Velocity building using full waveform inversion for deep hot dry rock

Key words:  hot dry rock  high-temperature granite body  full waveform inversion  velocity building for deep structure  local coherence

Abstract:

Severe seismic signal attenuation with increasing exploration depth renders conventional velocity building methods less accurate for high-precision exploration of deep hot dry rock (HDR) bodies. To address this challenge, the full waveform inversion (FWI) method is used to enhance the accuracy of velocity building. However, FWI encounters issues like slow convergence, limited inversion depth, and susceptibility to local optimal solutions due to constraints imposed by local optimization algorithms. To mitigate these challenges, we introduce a novel approach that utilizes preconditioning and smoothing gradients in conjunction with the conjugate gradient optimization algorithm. This strategy aims to rectify unbalanced illumination issues and speed up convergence. Additionally, to alleviate the issue of local minima, a local coherence misfit is integrated into FWI to update the velocity model. Numerical simulations conducted on typical HDR models show that the proposed method is capable of circumventing the negative effects of cycle skipping issues, ensuring stable iterative updating even with highly inaccurate initial model. Furthermore, the method significantly improves the depth and accuracy of inversion, providing a more precise depiction of high-velocity rock bodies, eventually obtaining high-accuracy velocity models in both shallow and deep layers. The FWI method proposed in this study provides a practical and efficient velocity building tool for detecting deep high-temperature granite bodies.