Waveform inversion for deep sea seismic data with limited-offset condition

Abstract

To alleviate the drawbacks of conventional frequency-domain inversion such as local minima or sensitivity to noise, Laplace-Fourier-domain full waveform inversion is considered one of the most reliable schemes for constructing a ve- locity model. By using a damped wavefield, we can reduce the possibility of converging to local minima to produce an accurate long-wavelength velocity model. Then, we can obtain final inversion results using high-frequency com- ponents and low damping coefficients. However, the imaging area is limited because this scheme uses a damped wavefield that makes the magnitudes of the gradient and residual small in deep water. Generally, the imaging depth of a Laplace-Fourier inversion is less than the half the streamer length. Thus, dealing with seismic data in the deep-sea layer is difficult. The deep-sea layer reduces the amplitude of signals and acts as an obstacle to computing an exact gradient image. To reduce the water layer effect, we extrapolated the wavefield with a downward continuation and performed refraction tomography. Then, we performed a Laplace-Fourier-domain inversion using refraction tomogra- phy results as an initial model. After obtaining a final velocity model, we veri- fied the inversion results using reverse-time migration. We applied this method to both synthetic (Marmousi) and field (Sumatra WG2 line) data. Through the test, we concluded that despite a relatively short streamer length compared to the water depth, the Laplace-Fourier inversion with refraction tomography of the downward-continued wavefield recovers the subsurface structure located below the conventional imaging depth.