Interpretation of project Magnet data (1959 to 1966) for Africa and the Mozambique Channel. Seismic ray tracing with applications to Africa

Abstract

PART I ABSTRACT INTERPRETATION OF PROJECT MAGNET DATA (1959 to 1966) FOR AFRICA AND THE MOZAMBIQUE CHANNEL This part of the thesis is concerned with the interpretation of Project MAGNET profiles over the African continent and Mozambique Channel. The paths of the continental profiles lie between latitudes 5°N. and 22°S., while the paths of the profiles which cross the Mozambique Channel lie between latitudes 0° and 35°S. (figure (1)). The geology and geochronology of 'equatorial' Africa are described briefly in chapter (1). In chapter (2) a new method of magnetic profile analysis is introduced and applied to a number of simulated profiles. This method forms the basis of the interpretation of the continental Project MAGNET profiles (chapter (4)). In chapter (3) the various details of the reduction of the Project MAGNET data (e.g. the interpolation of the data, removal of the regional magnetic field, removal of the diurnal magnetic field) are described. The continental Project MAGNET data are interpreted in chapter (4) while the 'oceanic' data are interpreted in chapter (5). PART II SEISMIC RAY TRACING WITH APPLICATIONS TO AFRICA Until recently most seismological data have been explained in terms of spherically symmetric Earth models. Improved recording techniques (e.g. seismic arrays) and the increased number of recording stations have resulted in the discovery of large lateral variations of the seismic velocity in the crust and mantle which are closely correlated with major tectonic features (e.g. ocean ridges and trenches). It has therefore become increasingly necessary to consider laterally. heterogeneous Earth models when inverting seismological data. This part of the thesis is concerned with the development and application of some computer programs to trace seismic rays through laterally heterogeneous velocity media. In the first chapter the basic principles of seismic wave theory and the ray approximation are outlined. This will include: (i) A derivation of the ray path equations for a seismic ray travelling through structures in which the velocity is a function of three dimensions and two dimensions respectively. The velocity structures may be continuously varying and/or defined in discrete units bounded by velocity discontinuities. (ii) A derivation of the equations describing the geometrical spreading effect in velocity structures which are a function of two coordinates. The equations which represent the energy partitioning of a seismic ray at a finite velocity boundary (Zoeppritz's equations) are stated. (iiI) The method of interpolation of the velocity data and the integration procedure. (iv) A brief description of the computer programs. In chapter (2) the two dimensional computer program (i.e. the computer program which traces seismic rays through velocity structures which are a function of two dimensions) is applied to some 'standard' Earth models and in the final chapter this program is used (a) to derive a spherically symmetric velocity depth model from body wave data, which represents the average velocity beneath east and southern Africa, and (b) to test some models of the East African Rift obtained from interpretations of gravity data.