Using Empirical Mode Decomposition (EMD) for the processing of marine MT data

Magnetotelluric (MT) method determines a frequency dependent impedance tensor using the spectra of associated time-varying horizontal electric and magnetic fields measured at the Earth’s surface. In this abstract, we present a dynamic time series analysis method dealing the non-stationary MT data to infer the impedance tensor. Most current methods to determine the spectra use Fourier transform based procedure and, therefore, assume that the signals are stationary over the record length. We introduce a new method for dealing with non-stationarity of the MT time series based upon empirical mode decomposition (EMD) method, a dynamic time series analysis method. Using EMD complicated data sets can be decomposed into a finite and small number of "intrinsic mode functions" (IMFs), which are mono-component signals and allow the calculation of physical meaningful instantaneous frequencies. EMD has no bias due to non-stationary of geomagnetic time series, since the IMFs are based entirely on signal characteristics and not on any given set of base functions such as sines and cosines in the Fourier transform or wavelets in the Wavelet transform. We use the EMD method to decompose MT data into IMFs and calculate the instantaneous frequencies and spectra to determine the impedance tensor. The method is tested in synthetic and real marine MT data sets, the obtained estimate results are reliable compared to frequently-used BIRRP processing method. Furthermore, new method has the possibility of noise visualization and filtering, which is especially important in marine applications, where noise free time segments maybe short

Joint inversion scheme with an adaptive coupling strategy - applications on synthetic and real data sets

Joint inversion strategies for geophysical data have become increasingly popular since they allow to combine complementary information from different data sets in an efficient way. However, for joint inversion algorithms that use methods that are sensitive to different parameters it is important that they are not restricted to specific survey arrays and subsurface conditions. Hence, joint inversion schemes are needed that 1) adequately balance data from the different methods and 2) use links between the parameter models that are suited for a wide range of applications. Here, we combine MT, seismic tomography and gravity data in a non-linear joint inversion that accounts for these critical issues. Data from the different methods are inverted separately and are joined through constrains accounting for parameter relationships. An advantage of performing the inversions separately (and not together in one matrix) is that no relative weighting between the data sets is required. To avoid that the convergence behavior of the inversions is profoundly disturbed by the coupling, the strengths of the associated constraints are re-adjusted at each iteration. As criteria to control the adaption of the coupling strengths we used a general version of the well-known discrepancy principle. Adaption of the coupling strengths makes the joint inversion scheme also applicable to subsurface conditions, for which the assumed relationships are only a rough first order approximation. So, the coupling between the different parameter models is automatically reduced if for some structures the true rock property behaviors differ significantly from the assumed relationships (e.g. the atypical density-velocity behavior of salt). We have tested our scheme first on different synthetic 2-D models for which the assumed parameter relationships are everywhere valid. We observe that the adaption of the coupling strengths makes the convergence of the inversions very robust and that the final results are close to the true models. In a next step the scheme has been applied on models for which the assumed parameter relationships are invalid for some structures. For these structures deviations from the relationships are present in the final results; however, for the remaining structures the relative behaviors of the physical parameters are still approximately described by the assumed relationship. Finally, we applied our joint inversion scheme on seismic, MT and gravity data collected offshore the Faroe Islands, where basalt intrusions are present

Towards 3D joint inversion of full tensor gravity, magnetotelluric and seismic refraction data

EGU2010-4184-2 Joint inversion of different datasets is emerging as an important tool to enhance resolution and decrease inversion artifacts in structurally complex areas. Performing the inversion in 3D allows us to investigate such complex structures but requires computationally efficient forward modeling and inversion methods. Furthermore we should be able to flexibly change inversion parameters, coupling approaches and forward modeling schemes in order to find a suitable approach for the given target. We present a 3D joint inversion framework for scalar and full tensor gravity, magnetotelluric and seismic data that allows us to investigate different approaches. It consists of two memory efficient gradient based optimization techniques, L-BFGS and NLCG, and optimized parallel forward solvers for the different datasets. In addition it provides the necessary flexibility in terms of model parametrization and coupling method by completely separating the inversion parameters and geometry from the parametrization of the individual method. This separation allows us to easily switch between completely different types of parameterizations and use structural coupling as well as coupling based on parameter relationships for the joint inversion. First tests on synthetic data with a fixed parameter relationship coupling show promising results and demonstrate that 3D joint inversion is becoming feasible for realistic size models

Adaption and GPU based parallelization of the code TEMDDD for the 3D modelling of CSEM data

