A 3D Joint Tomographic Inversion of First‐Arrival and Reflection Waves Based On the Adjoint State Method

ABSTRACT Seismic reflection traveltime tomography (RTT) is an effective technique for inverting subsurface low‐frequency velocity models for prestack depth migration and full‐waveform inversion of seismic data. However, the velocity model established using RTT demonstrates limited resolution for extremely shallow and deeply complex strata. Small‐offset first‐arrival wave effectively characterize velocity variations in shallow strata, whereas large‐offset first‐arrival wave can reflect the velocity distribution in deeper strata. Therefore, we propose a three‐dimensional joint tomographic inversion of first‐arrival and reflection waves based on the adjoint state method in this article. The method integrates first‐arrival traveltime data, reflection traveltime data and slope data for inversion, enhancing the accuracy of the inversion model from shallow to deep. The adjoint state method is employed to calculate the gradient of the misfit function with respect to velocity and the spatial coordinates of reflection points, thereby reducing the computational memory requirements and improving the efficiency of velocity modelling. The results of synthetic data tests based on a theoretical model verify the accuracy and effectiveness of the proposed joint inversion method. The proposed method is applied to ocean bottom node and ocean bottom cable wide‐line seismic data collected in an extremely shallow sea in eastern China, yielding a more accurate velocity model.