Three-dimensional anisotropic full-waveform inversion

Full-waveform inversion (FWI) is a powerful nonlinear tool for quantitative estimation of high-resolution high-fidelity models of subsurface seismic parameters, typically P-wave velocity. A solution is obtained via a series of iterative local linearised updates to a start model, requiring this model to lie within the basin of attraction of the solution space’s global minimum. The consideration of seismic anisotropy during FWI is vital, as it holds influence over both the kinematics and dynamics of seismic waveforms. If not appropriately taken into account, then inadequacies in the anisotropy model are likely to manifest as significant error in the recovered velocity model. Conventionally, anisotropic FWI either employs an a priori anisotropy model, held fixed during FWI, or uses a local inversion scheme to recover anisotropy as part of FWI; both of these methods can be problematic. Constructing an anisotropy model prior to FWI often involves intensive (and hence expensive) iterative procedures. On the other hand, introducing multiple parameters to FWI itself increases the complexity of what is already an underdetermined problem. As an alternative I propose here a novel approach referred to as combined FWI. This uses a global inversion for long-wavelength acoustic anisotropy, involving no start model, while simultaneously updating P-wave velocity using mono-parameter local FWI. Combined FWI is then followed by multi-parameter local FWI to recover the detailed final model. To validate the combined FWI scheme, I evaluate its performance with several 2D synthetic datasets, and apply it to a full 3D field dataset. The synthetic results establish the combined FWI, as part of a two-stage workflow, as more accurate than an equivalent conventional workflow. The solution obtained from the field data reconciles well with in situ borehole measurements. Although combined FWI includes a global inversion, I demonstrate that it is nonetheless affordable and commercially practical for 3D field data.Open Acces

Investigating the use of 3-D full-waveform inversion to characterize the host rock at a geological disposal site

The U.K. government has a policy to dispose of higher activity radioactive waste in a geological disposal facility (GDF), which will have multiple protective barriers to keep the waste isolated and to ensure no harmful quantities of radioactivity are able to reach the surface. Currently no specific GDF site in the United Kingdom has been chosen but, once it has, the site is likely to be investigated using seismic methods. In this study, we explore whether 3-D full-waveform inversion (FWI) of seismic data can be used to map changes in physical properties caused by the construction of the site, specifically tunnel-induced fracturing. We have built a synthetic model for a GDF located in granite at 1000 m depth below the surface, since granite is one of the candidate host rocks due to its high strength and low permeability and the GDF could be located at such a depth. We use an effective medium model to predict changes in P-wave velocity associated with tunnel-induced fracturing, within the spatial limits of an excavated disturbed zone (EdZ), modelled here as an increase in fracture density around the tunnel. We then generate synthetic seismic data using a number of different experimental geometries to investigate how they affect the performance of FWI in recovering subsurface P-wave velocity structure. We find that the location and velocity of the EdZ are recovered well, especially when data recorded on tunnel receivers are included in the inversion. Our findings show that 3-D FWI could be a useful tool for characterizing the subsurface and changes in fracture properties caused during construction, and make a suite of suggestions on how to proceed once a potential GDF site has been identified and the geological setting is known

Low-powered autonomous underwater vehicles for large-scale ocean-bottom acquisition

In this paper we propose the use of autonomous underwater vehicles to enable faster and cheaper ocean bottom seismic acquisition. Our effort is focused on buoyancy-driven vehicles as this method of propulsion is extremely low-powered, enabling the units to have very long endurance and, therefore, making them suited to seismic acquisition on a large scale. We show that, for development-style surveys, these units can potentially double the acquisition efficiency. Furthermore, their ease of deployment makes them suited to acquisitions purposely designed for velocity estimation via full-waveform inversion, either on their own or in hybrid configuration with a streamer vessel