electric and/or magnetic distortion are used in this growth. In a regularization with the minimum solution length criterion, the distortion parameters converge into a stable configuration after several iterations and attain reasonable values. The inversion algorithm was applied to a CSAMT field data set collected along a profile over a tunnel construction site at Hallandsåsen, Sweden. To avoid erroneous inverse models from strong anthropogenic effects on the data, two scalar transfer functions (one scalar impedance and one scalar vertical magnetic transfer function) were selected for inversion. Compared with a regularization of distortion parameters with the Marquardt-Levenberg method, the minimum-solution length criterion yielded smaller absolute values of distortion parameters and a horizontally more homogeneous distribution of electrical conductivity.

scholarly journals A nanocrystalline monoclinic CaCO3precursor of metastable aragonite

2018 ◽  
Vol 4 (12) ◽  
pp. eaau6178 ◽  
Author(s):  
Péter Németh ◽  
Enrico Mugnaioli ◽  
Mauro Gemmi ◽  
György Czuppon ◽  
Attila Demény ◽  
...  
Keyword(s):  
Electron Diffraction ◽  
Structure Determination ◽  
Diffuse Scattering ◽  
Diffraction Tomography ◽  
Synthetic Material ◽  
Near Surface ◽  
Surface Conditions ◽  
Terrestrial Sediments ◽  
Living Organisms ◽  
New Phase

Despite its thermodynamical metastability at near-surface conditions, aragonite is widespread in marine and terrestrial sediments. It abundantly forms in living organisms, and its abiotic formation is favored in waters of a Mg2+/Ca2+ratio > 1.5. Here, we provide crystallographic evidence of a nanocrystalline CaCO3polymorph, which precipitates before aragonite in a cave. The new phase, which we term monoclinic aragonite (mAra), is crystallographically related to ordinary, orthorhombic aragonite. Electron diffraction tomography combined with structure determination demonstrates that mAra has a layered aragonite structure, in which some carbonates can be replaced by hydroxyls and up to 10 atomic % of Mg can be incorporated. The diagnostic electron diffraction features of mAra are diffuse scattering and satellite reflections along aragonite {110}. Similar features have previously been reported—although unrecognized—from biogenic aragonite formed in stromatolites, mollusks, and cyanobacteria as well as from synthetic material. We propose that mAra is a widespread crystalline CaCO3that plays a hitherto unrecognized key role in metastable aragonite formation.

A generic soil velocity model that accounts for near-surface conditions and deeper geologic structure

2021 ◽  
Vol 140 ◽  
pp. 106461
Author(s):  
Nasser A. Marafi ◽  
Alex Grant ◽  
Brett W. Maurer ◽  
Gunjan Rateria ◽  
Marc O. Eberhard ◽  
...  
Keyword(s):  
Velocity Model ◽  
Geologic Structure ◽  
Near Surface ◽  
Surface Conditions

CFD Study of an Autonomous Submarine in Extraterrestrial Seas

2017 ◽  
Author(s):  
S. Carberry Mogan ◽  
P. Sawicki ◽  
C. J. Bernardo ◽  
D. Chen ◽  
I. Sahin ◽  
...  
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Computational Study ◽  
Liquid Hydrocarbon ◽  
Quantitative Agreement ◽  
Near Surface ◽  
Surface Conditions ◽  
Qualitative And Quantitative

A computational study is conducted to evaluate the performance of an extraterrestrial submarine operating in the liquid hydrocarbon seas of Saturn’s largest moon, Titan. To simulate the flow around the submarine and offer a prediction for thrust and power requirements, Computational Fluid Dynamics tools, ANSYS© FLUENT© and DualSPHysics, are utilized for the deeply submerged and near-surface conditions, respectively. Several operational scenarios are investigated and comparisons are made with other available results with a good qualitative and quantitative agreement.

Photosynthesis below the surface in a cryptic microbial mat

2002 ◽  
Vol 1 (4) ◽  
pp. 295-304 ◽  
Author(s):  
Lynn J. Rothschild ◽  
Lorraine J. Giver
Keyword(s):  
Carbon Fixation ◽  
Early Earth ◽  
Cyanobacterial Mat ◽  
Fixed Carbon ◽  
Daily Production ◽  
Near Surface ◽  
Surface Conditions ◽  
A Value ◽  
Subsurface Environments

The discovery of subsurface communities has encouraged speculation that such communities might be present on planetary bodies exposed to harsh surface conditions, including the early Earth. While the astrobiology community has focused on the deep subsurface, near-subsurface environments are unique in that they provide some protection while allowing partial access to photosynthetically active radiation. Previously we identified near-surface microbial communities based on photosynthesis. Here we assess the productivity of such an ecosystem by measuring in situ carbon fixation rates in an intertidal marine beach through a diurnal cycle, and find them surprisingly productive. Gross fixation along a transect (99×1 m) perpendicular to the shore was highly variable and depended on factors such as moisture and mat type, with a mean of ~41 mg C fixed m−2 day−1. In contrast, an adjacent well-established cyanobacterial mat dominated by Lyngbya aestuarii was ~12 times as productive (~500 mg C fixed m−2 day−1). Measurements made of the Lyngbya mat at several times per year revealed a correlation between total hours of daylight and gross daily production. From these data, annual gross fixation was estimated for the Lyngbya mat and yielded a value of ~1.3×105 g m−2 yr−1. An analysis of pulse-chase data obtained in the study in conjunction with published literature on similar ecosystems suggests that subsurface interstitial mats may be an overlooked endogenous source of organic carbon, mostly in the form of excreted fixed carbon.

