A bigger picture view of separated wavefields and marine broadband seismic
The advent of dual-sensor recording for towed seismic streamers enabled the true separation of the up-going and down-going pressure wavefields (Carlson et al., 2007), and heralded the ‘broadband’ seismic revolution seen in marine seismic exploration over the past decade (Widmaier et al., 2015). In reality, ‘broadband’ typically means deghosting accompanied by some additional forms of spectral conditioning. In most cases, the post-stack interpretation of deghosted data is enhanced by improved geological texture, improved event coherence and deeper signal penetration. Recent attention has focused on the benefits of low frequencies in broadband seismic data (ten Kroode et al., 2013), and indeed where ultra-low frequency phase integrity is preserved as a complement to accurate amplitude-versus-angle fidelity, deghosted data also enables more accurate pre-stack quantitative interpretation (Reiser et al., 2015a, 2015b). However, it has also become clear that some long-standing imperfections in the seismic method remain. The rapid decay in air gun output below about 7 Hz (Parkes and Hegna, 2011) is not satisfactorily addressed by deghosting, compensation for high-frequency attenuation remains a fundamental challenge, and traditional challenges to wavelet processing, denoise, multiple removal, velocity estimation and imaging may in fact be more complicated for some broadband methods, or unaffected for others. The key benefit of dual-sensor methods are accurate access to the various separated wavefields, in addition to enhanced recoverable frequency bandwidth. Reservoir monitoring is enhanced by the elimination of the down-going pressure wavefield from 4D differencing, reservoir illumination can be enhanced by the incorporation of the down-going pressure wavefield into wave theoretic imaging, reflectivity inversion can be enhanced by isolating the up-going pressure wavefield in imaging, and overall we expect to see ‘complete wavefield’ reservoir imaging and characterization solutions mature rapidly. Furthermore, increased focus on spatial frequency content courtesy of ‘wave equation inversion’ imaging solutions is also taking broadband seismic past the historical focus upon only temporal frequency content.