Processing and interpretation of near-earth satellite magnetic data

A thesis submitted to the faculty of science, University of the Witwatersrand, Johannesburg, In fulfilment of the requirements for the Degree of Doctor of Philosophy Johannesburg 1990The processing of near-earth satellite magnetometer data towards extracting an improved lithosphere anomaly map is addressed. The two aspects of the data reduction route where data quality has been compromised most by contemporary methods is researched, namely;[Abbreviated Abstract. Open document to view full version]GR201

Mantle structure and tectonic history of SE Asia

Seismic travel-time tomography of the mantle under SE Asia reveals patterns of subduction-related seismic P-wave velocity anomalies that are of great value in helping to understand the region's tectonic development. We discuss tomography and tectonic interpretations of an area centred on Indonesia and including Malaysia, parts of the Philippines, New Guinea and northern Australia. We begin with an explanation of seismic tomography and causes of velocity anomalies in the mantle, and discuss assessment of model quality for tomographic models created from P-wave travel times. We then introduce the global P-wave velocity anomaly model UU-P07 and the tectonic model used in this paper and give an overview of previous interpretations of mantle structure. The slab-related velocity anomalies we identify in the upper and lower mantle based on the UU-P07 model are interpreted in terms of the tectonic model and illustrated with figures and movies. Finally, we discuss where tomographic and tectonic models for SE Asia converge or diverge, and identify the most important conclusions concerning the history of the region. The tomographic images of the mantle record subduction beneath the SE Asian region to depths of approximately 1600. km. In the upper mantle anomalies mainly record subduction during the last 10 to 25. Ma, depending on the region considered. We interpret a vertical slab tear crossing the entire upper mantle north of west Sumatra where there is a strong lateral kink in slab morphology, slab holes between c.200-400. km below East Java and Sumbawa, and offer a new three-slab explanation for subduction in the North Sulawesi region. There is a different structure in the lower mantle compared to the upper mantle and the deep structure changes from west to east. What was imaged in earlier models as a broad and deep anomaly below SE Asia has a clear internal structure and we argue that many features can be identified as older subduction zones. We identify remnants of slabs that detached in the Early Miocene such as the Sula slab, now found in the lower mantle north of Lombok, and the Proto-South China Sea slab now at depths below 700. km curving from northern Borneo to the Philippines. Based on our tectonic model we interpret virtually all features seen in upper mantle and lower mantle to depths of at least 1200. km to be the result of Cenozoic subduction

Towards adjoint-based inversion of time-dependent mantle convection with non-linear viscosity

We develop and study an adjoint-based inversion method for the simultaneous recovery of initial temperature conditions and viscosity parameters in time-dependent mantle convection from the current mantle temperature and historic plate motion. Based on a realistic rheological model with temperature-dependent and strain-rate-dependent viscosity, we formulate the inversion as a PDE-constrained optimization problem. The objective functional includes the misfit of surface velocity (plate motion) history, the misfit of the current mantle temperature, and a regularization for the uncertain initial condition. The gradient of this functional with respect to the initial temperature and the uncertain viscosity parameters is computed by solving the adjoint of the mantle convection equations. This gradient is used in a pre-conditioned quasi-Newton minimization algorithm. We study the prospects and limitations of the inversion, as well as the computational performance of the method using two synthetic problems, a sinking cylinder and a realistic subduction model. The subduction model is characterized by the migration of a ridge toward a trench whereby both plate motions and subduction evolve. The results demonstrate: (1) for known viscosity parameters, the initial temperature can be well recovered, as in previous initial condition-only inversions where the effective viscosity was given; (2) for known initial temperature, viscosity parameters can be recovered accurately, despite the existence of trade-offs due to ill-conditioning; (3) for the joint inversion of initial condition and viscosity parameters, initial condition and effective viscosity can be reasonably recovered, but the high dimension of the parameter space and the resulting ill-posedness may limit recovery of viscosity parameters