scholarly journals On the Use of Electromagnetics for Earth Imaging of the Polar Regions

2019 ◽  
Vol 41 (1) ◽  
pp. 5-45 ◽  
Author(s):  
Graham J. Hill
Keyword(s):  
Electric Fields ◽  
Measurement Errors ◽  
Large Scale ◽  
Broad Band ◽  
High Energy ◽  
Polar Regions ◽  
Occupation Times ◽  
Magnetotelluric Data ◽  
Near Surface ◽  
Snow And Ice

Abstract The polar regions are host to fundamental unresolved challenges in Earth studies. The nature of these regions necessitates the use of geophysics to address these issues, with electromagnetic and, in particular, magnetotelluric studies finding favour and being applied over a number of different scales. The unique geography and climatic conditions of the polar regions means collecting magnetotelluric data at high latitudes, which presents challenges not typically encountered and may result in significant measurement errors. (1) The very high contact resistance between electrodes and the surficial snow and ice cover (commonly MΩ) can interfere with the electric field measurement. This is overcome by using custom-designed amplifiers placed at the active electrodes to buffer their high impedance contacts. (2) The proximity to the geomagnetic poles requires verification of the fundamental assumption in magnetotellurics that the magnetic source field is a vertically propagating, horizontally polarised plane wave. Behaviour of the polar electro-jet must be assessed to identify increased activity (high energy periods) that create strong current systems and may generate non-planar contributions. (3) The generation of ‘blizstatic’, localised random electric fields caused by the spin drift of moving charged snow and ice particles that produce significant noise in the electric fields during periods of strong winds. At wind speeds above ~ 10 m s−1, the effect of the distortion created by the moving snow is broad-band. Station occupation times need to be of sufficient length to ensure data are collected when wind speed is low. (4) Working on glaciated terrain introduces additional safety challenges, e.g., weather, crevasse hazards, etc. Inclusion of a mountaineer in the team, both during the site location planning and onsite operations, allows these hazards to be properly managed. Examples spanning studies covering development and application of novel electromagnetic approaches for the polar regions as well as results from studies addressing a variety of differing geologic questions are presented. Electromagnetic studies focusing on near-surface hydrologic systems, glacial and ice sheet dynamics, as well as large-scale volcanic and tectonic problems are discussed providing an overview of the use of electromagnetic methods to investigate fundamental questions in solid earth studies that have both been completed and are currently ongoing in polar regions.

scholarly journals Water budget and partial melting in an Archean crustal column: example from the Dharwar Craton, India

2019 ◽  
Vol 489 (1) ◽  
pp. 115-133 ◽  
Author(s):  
Gautier Nicoli
Keyword(s):  
Partial Melting ◽  
Water Budget ◽  
Critical Parameter ◽  
Bound Water ◽  
Dharwar Craton ◽  
Bulk Rock ◽  
Near Surface ◽  
Surface Conditions ◽  
Magma Compositions ◽  
Water Saturated

AbstractThe fluid budget of a composite crustal column is a critical parameter that influences many lithospheric processes. The amount of water introduced into the middle and lower crust can be quantified using phase equilibrium modelling. The Dharwar Craton, India, displays a now-exposed continuous crustal section from near-surface conditions to c. 30 km depth. This section records the different steps of a c. 15 myr-long high-temperature metamorphic event (60°C kbar−1) responsible for the formation of syn- to post-tectonic anatectic intrusions. The global water budget is assessed using thermodynamic modelling on bulk-rock compositions of an average early Proterozoic supracrustal unit and c. 3.0 Ga felsic basement, the Peninsular gneisses. Results show the fast burial of a water-saturated supracrustal package (1.6 wt%) will release c. 50% of its mineral-bound water, triggering water-fluxed partial melting of the basement. Modelled anatectic magma compositions match the observed granitoid chemistries, and distinction can be made between water-fluxed melting and water-absent melting in the origin of syn- to post-tectonic anatectic granites. Findings from this study show the importance of crustal pile heterogeneity in controlling the nature of partial melting reactions, the composition of the magmas and the rheology of the crust.

Crystallization history of a zoned plagioclase

1974 ◽  
Vol 39 (308) ◽  
pp. 867-877 ◽  
Author(s):  
G. J. Pringle ◽  
L. T. Trembath ◽  
G. E. Pajari
Keyword(s):  
Electron Microprobe ◽  
Near Surface ◽  
Surface Conditions ◽  
History Of ◽  
Textural Data ◽  
Zoned Plagioclase

SummaryElectron microprobe and textural data are used to deduce a crystallization history for plagioclase in the Grand Manan Island (Canada) diabase sheet. The plagioclase crystals consist of a prominent core (An79-67) surrounded by a distinctly more calcic zone (An82-76), which grades into a normally zoned mantle (An73-40). This zoning pattern is interpreted as reflecting the elevation of a dry diabase magma from at least 10 to 20 km to near surface conditions.

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