Jinli Li

Prismatic and full-waveform joint inversion

Prismatic wave is that it has three reflection paths and two reflection points, one of which is located at the reflection interface and the other is located at the steep dip angle reflection layer, so that contains a lot of the high and steep reflection interface information that primary cannot reach. Prismatic wave field information can be separated by applying Born approximation to traditional reverse time migration profile, and then the prismatic wave is used to update velocity to improve the inversion efficiency for the salt dame flanks and some other high and steep structure. Under the guidance of this idea, a prismatic waveform inversion method is proposed (abbreviated as PWI). PWI has a significant drawback that an iteration time of PWI is more than twice as that of FWI, meanwhile, the full wave field information cannot all be used, for this problem, we propose a joint inversion method to combine prismatic waveform inversion with full waveform inversion. In this method, FWI and PWI are applied alternately to invert the velocity. Model tests suggest that the joint inversion method is less dependence on the high and steep structure information in the initial model and improve high inversion efficiency and accuracy for the model with steep dip angle structure.

Multi-scale full waveform inversion for areas with irregular surface topography in an auxiliary coordinate system

For land exploration areas with irregular surface topography, there are many challenges and problems for full waveform inversion (FWI); for example, which type of wave equation should be used to calculate high-accuracy seismic wavefields, how to deal with diffraction of irregular surface topography, what initial velocity model should be utilised to improve the inversion accuracy and how to enhance the computational efficiency of iterative FWI. Aiming at these difficulties, we first simulate the seismic waves with the first-order acoustic wave equation in an auxiliary coordinate system, which easily describes irregular surface topography. Then, we apply this wavefield simulation frame to FWI to improve inversion quality of near-surface regions with strong elevation and velocity variation. Furthermore, to enhance the robustness and computational efficiency, a time-domain multi-scale decomposition method based on the Wiener filter and an optimised encoding strategy are introduced to the proposed inversion frame, and are critical to promoting the practical application of our method. Typical numerical tests prove that the proposed method can obtain more accurate inversion results than the traditional time-domain FWI. We implement full waveform inversion in an auxiliary coordinate system to improve inversion quality of near-surface regions with strong elevation and velocity variation. Furthermore, a time-domain multi-scale decomposition method and an optimised encoding strategy are introduced to the inversion frame to promote the practical application of our method.

An elastic full-waveform inversion based on wave-mode separation

Multi-parameter elastic full-waveform inversion (EFWI) attempts to find high resolution model parameters that are able to match observed data exactly by minimising residuals between the observed and predicted data. However, the coupling of Vp and Vs, and the cross-talk artefacts between P- and S-wave modes increase non-uniqueness and ill-conditionedness. We propose a new EFWI method based on P- and S-wave mode separation to mitigate these problems. In this method, we derive the gradient formulas with respect to various wave modes using a P- and S-wave mode separated first-order velocity-stress wave equation, and use a step search method in subspace to calculate the corresponding step lengths. The algorithm, called wave-mode separation EFWI (SEFWI), appears to be helpful to weaken non-uniqueness and ill-conditionedness of conventional EFWI by decoupling multiple parameters. Numerical examples conducted with a synthetic dataset modelled on a simple model with anomalies reveal that SEFWI can reduce the cross-talk artefacts between P- and S-wave modes. Synthetic tests on the Marmousi2 model demonstrate that SEFWI yields better inversion results than conventional EFWI. Although the computational cost of SEFWI per iteration is 1.81 times as much as that of EFWI, the total computational cost is almost at the same level, because of its faster convergence rate. We propose an elastic full-waveform inversion method based on P- and S-wave mode separation to mitigate the crosstalk artefacts between P- and S-wave modes by deriving the gradient formulas with respect to various wave modes using a P- and S-wave mode separated first-order velocity-stress wave equation.