oa Broadband Marine Seismic, Does Acquisition Make a Difference?

Expanding the bandwidth of surface seismic data, particularly towards low frequencies, is essential for many exploration and production objectives. Broader band signals, both in land and marine environments have marked benefits for imaging deeper targets, imaging through absorptive overburdens, and especially inversion for rock properties. Various methods have been proposed and implemented to expand seismic bandwidth; these include both acquisition and signal processing methods. A question that is often asked is how much difference does changing the acquisition geometry make? In this paper, we present a case study of a consistent, experimental offshore dataset in Southeast Asia. This data consists of a single boat pass of different cable depth configurations. These data were then processed with their appropriate deghosting methods and results compared. In addition, we examine methods for evaluating the success of these methods and their potential pitfalls. The key determinant of the eventual bandwidth of surface seismic data is the convolution of the source, near surface effects (free surface ghosts in the marine situation), the overall earth attenuation, and the level of additive environmental noise. Some of these effects can be modified by changes in the field acquisition geometry or at least, deterministically compensated for. The noise level may constrain or limit the capacity of signal processing tools to compensate for the “field” effects. When evaluating the raw and processed data it is wise to use various types of analysis and displays. Simple seismic amplitude stack sections and associated spectra can be misleading. Spectral “split” plots and inversion of the seismic data is often more indicative of success. In marine seismic acquisition, the free-surface ghost effect is one factor that can strongly impact the data bandwidth characteristics according to streamer and source depth. It is also a factor that could easily be adjusted in the survey design. Shallow towing favors the higher frequency response at the expense of low frequencies, whilst deeper towing favors the lower frequencies, at the expense of higher frequencies. Moreover, a deeper tow typically has lower levels of swell noise. At very low frequencies, the critical two octaves between two to eight Hertz, the level of ambient towing noise rises, the “DC notch” strengthens, and the power output of airgun sources declines. This combination provides both the biggest challenge and the opportunity of bandwidth expansion.