A comparison of ray-based traveltime and waveform inversion results for depth velocity model building in complex fold and thrust belts

Abstract

In onshore fold and thrust belt geology, seismic wave propagation leads to complex waveforms due to large lateral variations in the earth parameters and multiple scattering effects. Moreover, due to acquisition difficulties in mountainous terrain, data coverage can be limited. Although waveform inversion should help in imaging in this context, finding a sufficiently accurate initial model parameter is challenging. Efficient ray-based traveltime inversions could be used when we focus on the early arrivals. However, they rely on a high-frequency assumption that may not be justified in this context. Indeed, significant interferences may occur within the first Fresnel zone. To evaluate the potential challenges, we have inverted 3D passive and 2D active data sets recorded over a fold and thrust belt region in Albania. We compare ray-based high-frequency traveltime and finite-frequency waveform inversion results obtained by inverting local earthquake events and first arrivals from 2D active seismic data. We consider only acoustic propagation. The two results differ significantly. The ray-based traveltime inversion we used does not always give a velocity model that can be used as an initial model for waveform inversion. This could be due to a bias toward the initial model when the data coverage is limited because the inversions have a nonzero null space, or it could be a limitation of the ray-based traveltime inversion that assumes the phases of the picked events are linear in frequency over the considered frequency band.