Figure 5.42: Comparison of conventional (left) and BroadSeis data stacks (right). Variable-depth streamer data show better top of basement, better fault planes, clear indication of stratigraphic unit, and intra-basement reflections. From Zhang et al. (2013)


to the seismic signal that shapes the larger-scale impedance variations, or in geological terms, the major lithology variations. Tis increases confidence in correlating seismic interpretation across faults and other major structural features, allowing layers to be easily distinguished. Te Santos Basin data (Figure 5.40) demonstrate these advantages by providing clear differentiation of sedimentary packages and high-resolution shallow images, as well as clear imaging of the base of salt and the pre-salt sequences. Tree octaves of signal are generally con sidered necessary


for adequate seis mic resolution. Terefore, in order to achieve sufficient bandwidth in areas where high frequencies are limited, such as below salt where frequencies over 20 Hz tend not to penetrate, it is necessary to extend the low frequencies, which are affected less by attenuation and provide greater accuracy and stability for seismic inversion. Conventional marine streamer acquisition usually lacks sufficient signal-to-noise ratio in the 2–7 Hz bandwidth due to streamer depth, streamer tow noise, source array configuration, source depth and source bubble. Using BroadSeis, it is possible to routinely record frequencies with good signal-to-noise ratio down to 2.5–3 Hz, providing three octaves below 20 Hz as shown in Figure 5.41.


Figure 5.43: Amplitude spectra gain of a synchronised two-level source array with airguns at 6m and 9m, showing how the source ghost notch becomes filled in.


BroadSource two- layer source


Conventional one-layer source


Te uses of broadband seismic in exploration are not limited


to deep waters. CGG has acquired BroadSeis data in shallow water in the North Sea, offshore Vietnam and China, in water depths as shallow as 37.5m. In these areas the streamer shape is adapted to fit the circumstances, raising the deeper part of the streamer as the water gets shallower. Figure 5.42 shows data recorded offshore China in water depths varying from 85 to 115m, which provide excellent results, both in the shallow detail and in the fractured basement.


5.7.1.1 Ghost-Free Data


BroadSource is a synchronised source with two gun arrays deployed at different depths. Te downgoing wavefields from these gun arrays are synchronised while their respective ghosts become de-synchronised. Te advantage of obtaining de-synchronised ghosts with well-tuned time delays lies in the resulting notch-free amplitude spectrum of the source (Siliqi, 2013). A synchronised multi-level source is a compact solution for


acquiring broad band data with variable-depth streamers. It can be deployed in the flip-flop mode necessary for 3D broadband surveys and has all the benefits of a standard marine source in


Synchronised primaries Desynchronised ghosts


9m notch


6m notch


219


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