DSpace Collection:
http://hdl.handle.net/2440/13593
2014-09-02T23:44:00ZGeoelectric experimental design — Efficient acquisition and exploitation of complete pole-bipole data sets
http://hdl.handle.net/2440/84891
Title: Geoelectric experimental design — Efficient acquisition and exploitation of complete pole-bipole data sets
Author: Blome, M.; Maurer, H.; Greenhalgh, S.
Abstract: Exploiting the information content offered by geoelectric data in an efficient manner requires careful selection of the electrode configurations to be used. This can be achieved using sequential experimental design techniques proposed over the past few years. However, these techniques become impractical when large-scale 2D or 3D experiments have to be designed. Even if sequential experimental design were applicable, acquisition of the resulting data sets would require an unreasonably large effort using traditional multielectrode arrays. We present a new, fully parallelized pole-bipole measuring strategy by which large amounts of data can be acquired swiftly. Furthermore, we introduce a new experimental design concept that is based on “complete” data sets in terms of linear independence. Complete data sets include a relatively small number of basis electrode configurations, from which any other configuration can be reconstructedby superposition. The totality of possible configurations is referred to as the comprehensive data set. We demonstrate the benefits of such reconstructions using eigenvalue analyses for the case of noise-free data. In the presence of realistic noise, such reconstructions lead to unstable results when only four-point (bipole-bipole) configurations are considered. In contrast, complete three-point (pole-bipole) data sets allow more stable reconstructions. Moreover, complete pole-bipole data sets can be acquired very efficiently with a fully parallelized system. Resolution properties of complete pole-bipole data sets are illustrated using both noise-free and noisy synthetic data sets. We also show results from a field survey performed over a buried waste disposal site, which further demonstrates the usefulness of our approach. Although this paper is restricted to 2D examples, it is trivial to extend the concept to 3D surveys, where the advantages of parallelized pole-bipole data acquisition become very significant.2010-12-31T13:30:00ZBoundary element solutions for broad-band 3-D geo-electromagnetic problems accelerated by an adaptive multilevel fast multipole method
http://hdl.handle.net/2440/84862
Title: Boundary element solutions for broad-band 3-D geo-electromagnetic problems accelerated by an adaptive multilevel fast multipole method
Author: Ren, Z.; Kalscheuer, T.; Greenhalgh, S.; Maurer, H.
Abstract: We have developed a generalized and stable surface integral formula for 3-D uniform inducing field and plane wave electromagnetic induction problems, which works reliably over a wide frequency range. Vector surface electric currents and magnetic currents, scalar surface electric charges and magnetic charges are treated as the variables. This surface integral formula is successfully applied to compute the electromagnetic responses of 3-D topography to low frequency magnetotelluric and high frequency radio-magnetotelluric fields. The standard boundary element method which is used to solve this surface integral formula quickly exceeds the memory capacity of modern computers for problems involving hundreds of thousands of unknowns. To make the surface integral formulation applicable and capable of dealing with large-scale 3-D geo-electromagnetic problems, we have developed a matrix-free adaptive multilevel fast multipole boundary element solver. By means of the fast multipole approach, the time-complexity of solving the final system of linear equations is reduced to O(m log m) and the memory cost is reduced to O(m), where m is the number of unknowns. The analytical solutions for a half-space model were used to verify our numerical solutions over the frequency range 0.001–300 kHz. In addition, our numerical solution shows excellent agreement with a published numerical solution for an edge-based finite-element method on a trapezoidal hill model at a frequency of 2 Hz. Then, a high frequency simulation for a similar trapezoidal hill model was used to study the effects of displacement currents in the radio-magnetotelluric frequency range. Finally, the newly developed algorithm was applied to study the effect of moderate topography and to evaluate the applicability of a 2-D RMT inversion code that assumes a flat air–Earth interface, on RMT field data collected at Smørgrav, southern Norway. This paper constitutes the first part of a hybrid boundary element-finite element approach to compute the electromagnetic field inside structures involving complex 3-D conductivity and permittivity distributions.2012-12-31T13:30:00ZA critical appraisal of asymptotic 3D-to-2D data transformation in full-waveform seismic crosshole tomography
http://hdl.handle.net/2440/84861
Title: A critical appraisal of asymptotic 3D-to-2D data transformation in full-waveform seismic crosshole tomography
Author: Auer, L.; Nuber, A.M.; Greenhalgh, S.A.; Maurer, H.; Marelli, S.
Abstract: Seismic full-waveform inversion (FWI) is often based on forward modeling in the computationally attractive 2D domain. This implies the assumption of a line source extended in the out-of-plane medium invariant direction, with far-field amplitudes decaying inversely with the square root of distance. Realistic point sources, however, generate amplitudes that decay approximately with the inverse of distance. Conventionally, practitioners correct for this amplitude difference and the associated phase shift by transforming the recorded 3D field data to the approximate 2D equivalent by using simplistic asymptotic filter algorithms. Such filters assume straight raypaths, a constant velocity medium, and far-field recordings. We have assessed the validity of 3D-to-2D data transformation in the context of crosshole seismic full-waveform tomography by propagating 3D and 2D wavefields through 2D media and comparing 2D reference synthetics with their filtered 3D equivalent. The filter performs well in simple situations, which confirms the general applicability of the conventional asymptotic approach. However, we have observed substantial errors in more complex elastic models, associated with overlapping arrivals and strongly curved raypaths. To test if this error translates into deficient model reconstruction in FWI, we performed complementary inversion experiments using a frequency-domain algorithm. Purely acoustic waveform inversions of 3D-to-2D filtered data are only weakly affected, but in the case of elastic FWI, in which an S-wave influence is present, adverse effects increase substantially. Two-dimensional FWI in combination with filtering seems to be an acceptable strategy as long as the model is two-dimensional, the recording geometry is straight and perpendicular to strike, and only slight S-wave energy is contained in the data. The latter two conditions are generally met in exploration-type marine seismic surveys at short offsets and in some crosswell applications using explosive sources and nondirectional pressure receivers.2012-12-31T13:30:00ZFast and accurate global multiphase arrival tracking: the irregular shortest-path method in a 3-D spherical earth model
http://hdl.handle.net/2440/84860
Title: Fast and accurate global multiphase arrival tracking: the irregular shortest-path method in a 3-D spherical earth model
Author: Huang, G.J.; Bai, C.Y.; Greenhalgh, S.
Abstract: The traditional grid/cell-based wavefront expansion algorithms, such as the shortest path algorithm, can only find the first arrivals or multiply reflected (or mode converted) waves transmitted from subsurface interfaces, but cannot calculate the other later reflections/conversions having a minimax time path. In order to overcome the above limitations, we introduce the concept of a stationary minimax time path of Fermat's Principle into the multistage irregular shortest path method. Here we extend it from Cartesian coordinates for a flat earth model to global ray tracing of multiple phases in a 3-D complex spherical earth model. The ray tracing results for 49 different kinds of crustal, mantle and core phases show that the maximum absolute traveltime error is less than 0.12 s and the average absolute traveltime error is within 0.09 s when compared with the AK135 theoretical traveltime tables for a 1-D reference model. Numerical tests in terms of computational accuracy and CPU time consumption indicate that the new scheme is an accurate, efficient and a practical way to perform 3-D multiphase arrival tracking in regional or global traveltime tomography.2012-12-31T13:30:00Z