2D Elastic Full Waveform Inversion as a Tool for Exploring Gas Hydrates and Investigating Fluid Flow on the Southern Hikurangi Margin, New Zealand

Abstract

New Zealand’s North Island marks the plate boundary where the Pacific plate subducts westward beneath the Australian plate. The 25 Ma Hikurangi Margin epitomizes the variability and geologic complexity of subduction zones. The Hikurangi Margin hosts significant gas hydrates and a range of seismic events. The mechanisms controlling gas hydrates formation and seismicity are often related to fluid flow. In recent years, seismic attributes and velocity analysis of active source seismic data have often been used to infer the subsurface elastic properties. However, traditional velocity analysis and ray-base tomography approaches produce velocity models that usually cannot resolve fine-scale features because of their resolution and accuracy limitations. Full waveform inversion (FWI) is a powerful alternative that uses phase and amplitude information to produce geologically realistic high-resolution physical models of the earth. In this thesis, I apply 2D elastic FWI on two multichannel seismic profiles crossing the southern Hikurangi Margin, New Zealand to shed light on the “plumbing” system of gas hydrates and to investigate fluid flow within the Hikurangi subduction zone. The FWI processing sequence includes: (1) downward continuation of the seismic streamer data, (2) 2D traveltime tomography, and (3) FWI of refracted and reflected energy of both the downward continued and surface seismic reflection data. Gas hydrates are ice-like crystalline materials that require moderate pressure, low temperature and sufficient gas supply to form in submarine environments. Detailed imaging and velocity analysis of the plumbing system of gas hydrates can provide confidence that amplitude anomalies in seismic data are related to gas hydrate accumulations. I conducted FWI along a 14-km long segment of a 2D multichannel seismic profile to: (1) obtain a high-resolution velocity model of a hydrate system on the southern Hikurangi Margin and (2) test the method against conventional velocity analyses by comparing the FWI velocity model to previously published semblance- and tomography-based velocity models from the same data. The FWI yielded a structurally more accurate velocity model that better delineated the low velocity zone associated with free gas beneath the bottom simulating reflector compared to the semblanceand tomography-based velocity models. The results also show a lateral velocity inversion, i.e., a narrow low-velocity zone surrounded by bands of higher velocities at a seaward-verging proto-thrust fault, which the two other methodologies failed to resolve. The FWI result showcased a major benefit of the method that is producing velocity models with an improved lateral resolution making it an important tool when imaging the “plumbing” systems of gas hydrate reservoirs. In the southeastern limb of the anticline, the FWI results show that the closely spaced landward-vergent proto-thrusts provide gas-charged fluids for hydrate formation above the bottom simulating reflector. Moreover, at the centre of the anticline, my results show that a seaward-vergent proto-thrust fault appears to be acting as a conduit for gasrich fluids into strata, though there is no accumulation of any significant hydrate above the bottom simulating reflector at the apex of the anticline. This finding emphasizes the significance of densely spaced faults and fractures for providing gas for hydrate formation in the hydrate stability zone. I also performed FWI on a second seismic line on the southern Hikurangi Margin to investigate upper plate dewatering and fluid flow. The FWI produced a structurally accurate, highresolution velocity model, and reveals widespread zones of anonymously low velocities in the overthrusting plate (~300 to ~600 m/s lower than neighbouring strata). Near the deformation front, an anticline within the incoming trench sediments extends down to the décollement and is characterized by low velocities, suggestive of upward migration of gas-rich fluids towards the gas hydrates stability zone. Vertical streaks of high and low velocities in the upper part of the anticline may reflect fluid migrating into and through the gas hydrate stability zone. The décollement is represented by an interface with a high impedance contrast separating Miocene sediments above from a highly compacted section of Late Cretaceous-Early Oligocene sediments and the basement of the Chatham Rise. The velocity character of this interface and the underlying sediments reveal that it is over-pressurized within the Hikurangi Trough, but well drained at the Chatham Rise. The results also show that overlying Miocene sediments do not act as a seal in this region as was previously thought. At the Chatham Rise, I suggest that the anonymously high velocities present at the walls of the faults are most likely due to a diagenetic process but we cannot rule out alternative mechanisms. Furthermore, my results provide evidence of lateral fluid migration and show that faults are the preferred paths for dewatering. My results demonstrated that the 2D elastic FWI is ideal for investigating lateral velocity variations at high resolution because of its use of wide-angle arrivals (refractions, in particular) making it a robust tool in imaging the “plumbing” system of gas hydrate systems as well as in shedding light on fluid flow in subduction zones.