Seismological forward and inverse modelling for upper mantle seismic anisotropy studies

Seismic anisotropy is the dependence of seismic wave velocity on the propagation direction and it is mainly generated by strain-induced lattice preferred orientation (LPO) of intrinsically anisotropic minerals. Despite previous studies have demonstrated that neglecting anisotropy introduces notable imaging artifacts, most tomographic methods rely on the assumption of isotropy, interpreting fast and slow velocity anomalies as related to seismically isotropic sources (e.g., temperature anomalies, presence of a liquid phase, etc). In this Thesis we carried out numerical simulations aiming at improving strain-induced fabric estimates and predicting realistic elastic properties in 2-D and 3-D synthetic domains. We generated synthetic datasets with forward waveform modelling and explored different inverse methodologies (e.g., P- and S-wave travel time tomography, automatic partitioned waveform inversion of surface waves) both with real and synthetic data. Among the results, we present ani-NEWTON21, the first 3D anisotropic teleseismic P-wave tomography revealing upper mantle structures and dynamics beneath the Central Mediterranean. By performing synthetic seismic data inversions we tested how ray density, data quality and regularization (i.e., damping and smoothing factors) influence the tomographic image. Finally, from the comparison of purely isotropic and anisotropic tests, we observed that the first-order effect of including anisotropy in the inversion is to reduce the magnitude of isotropic anomalies, more significantly for low-velocity zones relative to high-velocity zones. The research activities described in this Thesis altogether provide important insights for predicting and isolating seismic anisotropy, and for obtaining more reliable and physically consistent imaging of the Earth’s internal structure.Seismic anisotropy is the dependence of seismic wave velocity on the propagation direction and it is mainly generated by strain-induced lattice preferred orientation (LPO) of intrinsically anisotropic minerals. Despite previous studies have demonstrated that neglecting anisotropy introduces notable imaging artifacts, most tomographic methods rely on the assumption of isotropy, interpreting fast and slow velocity anomalies as related to seismically isotropic sources (e.g., temperature anomalies, presence of a liquid phase, etc). In this Thesis we carried out numerical simulations aiming at improving strain-induced fabric estimates and predicting realistic elastic properties in 2-D and 3-D synthetic domains. We generated synthetic datasets with forward waveform modelling and explored different inverse methodologies (e.g., P- and S-wave travel time tomography, automatic partitioned waveform inversion of surface waves) both with real and synthetic data. Among the results, we present ani-NEWTON21, the first 3D anisotropic teleseismic P-wave tomography revealing upper mantle structures and dynamics beneath the Central Mediterranean. By performing synthetic seismic data inversions we tested how ray density, data quality and regularization (i.e., damping and smoothing factors) influence the tomographic image. Finally, from the comparison of purely isotropic and anisotropic tests, we observed that the first-order effect of including anisotropy in the inversion is to reduce the magnitude of isotropic anomalies, more significantly for low-velocity zones relative to high-velocity zones. The research activities described in this Thesis altogether provide important insights for predicting and isolating seismic anisotropy, and for obtaining more reliable and physically consistent imaging of the Earth’s internal structure

Adaptation of back projection tomography to seismic travel time problems

A back projection method of reconstruction is adapted to invert seismic travel time data for velocity structure. Adaptations are made so that the inhomogeneous and anisotropic ray sets and the three-dimensional geometries commonly dealt with in seismic experiments can be handled with greater success. Jacobi iteration, deconvolution, and ray weighting work well in augmenting the basic back projection method to produce a well-focused image. These methods succeed by amounts that depend on the quality of the ray coverage. Also, the ability to reconstruct an accurate image when the data include moderate amounts of noise is shown to be good. Comparison of inversions produced with back projection tomography and with damped least squares indicate that the two methods are comparable in their ability to reconstruct an image of the actual structure. The back projection approach, however, is much more computer efficient. In practice, this allows for the construction of more detailed inversions

Book of Abstracts

no abstrac