Miko Fohrmann
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Miko Fohrmannemail: fohmi830@student.otago.ac.nz SupervisorsDr. Andrew R. Gorman See also: Dr. Ingo A. Pecher |
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PhD title
Examination of marine seismic reflection data for the occurrence and distribution of gas hydrates on the continental margins of New Zealand
Brief summary of research topic
Gas hydrate, an ice-like compound of water and gas molecules, trapped in marine sediments stores immense amounts of methane, which must be taken carefully into account as a climate-relevant greenhouse gas and as a possible cause of seafloor instability. In addition, methane from gas hydrates could play a dominant role as a major energy resource in the future. So far, the origin and the natural controls on hydrates and their impacts on the environment are poorly understood.
The occurrence of gas hydrates is most often indicated by the geophysical registration of a bottom-simulating reflection (BSR). The BSR is a seismic reflection which can be recognised by a negative reflection-coefficient. It evolves at a boundary between shallow sediments that contain gas hydrates and deeper sediments storing free methane gas. The existence of free gas below the BSR greatly reduces the seismic velocity in those sediments, thereby creating a seismic reflection at the boundary. Further, it is characteristic for BSR structures to follow isotherms which are nearly parallel to the morphology of the sea floor, as opposed to following a stratigraphic horizon (Figures 1&2).
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Figure 1: Brute stack of a six channel seismic profile acquired by R/V L’Atalante in 1993 on the GeodyNZ-Sud cruise (normal-moveout velocity used: 1500-1700 m/s). |
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Figure 2: Instantaneous amplitude section of Figure 1, after a broad bandpass (1/5-150/200 Hz) was applied. |
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Figure 3: Fiordland research area and the different surveys that are covering it. The numbers indicate line numbers (yellow & red) and shotpoints (green), respectively. |
Nowadays, methane -and therefore gas hydrates as one of the major sources- is increasingly held responsible for climate changes and mass extinctions of flora and fauna in earth's history. In the event of a fast destabilisation of gas hydrates, the free gas may become an important agent of climate change. For example, releases of methane from hydrates seem to have played a driving force in rising temperatures since the last glacial maximum.
Recent studies show that free methane gas below the BSR migrates upwards through overlying sediments. Therefore an aim of the project will be to examine the dynamics of methane migration in New Zealand's margins. Any free gas that escapes into the ocean and further on into the atmosphere would have a dominant impact on the environment.
Additionally, knowledge of the distribution of gas hydrates will contribute to natural hazard prevention. Recent investigations indicate that hydrate instability may cause subsea landslides on the continental slope and therefore must be considered as a possible triggering mechanism for tsunamis.
My PhD project will investigate the occurrence and distribution of gas hydrates on the continental margins of Fiordland (southwest of New Zealand's South Island) and Hikurangi (east of New Zealand's North Island). An extensive marine seismic reflection data set covering those areas already exists, acquired by various government and research agencies and the oil industry (Figure 3). They will be reprocessed with the GLOBE Claritas software. By evaluating the data, it is possible to compute a 3D model of the underground. Hence, the gas hydrate resources on New Zealand's continental shelves will be estimated. Further, the impact of gas hydrates on the slope stability on convergent margins and the mechanisms controlling gas hydrate formation will be investigated.
In summary, the goals of this project are twofold: first to locate and delineate new hydrate deposits and second to understand the interaction of sub-seafloor methane with the ocean.