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

Application of acoustic full waveform inversion to the synthetic {V}alhall velocity model

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Multiparameter full waveform inversion of multicomponent ocean-bottom-cable data from Valhall. Part 2: imaging compressive-wave and shear-wave velocities,

International audienceMultiparameter elastic full waveform inversion (FWI) is a promising technology that allows inferences to be made on rock and fluid properties, which thus narrows the gap between seismic imaging and reservoir characterization. Here, we assess the feasibility of 2-D vertical transverse isotropic visco-elastic FWI of wide-aperture multicomponent ocean-bottom-cable data from the Valhall oil field. A key issue is to design a suitable hierarchical data-driven and model-driven FWI workflow, the aim of which is to reduce the nonlinearity of the FWI. This nonlinearity partly arises because the shear (S) wavespeed can have a limited influence on seismic data in marine environments. In a preliminary stage, visco-acoustic FWI of the hydrophone component is performed to build a compressional (P)-wave velocity model, a density model and a quality-factor model, which provide the necessary background models for the subsequent elastic inversion. During the elastic FWI, the P and S wavespeeds are jointly updated in two steps. First, the hydrophone data are inverted to mainly update the long-to-intermediate wavelengths of the S wavespeeds from the amplitude-versus-offset variations of the P-P reflections. This S-wave velocity model is used as an improved starting model for the subsequent inversion of the better-resolving data recorded by the geophones. During these two steps, the P-wave velocity model is marginally updated, which supports the relevance of the visco-acoustic FWI results. Through seismic modelling, we show that the resulting visco-elastic model allows several P-to-S converted phases recorded on the horizontal-geophone component to be matched. Several elastic quantities, such as the Poisson ratio, and the ratio and product between the P and S wavespeeds, are inferred from the P-wave and S-wave velocity models. These attributes provide hints for the interpretation of an accumulation of gas below lithological barriers

Building starting model for full waveform inversion from wide-aperture data by stereotomography

International audienceBuilding reliable starting model remains one of the most topical issue for successful application of full waveform inversion (FWI). In this study, we assess the stereotomography as a tool to build reliable starting model for frequency-domain FWI from long offset (i.e., wide-aperture) data. Stereotomography is a slope tomography method based on the use of traveltimes and slopes of locally-coherent events in the data cube. We assessed a tomographic workflow based on stereotomography and frequency-domain FWI on the 2D acoustic synthetic Val- hall case study. We first computed an acoustic full-wavefield dataset using a finite-difference time-domain modeling engine for a wide-aperture survey with a maximum offset of 24 km. The source bandwidth is between 10 and 45 Hz. Compared to a conventional application of stereotomography, we investigate in this study the benefits provided by the joint inversion of refraction and reflection traveltimes from long-offset data. The use of refraction traveltimes is expected to stabilize and improve the reconstruction of the shallow part of the model. In a similar way as for frequency-domain FWI, we design a multiscale approach which proceeds hierarchically from the wide-aperture to the short-aperture angles to mitigate the non linearity of the inversion. The starting models for FWI were built by stereotomography using three sets of picked events. For the first data set, the picking is limited to reflection travel- times for a maximum offset of 4 km. For the second data set, both refracted and reflected events were picked using an acquisition with a maximum offsets of ± 16 km. In a third test, we extended the maximum offsets to ± 24 km offset for first- arrival travel time picking. We highlight the improvements of the FWI results obtained from the starting stereotomographic model built from the long-offset data set. The improvements due to the use of long offsets data are observed at the reservoir level below the gas layers but also in the upper part of the model where the joint use of refraction and reflection travel- times is helpful to improve the ray illumination. ©2010 Society of Exploration Geophysicist

Application of acoustic full waveform inversion to the synthetic {V}alhall velocity model

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Multiparameter full waveform inversion of multicomponent ocean-bottom-cable data from Valhall. Part 1: imaging compressional wavespeed, density and attenuation,

International audienceMultiparameter full waveform inversion (FWI) is a challenging quantitative seismic imaging method for lithological characterization and reservoir monitoring. The difficulties in multiparameter FWI arise from the variable influence of the different parameter classes on the phase and amplitude of the data, and the trade-off between these. In this framework, choosing a suitable parametrization of the subsurface and designing the suitable FWI workflow are two key methodological issues in non-linear waveform inversion. We assess frequency-domain visco-acoustic FWI to reconstruct the compressive velocity (VP), the density (ρ) or the impedance (IP) and the quality factor (QP), from the hydrophone component, using a synthetic data set that is representative of the Valhall oil field in the North Sea. We first assess which of the (VP, ρ) and (VP, IP) parametrizations provides the most reliable FWI results when dealing with wide-aperture data. Contrary to widely accepted ideas, we show that the (VP, ρ) parametrization allows a better reconstruction of both the VP, ρ and IP parameters, first because it favours the broad-band reconstruction of the dominant VP parameter, and secondly because the trade-off effects between velocity and density at short-to-intermediate scattering angles can be removed by multiplication, to build an impedance model. This allows for the matching of the reflection amplitudes, while the broad-band velocity model accurately describes the kinematic attributes of both the diving waves and reflections. Then, we assess different inversion strategies to recover the quality factor QP, in addition to parameters VP and ρ. A difficulty related to attenuation estimation arises because, on the one hand the values of QP are on average one order of magnitude smaller than those of VP and ρ, and on the other hands model perturbations relative to the starting models can be much higher for QP than for VP and ρ during FWI. In this framework, we show that an empirical tuning of the FWI regularization, which is adapted to each parameter class, is a key issue to correctly account for the attenuation in the inversion. We promote a hierarchical approach where the dominant parameter VP is reconstructed first from the full data set (i.e. without any data preconditioning) to build a velocity model as kinematically accurate as possible before performing the joint update of the three parameter classes during a second step. This hierarchical imaging of compressive wave speed, density and attenuation is applied to a real wide-aperture ocean-bottom-cable data set from the Valhall oil field. Several geological features, such as accumulation of gas below barriers of claystone and soft quaternary sediment are interpreted in the FWI models of density and attenuation. The models of VP, ρ and QP that have been developed by visco-acoustic FWI of the hydrophone data can be used as initial models to perform visco-elastic FWI of the geophone data for the joint update of the compressive and shear wave speeds

