| Diffraction theory predicts that traveltimes of seismic waves with a finite dominant frequency are affected by heterogeneities within a narrow region around the ray-theoretical path, called Fresnel zone. Because of wavefront healing, heterogeneities whose scale-length is smaller than the size of the Fresnel zone have a reduced effect on the traveltime of a wave. Dahlen et al. (2000) show how is possible to correct for effects of wavefront healing using a new method, referred to as finite-frequency modeling.; In this thesis, we investigate the effects of using finite-frequency modeling in global seismic tomography. To reduce the consequence of the heterogeneous ray path coverage of the earth we also introduce an irregular model parameterization.; To establish the influence of diffraction phenomena on the final tomographic image, we use a high quality long-period data set obtained from Prof Masters (Scripps), that we invert both with standard ray theory and by using finite-frequency modeling. The comparison of the two tomographic images shows that the velocity anomalies are underestimated by 30–60% when interpreted with classical ray theory.; In order to enhance the resolution of the tomographic image even further, we combine the long period arrival times, with the very best high-frequency measurements contained in Prof Engdahl's NEIC data set. A comparison of both data sets, as well as the results from separate inversions, show a high level of compatibility.; All the models show a strong correlation between low velocity anomalies and the location of a large number of known hotspots. The reliability of these anomalies was confirmed by extensive resolution analysis. As a result, we can confidently say that deep mantle plumes are located beneath Ascension, Azores, Canary, Easter, Hawaii, Samoa and Tahiti. Many others, among which is Iceland, are of more shallow origin. Newly discovered plume-like features are clearly visible beneath the Mid-Atlantic Ridge, the South-east Indian ridge, Seychelles and the Coral Sea. |
