A Multi-physics Alternating Coupled Inversion Using Gravity and Full Waveform Data in Salt Dome

Abstract

Complex salt geometries and strong velocity contrasts pose significant challenges for velocity model building and subsalt imaging. Although full waveform inversion (FWI) provides high-resolution velocity models, its performance strongly depends on the accuracy of initial model. On the other hand, gravity focusing inversion (GFI) can recover compact density distributions and provide reliable long-wavelength structural information for seismic exploration, but it suffers from poor depth resolution and inherent non-uniqueness. To better invert salt structure by leveraging the complementary advantages of full waveform and gravity data, we propose a multi-physics alternating coupled inversion strategy for salt dome model. The proposed strategy mainly includes three parts. First, we perform FWI using a simple layered velocity model to obtain preliminary velocity updates and extract the salt top boundary. Second, this structural information is used as a constraint in GFI to recover a compact salt density distribution beneath the salt top. Third, the resulting salt geometry is used to construct an improved velocity model for the next stage of FWI. Through iterative alternation, FWI provides reliable structural constraints for GFI, while GFI supplies a more reasonable macroscopic salt model for FWI, effectively mitigating the strong dependence on the initial model. In addition, a depth-varying density contrast is introduced in GFI to better represent sediment compaction effects. Compared with unconstrained GFI and conventional FWI using a horizontally layered initial model, the proposed method effectively improves both velocity and density reconstruction in the modified BP salt model and SEG/EAGE salt model.

Figures (14)

• Figure 1: Flowchart of alternating coupled inversion.
• Figure 2: (a) True velocity, and (b) true density of the modified BP salt model. $\rm {W}_{1-1}$ is a hypothetical well used to construct the initial layered model for FWI-Stage-I.
• Figure 3: Measurement data of the modified BP salt model. The color scale of waveform data is clipped between its 5% and 95% quantiles for enhanced visualization. (a) Waveform data for the 25 th source of 50 with 6% Gaussian noise, (b) gravity data $\bm g_{\rm z}$ with 6% Gaussian noise, (c) residual density $\bm \rho_{\rm salt}$ of the salt body.
• Figure 4: FWI-Stage-I of the modified BP salt model. (a) initial velocity model, (b) inversion result, and the white dashed line represents the locally enlarged range in (c) to demonstrate the boundary extraction effect.
• Figure 5: (a) Initial density model, and (b) inversion result of GFI-Stage-I. (c) GFI result without salt top structural constrain.