Seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the land surface or seafloor. Among other things, seismic data acquisition involves the generation of acoustic waves and the collection of reflected/refracted versions of those acoustic waves to generate the image. This image does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process in the field of seismic surveying.
A significant problem in marine-based seismic data analysis involves how to compensate for ghost effects associated with the free-surface interactions of the acoustic waves which are generated to image the subsurface. Considering first the so-called receiver ghosts, in marine-based seismic data acquisition the up-going acoustic waves reflected from subsurface reflectors are first recorded by the receivers. Next, the acoustic waves continue to propagate to the surface where they are reflected back down and are recorded again by the receivers as ghosts. The reflectivity at the free surface is close to negative one and, based on this property, the down-going acoustic waves have similar amplitudes as the previously described up-going acoustic waves but with an opposite polarity. Accordingly, some of the frequencies in the recorded acoustic wave data are attenuated near the ghost notches and the removal of the receiver ghosts can provide the benefit of infilling the ghost notches and providing higher quality images in terms of frequency band and signal-to-noise ratio.
Similarly, sources create ghosts of their own. For example, when a shot is fired by a source, there is both a direct wave which emanates directly from the source toward the subsurface being imaged, and a wave which is reflected from the ocean's surface toward the subsurface.
Based on both low and high frequency requirements for imaging subtle geologic features, interest has developed for widening the seismic bandwidth associated with marine data acquisition. Bandwidth limitations caused by source and receiver ghosts represent one of the major obstacles to accomplishing the goal of wider seismic bandwidth. The ghosts, both source and receiver, generated by the free surface reflection are angle dependent effects which change both the amplitude and the phase of the wavelets being recorded.
A number of recent efforts for ghost compensation have focused on a combination of acquisition and processing methodologies. For example, R. Soubaras and P. Whiting in their 2011 article entitled “Variable Depth Streamer—The New Broadband Acquisition System,” published in the 81st Annual International Meeting, SEG, Expanded Abstracts, pages 4349-4353 and incorporated herein by reference, describes variable depth streamer data acquisition. Further, R. Soubaras in his 2010 article entitled “De-Ghosting by Joint Deconvolution of a Migration and a Mirror Migration,” published in the 81st Annual International Meeting, SEG, Expanded Abstracts, pages 3406-3410 and incorporated herein by reference and R. Soubaras and Y. Lafet in their 2011 article entitled “Variable-Depth Streamer Acquisition: Broadband Data for Imaging and Inversion,” published in the 81st Annual International Meeting, SEG, Expanded Abstracts, pages 2364-2368 and incorporated herein by reference, describe variable receiver depth which introduces ghost notch diversity that can be handled by new processing techniques. The aforementioned techniques provide a high quality broadband image.
Other efforts have revolved around widening the bandwidth on conventionally acquired marine data, i.e., receivers located at approximately constant shallow depths. These efforts include compensating for the source and receiver ghost before migration as described by P. Wang and C. Peng in their 2012 article entitled “Premigration Deghosting for Marine Towed Streamer Data Using a Bootstrap Approach,” published in the 82nd Annual International Meeting, SEG, Expanded Abstracts, ACQ 4.4 and incorporated herein by reference, and compensating for the source and receiver ghost during migration as described by Y. Zhang, G. Roberts and A. Khalil in their 2012 article entitled “Compensating for Source and Receiver Ghost Effects in Reverse Time Migration,” published in the 82nd Annual International Meeting, SEG, Expanded Abstracts, SPMI 3.5 and incorporated herein by reference.
Despite these previous efforts it is still desirable to develop new and improved techniques for compensating for the effects of receiver and/or source ghosts in received seismic data in order to improve the image of the subsurface which is developed for a target area in order to better identify regions for potential natural resource exploration.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks associated with bandwidth limitations caused by source and receiver ghosts.