Research And Application Of Broadband Marine Seismic Exploration

For the reason of ghost, amplitude attenuation and dispersion, conventional marine seismic exploration suffers from several problems of low-frequency loss,notches, high-frequency attenuation, which make it unable to acquire sufficient reflection from the target layer accurately. In order to achieve high precision and high resolution imaging, this paper deeply research about broadband marine. We propose some solutions about broadband numerical simulation, deghosting,low-frequency reconstruction, high-frequency attenuation compensation and broadband acquisition strategy. Examples of numerical and field data prove our theory.This paper propose the complicated structure grid discretization and high precision spectral element numerical simulation technology. It combines the precision of pseudospectral method and the flexibility of finite-element method. First of all, the simulation area is split into a number of triangular meshes based on Delaunay triangular mesh method. Then by using indirect method, frontal method and isoparametric transformation, we can get quadrilateral meshes with uniform size and high quality. The meshes can flexibly adapt to changes of speed border. It provides forward operator and theory for broadband marine processing and interpretation.Aiming at conventional acquisition data, this paper summarizes deconvolution and inverse scattering series method and proposes the idea of deghosting method based on Green theory. By using Fresnel principle and “Double Dirichlet” boundary conditions, we can predict the pressure and vertical velocity fields at any depth.Without any need of subsurface information, it can successfully achieve data-driven ghost removal. The band especially the low-frequency component is broadened.Besides, stacking process suppresses random noise and improves the signal-noise ratio(SNR) effectively.Due to seismic wave dispersion, it is known that the amplitude will show strong attenuation when seismic signal penetrates through underground media. This paper studies methods of inverse Q filtering and spectral whitening and propose the idea of high-frequency attenuation compensation method based on Synchrosqueezing wavelet transform(SWT), it focus the time-frequency representation by squeezing its value along the frequency axis and offers an exact reconstruction formula for signals.The method broad the band and improve the resolution without destorying the energy relationship between relative phase axis.Ghost and amplitude attenuation will cause the loss of low-frequency energy.This paper combines sparse inversion with compressed sensing, and it recovers full frequency band. The low-frequency signal can be reconstructed with improved energy.In addition to broadband data processing, we focus on the research of broadband acquisition(over/under streamers, dual-sensor and variable-depth streamer) to acquire original broadband data from the beginnings.Over/under streamers can take full advantages of over streamer’s wide band,high resolution and under streamer’s strong low-frequency energy, high SNR. By wavefield separation, the upgoing wavefield without ghost can be calculated. We derive “dephase and sum”(DPS), “Optimized deghosting”(ODG), “Shift and subtract”combination method. The combination of over/under streamers data depends on the combination operator and acquisition environment. Reversly thinking,based on the actual demands and acquisition conditions, selecting the towing depth of the couple streamer cables determines the pros and cons of acquisition effect. The results demonstrate a good effect.Dual-sensor acquisition makes full use of the different frequency, amplitude and phase output of geophone and hydrophone. As the only way to measure both pressure and vertical velocity fields, it gets enough stringency. Also a deghosting method based on Green theory is deeply discussed, without any need of subsurface information, it can successfully achieve dual-sensors combination and adapt for complicated marine topography and geology.Variable-depth streamer allows receivers towed at a continuous depth, and it introduces several various ghosts, the seismic profiles can get wider band after deghosting. Variable-depth streamer contains adequate low frequency information.The events are continuous, and stratigraphic contact relationship is reasonable.This paper first propose a data merge strategy for fluctuate double streamers in frequency domain. By estimating the ghost operator in every seismic hydrophone,the new strategy self-adaptively merges the two streamer data and reduces the equivalent notch effect to a theoretical minimum, while the information of all receiver points are utilized to improve the resolution and signal-noise ratio of merged results. For the case of singular ghost operator, the white noise coefficient is introduced and the stability of the calculation is improved. Variable-depth streamer could also be combined with a shallow streamer to strengthen its high-frequency energy, which provides a new thinking and well supplement in real high precision marine seismic exploration.