arXiv subjects

Maik Neukirch

Publications and source records attributed to Maik Neukirch.

3 recordsLinked to original sources

arXiv2023

Nowcasting Geoelectric Fields in Ireland using Magnetotelluric Transfer Functions

Geomagnetically induced currents (GIC) driven by geoelectric fields pose a hazard to ground-based infrastructure, such as power grids and pipelines. Here, a new method is presented for modelling geoelectric fields in near real time, with the aim of providing valuable information to help mitigate the impact of GIC. The method uses magnetic field measurements from the Magnetometer Network of Ireland (MagIE; \url{www.magie.ie}), interpolates the geomagnetic field variations between magnetometers using spherical elementary current systems (SECS), and estimates the local electric field using a high density ($<~40~km$) network of magnetotelluric transfer functions (MT-TF) encompassing the island. The model was optimised to work in near real time, with a correction curve applied to the geoelectric field time series. This approach was successfully validated with measured electric fields at four sites for a number of geomagnetic storms, providing accurate electric fields up to a 1-minute delay from real time, with high coherence ($0.70 - 0.85$) and signal-to-noise ratio (SNR; $3.2 - 6.5$) relative to measured electric field validation time series .This was comparable to a standard non real-time geoelectric field model (coherence$~=~0.80 - 0.89$ and SNR$~=~4.0 - 7.0$). The impact of galvanic distortion on the model was also briefly evaluated, with a galvanic distortion correction leading to a more homogeneous representation of the direction of the electric field, at a regional scale.

physics.space-ph↗

arXiv2017

The Amplitude-Phase Decomposition for the Magnetotelluric Impedance Tensor

The Phase Tensor (PT) marked a breakthrough in understanding and analysis of electric galvanic distortion but does not contain any impedance amplitude information and therefore cannot quantify resistivity without complementary data. We formulate a complete impedance tensor decomposition into the PT and a new Amplitude Tensor (AT) that is shown to be complementary and mathematically independent to the PT. We show that for the special cases of 1D and 2D models, the geometric AT parameters (strike and skew angles) converge to PT parameters and the singular values of the AT correspond to the impedance amplitudes of the transverse electric and transverse magnetic modes. In all cases, we show that the AT contains both galvanic and inductive amplitudes, the latter of which is argued to be physically related to the inductive information of the PT. The geometric parameters of the inductive AT and the PT represent the same geometry of the subsurface conductivity distribution that is affected by induction processes, and therefore we hypothesise that geometric PT parameters can be used to approximate the inductive AT. Then, this hypothesis leads to the estimation of the galvanic AT which is equal to the galvanic electric distortion tensor at the lowest measured period. This estimation of the galvanic distortion departs from the common assumption to consider 1D or 2D regional structures and can be applied for general 3D subsurfaces. We demonstrate exemplarily with an explicit formulation how our hypothesis can be used to recover the galvanic electric anisotropic distortion for 2D subsurfaces, which was, until now, believed to be indeterminable for 2D data. Moreover, we illustrate the AT as a mapping tool and we compare it to the PT with both synthetic and real data examples. Lastly, we argue that the AT can provide important non-redundant amplitude information to PT inversions.

physics.geo-ph↗

arXiv2017

Appraisal of the Magnetotelluric Galvanic Electric Distortion by Optimisation of the Relation between Amplitude and Phase Tensors

The introduction of the Phase Tensor marked a major breakthrough in the understanding, analysis and treatment of galvanic distortion of the electric field in the Magnetotelluric (MT) method. We build upon a recently formulated impedance tensor decomposition into the known Phase Tensor and an Amplitude Tensor that is shown to be complementary and algebraically independent of the Phase Tensor. This recent decomposition demonstrates that the Amplitude Tensor contains inductive and galvanic information of the subsurface and that the inductive information is physically coupled to the one contained in the Phase Tensor. In this work we present an algorithm that employs this last finding to show that the MT galvanic electric distortion tensor can be separated from the inductive Amplitude Tensor and hence, that this distortion can be recovered for any given data up to a single constant usually denoted as galvanic shift. Firstly, to illustrate distortion effects on the Amplitude Tensor, we manually apply distortion by matrix multiplication to synthetic impedance tensor data. Then, we use the observations of that analysis to define an objective function, which minimises when there is no distortion present in the Amplitude Tensor. Secondly, we describe our algorithm that employs a genetic algorithm to find the optimal distortion tensor needed to correct the Amplitude Tensor, and therewith the impedance tensor. Lastly, we test the performance of the proposed methodology on synthetic data of known distortion and random distortion, and on four real data sets. The real data sets, lit007/lit008 and lit901/lit902, demonstrate the utility of the proposed algorithm by revealing geological expected results in the impedance data for the first time, which could not be achieved before by alternative methods.

physics.geo-ph↗