Discrete frequency inequalities for magnetotelluric impedances of one-dimensional conductors

For the one-dimensional magnetotelluric inverse problem the ties between the impedances at neighbouring frequencies, reflecting the analytical properties of the transfer function, are expressed in terms of inequalities between the data. After the derivation of some elementary necessary constraints for data sets with two or three frequencies, a set of necessary and sufficient conditions warranting the existence of a one-dimensional conductivity model in the general M-frequency case is given. This set of constraints characterizes a 1-D data set by the signs of 2M determinants derived from the data.           ARK: https://n2t.net/ark:/88439/y087338 Permalink: https://geophysicsjournal.com/article/228 &nbsp

Preferential adsorption of para and ortho water molecules on charged nanoparticles in planetary ice clouds

In the Earth mesopause, nanometer-size singly charged particles form by condensation of evaporated meteorite material. Tey exhibit an enhanced water adsorption cross section due to the strong charge-dipole-interaction. In this work, we study how the nuclear spin state of water molecules afects this enhancement and whether there are conditions that could lead to the formation of spin-polarized ice. Due to symmetry constraints on the total molecular wave function, ortho (proton spins parallel) and para (spins antiparallel) water occupy diferent rotational states, resulting in a diferent average dipole orientation in electric felds. Terefore, we expect ortho and para water to exhibit distinct ad- sorption enhancement factors onto charged nanoparticles. Based on Stark-shifs of individual rotational states of water, average dipole orientations of a molecu- lar ensemble and the resulting collision cross section was calculated for various temperatures and particle sizes. We found that in the mesosphere of the Earth (T~150K) the adsorption enhancement of ortho- and para- water is approxi- mately equal while at lower temperatures prevailing around ice giant planets and their moons, signifcant spin polarizations up to 15% occur

On mapping seafloor mineral deposits with central loop transient electromagnetics

Electromagnetic methods are commonly employed in exploration for land-based mineral deposits. A suite of airborne, land, and borehole electromagnetic techniques consisting of different coil and dipole configurations have been developed over the last few decades for this purpose. In contrast, although the commercial value of marine mineral deposits has been recognized for decades, the development of suitable marine electromagnetic methods for mineral exploration at sea is still in its infancy. One particularly interesting electromagnetic method, which could be used to image a mineral deposit on the ocean floor, is the central loop configuration. Central loop systems consist of concentric transmitting and receiving loops of wire. While these types of systems are frequently used in land-based or airborne surveys, to our knowledge neither system has been used for marine mineral exploration. The advantages of using central loop systems at sea are twofold: (1) simplified navigation, because the transmitter and receiver are concentric, and (2) simplified operation because only one compact unit must be deployed. We produced layered seafloor type curves for two particular types of central loop methods: the in-loop and coincident loop configurations. In particular, we consider models inspired by real marine mineral exploration scenarios consisting of overburdens 0 to 5 m thick overlying a conductive ore body 5 to 30 m thick. Modeling and resolution analyses showed that, using a 50 m(2) transmitting loop with 20 A of current, these two configurations are useful tools to determine the overburden depth to a conductive ore deposit and its thickness. In the most extreme case, absolute voltage errors on the order of 10 nV are required to resolve the base of a 30 m thick ore deposit. Whether such noise floors can be achieved in real marine environments remains to be seen

On the electromagnetic fields produced by marine frequency domain controlled sources

Author Posting. © The Author, 2009. This article is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 179 (2009): 1429-1457, doi:10.1111/j.1365-246X.2009.04367.x.In recent years, marine controlled source electromagnetics (CSEM) has found increasing use in hydrocarbon exploration due to its ability to detect thin resistive zones beneath the seafloor. Although it must be recognized that the quantitative interpretation of marine CSEM data over petroleum-bearing formations will typically require 2-D surveys and 2-D or 3-D modelling, the use of the 1-D approximation is useful under some circumstances and provides considerable insight into the physics of marine CSEM. It is the purpose of this paper to thoroughly explore the 1-D solutions for all four fundamental source types—vertical and horizontal, electric and magnetic dipole (VED, HED, VMD and HMD)—using a set of canonical reservoir models that encompass brine to weak to strong hydrocarbon types. The paper introduces the formalism to solve the Maxwell equations for a 1-D structure in terms of independent and unique toroidal and poloidal magnetic modes that circumscribe the salient diffusion physics. Green's functions for the two modes from which solutions for arbitrary source current distributions can be constructed are derived and used to obtain the electromagnetic (EM) fields produced by finite VED, HED, VMD and HMD sources overlying an arbitrary 1-D electrical structure. Field behaviour is analysed using the Poynting vector that represents the time-averaged flow of energy through the structure and a polarization ellipse decomposition of the triaxial seafloor EM field that is a complete field description. The behaviour of the two EM modes using unimodal VED and VMD sources is presented. The paper closes by extending these results to the bimodal HED and HMD sources

Upper mantle electrical resistivity structure beneath the central Mariana subduction system

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 11 (2010): Q09003, doi:10.1029/2010GC003101.This paper reports on a magnetotelluric (MT) survey across the central Mariana subduction system, providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back-arc spreading ridge. After calculation of MT response functions and their correction for topographic distortion, two-dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab. The resultant isotropic electrical resistivity structure contains several key features. There is an uppermost resistive layer with a thickness of up to 150 km beneath the Pacific Ocean Basin, 80–100 km beneath the Mariana Trough, and 60 km beneath the Parece Vela Basin along with a conductive mantle beneath the resistive layer. A resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge. There is no evidence for a conductive feature beneath the back-arc spreading center. Sensitivity tests were applied to these features through inversion of synthetic data. The uppermost resistive layer is the cool, dry residual from the plate accretion process. Its thickness beneath the Pacific Ocean Basin is controlled mainly by temperature, whereas the roughly constant thickness beneath the Mariana Trough and beneath the Parece Vela Basin regardless of seafloor age is controlled by composition. The conductive mantle beneath the uppermost resistive layer requires hydration of olivine and/or melting of the mantle. The resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three-dimensional and of limited size. In contrast, the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab. The resistive region beneath the back-arc spreading center can be explained by dry mantle with typical temperatures, suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge. Evidence for electrical anisotropy in the central Mariana upper mantle is weak.Japanese participation in the Marianas experiment was supported by Japan Society for the Promotion of Science for Grant-In-Aid for Scientific Research (15340149 and 12440116), Japan-U.S. Integrated Action Program and the 21st Century COE Program of Origin and Evolution of Planetary Systems, and by the Ministry of Education, Culture, Sports, Science, and Technology for the Stagnant Slab Project, Grant-in Aid for Scientific Research on Priority Areas (17037003 and 16075204). U.S. participation was supported by NSF grant OCE0405641. Australian support came from Flinders University. T. M. is supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Deep Ocean Exploration Institute