- Title
- Using ambient noise tomography to image the Eastern Cape-Karoo and Karoo regions, South Africa
- Creator
- Bezuidenhout, Lucian John-Ross
- Subject
- Geometric tomography -- South Africa
- Subject
- Geobiology Geology -- South Africa
- Date Issued
- 2018
- Date
- 2018
- Type
- Thesis
- Type
- Doctoral
- Type
- DPhil
- Identifier
- http://hdl.handle.net/10948/23602
- Identifier
- vital:30585
- Description
- The primary aim of this thesis was to explore the use of ambient seismic noise as a tool to map the subsurface of the Cape-Karoo and Karoo region of South Africa. The Karoo is an ideal laboratory to use ambient seismic signal to map the shallow subsurface, as it is a quiet and pristine environment with a relatively well known geology. Ambient seismic signals were continuously recorded at three independent networks (ArrayA, ArrayB and ArrayC). ArrayA and ArrayB comprised 17 temporary stand-alone seismic stations each and recorded ambient noise wavefields for a ten week period between August and October 2015. ArrayC comprised 19 temporary stand-alone seismic stations, recording ambient seismic noise for a period of six weeks between June and July 2016. ArrayA and ArrayB were installed in the south-eastern Cape-Karoo region, near the town of Jansenville and ArrayC was installed near the Cradock-Tarkastad region of South Africa. This thesis is made up of two separate studies. Firstly, the retrieval and coherency of Rayleigh surface waves extracted from the vertical component recordings. For the first time in the south-eastern Cape-Karoo and Karoo area, estimates of Green’s function from cross-correlating ambient noise data between stations pairs were reconstructed and shown, which can be successfully used to image the subsurface. The stacked cross-correlations between all station pairs show clear arrivals of the Rayleigh surface waves. The group velocities of the Rayleigh waves in the 2 to 7 seconds period range were picked and inverted to compute the 2-D group velocity maps. For ArrayA and ArrayB, the resulting 2-D group velocity maps at different periods resulted in a group velocity model from approximately 2 to 7 km depth, which generally show a high velocity anomaly in the north of the study area, most likely imaging the denser, thick sedimentary basin of the Karoo (Carboniferous-Permian). To the south, the low velocity anomaly likely corresponds to the overlying Jurassic- Cretaceous sequences of the younger Algoa Basin (Uitenhage Group). For ArrayC, the group velocity maps showed high velocity regions, which is consistent with the dolerite sill intrusions in the Karoo and the low velocity structures, which was interpreted as the Karoo sediments. Secondly, the study comprised of characterizing the ambient seismic noise source. The first order analysis of the symmetry of the cross-correlation function showed that, although the ambient noise sources are relatively homogeneously distributed in the study area, most (energetic) of the ambient seismic noise propagates from the coast of South Africa. This was verified by analyzing the azimuthal distribution of the ambient seismic noise.
- Format
- xv, 170 leaves
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
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