博士 / 國立中正大學 / 地震研究所 / 100 / An in-house development of parallel prestack full waveform inversion scheme which utilizes iterative approach for velocity structure investigation and refinement related to gas hydrate exploration at offshore SW Taiwan. The generalized linear inversion uses optimization strategies including steepest descent and conjugate gradient methods. For full waveform inversion (FWI), the initial reference model can be created either from conventional semblance analysis or through tomography study. The computation involved in FWI is based upon adjoint-state wavefield method (AW) by cross-correlating the synthetic forward propagating wave fields with backward propagating residual wave fields. Reverse-time wave field extrapolation is the key computational engine to construct sensitivity kernel and to estimate the amount of velocity perturbation for the final depth-velocity profile.
In study of sensitivity kernels, through synthetic verifications the velocity adjustments are directly associated with predominant frequency and wave-paths which is different from ray-centered approach. Checkerboard tests provide critical test that illustrate the limits on seismically defined scatter size and amount of velocity perturbation. The method is well behaved through tests including the effects related to noise level, near-surface, topography changes, strong lateral velocity variations and arbitrary survey geometries. For illumination/sensitivity analyses of realistic MCS/OBS marine acquisitions, particle velocity records are more sensitive than the pressure records for detecting and updating lateral velocity structure through full waveform inversion.
Field data application is performed for a set of OBS data collected along MCS8810-42 during a pseudo-3D survey around Yuan-An ridge in 2008. Implementation of full waveform inversion is done with dominant frequency of 3-12 Hz. FWI results indicate that BSR is located around 200-500 meter below sea-floor. Average velocity for BSR is 1.7 km/s with free gas velocity varying between 1.3-1.5 km/s. Because of constructive interference and wav-paths effects, the rather closely deployed OBS instrumentations can clearly detected and depicted the lateral velocity variation, unconformity, faults, and BSRs. Three faults intersect at depth of 2 km provide a useful fluid migration paths and form fairly distinct BSR signatures.
| Identifer | oai:union.ndltd.org:TW/100CCU00140008 |
| Date | January 2012 |
| Creators | Li, Kun-Sung, 李坤松 |
| Contributors | Chen, How-Wei, 陳浩維 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 117 |
Page generated in 0.0016 seconds