The finite difference time domain code TEMDDD was modified for the 3D forward modeling of marine CSEM data. After changes in the code, which make it possible to create model geometries typically encountered in marine CSEM experiments, parts of the code have been parallelized using massive parallelization on graphic cards. Parts of the singular value decomposition, which is the most time consuming part of the code, have been successfully ported with massive speed-ups (8-12x faster) observed as compared to the standard code. The full parallelization of the code is still work in progress

Using empirical mode decomposition to process marine MT data

Magnetotellurics (MT) uses a frequency-dependent impedance tensor estimated from the spectra of associated time-varying horizontal electric and magnetic fields measured at the Earth's surface to image the sub-surface of the Earth. Most current methods use Fourier transform based procedures to estimate power spectral densities and, therefore, assume that the signals are stationary over the record length. Stationarity in geomagnetic data, however, is not always ensured given the variety of source mechanism causing the geomagnetic variations at different time and spatial scales. Additional complication and bias may arise from the presence of noise in the recorded electric and magnetic file data. Sophisticated MT data processing account for a potential bias through windowing of the time series as well as robust estimates of the impedance. We explore a new heuristic method for dealing with the non-stationarity of MT time series based on empirical mode decomposition. It is a dynamic time series analysis method, in which complicated data sets can be decomposed into a finite and small number of simple intrinsic mode functions. In this paper, we use the empirical mode decomposition method to decompose MT data into intrinsic mode functions and calculate the instantaneous frequencies and spectra to determine the impedance tensor. We investigate the reliability of the impedance estimates on synthetic data by comparing the results to those obtained by analytical methods. Finally, we apply our processing scheme to data measured from the Costa Rica subduction zone, and compare the results from our new method to the frequently-used BIRRP processing method

Integration of Hyperspectral and Magnetic Data for Geological Characterization of the Niaqornarssuit Ultramafic Complex in West-Greenland

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Combined three-dimensional electric and seismic tomography study on the Aknes rockslide in western Norway

We present a combined 3-D geoelectric and seismic tomography study conducted on the large Aknes rockslide in western Norway. Movements on the slope are strongly influenced by water infiltration, such that the hydrogeological regime is considered as a critical factor affecting the slope stability. The aim of our combined geophysical study was to identify and visualize the main shallow tension fractures and to determine their effect on hydraulic processes by comparing the geophysical results with information from borehole logging and tracer tests. To resolve the complex subsurface conditions of the highly fractured rock mass, a three-dimensional set-up was chosen for our seismic survey. To map the water distribution within the rock mass, a pattern of nine intersecting 2-D geoelectric profiles covered the complete unstable slope. Six of them that crossed the seismic survey area were considered as a single data set in a 3-D inversion. For both methods, smoothing-constraint inversion algorithms were used, and the forward calculations and parameterizations were based on unstructured triangular meshes. A pair of parallel shallow low-velocity anomalies (<1400 m/s) observed in the final seismic tomogram was immediately underlain by two anomalies with resistivities <13 k Omega m in the resistivity tomogram. In combination with borehole logging results, the low-velocity and resistivity anomalies could be associated with the drained and water-filled part of the tension fractures, respectively. There were indications from impeller flowmeter measurements and tracer tests that such tension fractures intersected several other water-filled fractures and were responsible for distinct changes of the main groundwater flow paths. (C) 2009 Elsevier B.V. All rights reserved

3-D Magnetotelluric Image of Offshore Magmatism at the Walvis Ridge and Rift Basin

Highlights • We report on marine 3D Magnetotelluric study on Walvis Ridge • Derived 3D electrical resistivity model shows a large scale resistive zone, which we link to crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting • Smaller scale resistive region is attributed to magma ascent during rifting • Rift basin is identified by low resistivity region The Namibian continental margin marks the starting point of the Tristan da Cunha hotspot trail, the Walvis Ridge. This section of the volcanic southwestern African margin is therefore ideal to study the interaction of hotspot volcanism and rifting, which occurred in the late Jurassic/early Cretaceous. Offshore magnetotelluric data image electromagnetically the landfall of Walvis Ridge. Two large-scale high resistivity anomalies in the 3-D resistivity model indicate old magmatic intrusions related to hot-spot volcanism and rifting. The large-scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2-D offshore seismic profiles. One of the high resistivity anomalies (above 500 Ωm) has three arms of approximately 100 km width and 300 km to 400 km length at 120 degree angles in the lower crust. One of the arms stretches underneath Walvis Ridge. The shape is suggestive of crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting. A second, smaller anomaly of 50 km width underneath the continent ocean boundary may be attributed to magma ascent during rifting. We attribute a low resistivity anomaly east of the continent ocean boundary and south of Walvis Ridge to the presence of a rift basin that formed prior to the rifting