Building starting models for full waveform inversion from wide-aperture data by stereotomography,

International audienceBuilding an accurate initial velocity model for full waveform inversion (FWI) is a key issue to guarantee convergence of full waveform inversion towards the global minimum of a misfit function. In this study, we assess joint refraction and reflection stereotomography as a tool to build a reliable starting model for frequency-domain full waveform inversion from long-offset (i.e., wide-aperture) data. Stereotomography is a slope tomographic method that is based on the inversion of traveltimes and slopes of locally-coherent events in a data cube. One advantage of stereotomography compared to conventional traveltime reflection tomography is the semi-automatic picking procedure of locally-coherent events, which is easier than the picking of continuous events, and can lead to a higher density of picks. While conventional applications of stereotomography only consider short-offset reflected waves, we assess the benefits provided by the joint inversion of reflected and refracted arrivals. Introduction of the refracted waves allows the construction of a starting model that kinematically fits the first arrivals, a necessary requirement for full waveform inversion. In a similar way to frequency-domain full waveform inversion, we design a multiscale approach of stereotomography, which proceeds hierarchically from the wide-aperture to the short-aperture components of the data, to reduce the non-linearity of the stereotomographic inversion of long-offset data. This workflow which combines stereotomography and full waveform inversion, is applied to synthetic and real data case studies for the Valhall oil-field target. The synthetic results show that the joint refraction and reflection stereotomography for a 24-km maximum offset data set provides a more reliable initial model for full waveform inversion than reflection stereotomography performed for a 4-km maximum offset data set, in particular in low-velocity gas layers and in the deep part of a structure below a reservoir. Application of joint stereotomography, full waveform inversion and reverse-time migration to real data reveals that the FWI models and the reverse-time migration images computed from the stereotomography model shares several features with FWI velocity models and migrated images computed from an anisotropic reflection-traveltime tomography model, although stereotomography was performed in the isotropic approximation. Implementation of anisotropy in joint refraction and reflection stereotomography of long-offset data is a key issue to further improve the accuracy of the method

Two-dimensional Anisotropic Visco-elastic Full Waveform Inversion of Wide-aperture 4C OBC Data from the Valhall Field

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Application of 2D acoustic frequency-domain full-waveform inversion to OBC wide-aperture data from the Valhall field

International audienceWe present an application of 2D acoustic frequency-domain Full Waveform Inversion (FWI) to the hydrophone component of 4-C ocean bottom cable (OBC) data recorded from the Valhall field in North sea. The starting model for FWI was built by reflection traveltime tomography (RTT). Although this starting model leads to flat common-image gathers (CIGs), it does not allow us to match first-arrival traveltimes of diving waves from above the gas layers. This mismatch between vertical and horizontal velocities is likely the footprint of anisotropy. We updated the RTT model by first-arrival traveltime tomography (FATT) to build a new starting model for FWI. The velocities above the gas layers of the updated model are significantly higher than velocities from in-well seismic (VSP) data. FWI models were computed from the two starting models just mentioned. More stable results were obtained with the starting model updated by FATT. The resulting FWI model shows a reasonable agreement with a former model developed by 3D FWI. A reasonable match of both short-aperture and wide- aperture components of the data was obtained by isotropic FWI. This might indicate that layer-induced anisotropy was created by FWI in the gas layers to balance the increase of the shallow velocities created by the inversion of the wide-aperture data components. ©2010 Society of Exploration Geophysicist

Which parameterization is suitable for acoustic vertical transverse isotropic full waveform inversion ? : Part 2. Synthetic and real data case studies from Valhall

International audienceIt is necessary to account for anisotropy in full waveform inversion (FWI) of wide-azimuth and wide-aperture seismic data in most geologic environments, for correct depth positioning of reflectors, and for reliable estimations of wave speeds as a function of the direction of propagation. In this framework, choosing a suitable anisotropic subsurface parameterization is a central issue in monoparameter and multiparameter FWI. This is because this parameterization defines the influence of each physical parameter class on the data as a function of the scattering angle, and hence the resolution of the parameter reconstruction, and on the potential trade-off between different parameter classes. We apply monoparameter and multiparameter frequency-domain acoustic vertical transverse isotropic FWI to synthetic and real wide-aperture data, representative of the Valhall oil field. We first show that reliable monoparameter FWI can be performed to build a high-resolution velocity model (for the vertical, the horizontal, or normal move-out velocity), provided that the background models of two Thomsen parameters describe the long wavelengths of the subsurface sufficiently accurately. Alternatively, we show the feasibility of the joint reconstruction of two wave speeds (e.g., the vertical and horizontal wave speeds) with limited trade-off effects, while Thomsen parameter δ is kept fixed during the inversion. The influence of the wave speeds on the data for a limited range of scattering angles when combined each other can, however, significantly hamper the resolution with which the two wave speeds are imaged. These conclusions inferred from the application to the real data are fully consistent with those inferred from the theoretical parameterization analysis of acoustic vertical transverse isotropic FWI performed in the companion report. Read More: http://library.seg.org/doi/abs/10.1190/geo2012-0203.