The challenge of imaging below complex salt canopies is well known, as are the solutions – full azimuths for better illumination and low frequencies for improved penetration. Unfortunately, until recently neither of these has been easy to achieve, or they have resulted in compromises to other factors that are required for high-quality imaging.

Before the advent of broadband seismic, low frequencies were achieved by towing streamers deep to steepen the slope of the 0 Hz receiver ghost notch and so record more low-frequency signal. Towing deep had the additional advantage of reducing the susceptibility to sea-state noise but also brought the first and second receiver ghost notches into the range of useful seismic frequencies, thereby compromising the mid- and high-frequency ranges. Although the low frequencies are the most important for imaging beneath the salt, higher frequencies are required for accurate velocity modeling of the shallow data to create reliable models of the deeper subsalt.

New broadband methodologies

Broadband seismic has enabled both low and high frequencies to be recorded using BroadSeis variable-depth streamer acquisition combined with deghosting by mirror migration and joint deconvolution. This technique uses a curved streamer shape that is customized for each survey to optimize the resolution at the target and is typically towed with a maximum depth of 50 m (164 ft), providing a deeper average streamer depth (and hence better low frequencies) than other methods. The variable receiver depths produce receiver ghost notch diversity, which is exploited by the joint deconvolution deghosting to achieve more than six octaves of signal with frequencies down to 2.5 Hz and up to the source ghost notch. When combined with CGG’s broadband source, BroadSource, the upper frequency boundary is limited only by the sample rate anti-aliasing filter.

CGG Figure 1

FIGURE 1. Preliminary fast-track prestack depth-migrated salt-flood Kirchhoff data from a Santos basin BroadSeis multiclient survey show high-frequency faulting with shallow texture and layering and exceptional clarity of faulted presalt events, with a structural high into a fault. (Images courtesy of CGG Multi-Client Data Library)

Santos basin

Variable-depth streamers have been used to acquire multi-client data in the deepwater Santos basin offshore Brazil to obtain exceptional presalt and post-salt images (Figure 1). Broad-bandwidth data containing very low frequencies are characterized by sharp wavelets without side lobes, producing events that are single peaks or troughs and corresponding to genuine geological layers. This clarifies impedance contrasts, creates sharp images of small features, and provides clear differentiation between different sedimentary packages. The extra-low frequencies give an envelope to the seismic signal that shapes the larger scale impedance variations or major lithology variations. This increases confidence in correlating seismic interpretation across faults and other major structural features, allowing layers to be easily differentiated. This layer differentiation is clearly visible in Figure 1 as is the detailed shallow faulting. The base of the salt also is easily identifiable as well as presalt faulting and a structural high.

Additional vessels

The success of variable-depth streamer acquisition for the presalt offshore Brazil has been replicated in the conjugate margin offshore Angola. Two multiclient variable-depth streamer surveys have recently been recorded in this area, with preliminary results showing the same promise of excellent presalt imaging.

In areas with complex overburden such as the subsalt plays of the Gulf of Mexico (GoM), wide-azimuth (WAZ) acquisition, multi-azimuth acquisition, or preferably full-azimuth acquisition (FAZ) are recognized as providing enhanced illumination as well as improved natural noise and multiple attenuation, especially when combined with ultra-long offsets. Combining this acquisition geometry with the broadband benefits of variable-depth streamer acquisition provides the ultimate in deep imaging in complex geological areas.

A new technique for efficiently acquiring FAZ, towed-streamer data recently has been developed and used to acquire a multiclient survey in the GoM. This technique, known as StagSeis, uses two multistreamer vessels with three additional source vessels in a staggered formation to acquire FAZ, long-offset data in two orthogonal passes. The configuration maintains the advantages of a linear tow in providing consistent azimuth, fold, and offset distribution while being compatible with conventional WAZ processing techniques. The linear tow enables faster turnaround times for the combined acquisition and processing and means that field data are immediately available.

Initial fast-track processing results were available from the GoM survey four months after the firing of the first shot. Unlike some other FAZ techniques, this geometry can easily be infilled to a finer line spacing for development, extended to cover a larger area, or repeated for 4-D.

This staggered-vessel acquisition technique provides the benefit of ultra-long offsets up to 20 km (12 miles) for improved subsalt velocity model accuracy and imaging of steep dips. The effective FAZ coverage enables accurate anisotropic velocity model-building and was developed to address the challenges of the illumination of complex geologies where conventional WAZ acquisition fails to illuminate the target.

CGG Figure 2

FIGURE 2. StagSeis improves illumination beneath complex overburdens compared with standard WAZ data. This fast-track raw migration shows considerably better continuity subsalt than the previous WAZ data.

A comparison of conventional WAZ data vs. this staggered spread is shown in Figure 2. The improved continuity and greater illumination beneath the salt are clearly visible. The success of this initial GoM survey and the interest it has generated have led to the commencement of a major extension, doubling the initial area. The flexibility of this staggered acquisition geometry makes it ideally suited to other complex geologies as well. It already is being considered offshore Brazil and Angola and is being evaluated for the challenging geology of the Red Sea using shorter offsets.

Processing

Of course acquisition is only a part of the story. True 3-D processing algorithms such as 3-D surface-related multiple elimination and 3-D radon demultiple that make proper use of the FAZ information can be applied for optimized multiple attenuation. The FAZ range also is ideal for modeling complex orthorhombic anisotropy for use with the latest migration algorithms. Full waveform inversion can make use of the extremely low frequencies and long offsets to provide accurate velocity models of the near surface, ideal for shallow hazard identification. The use of mirror migration and joint deconvolution for deghosting the variable-depth streamer data not only provides accurate 3-D deghosting in image space that is able to handle the most complex geology but also doubles the illumination since both the upgoing primary and downgoing ghost wavefields are fully imaged.

The step-change in imaging achieved by broad bandwidths in recent years is now being matched by the power of FAZ geometries. The benefits of FAZ acquisition have been known for many years and are being employed as standard for onshore acquisition projects. Offshore, the problem has been one of logistics and economics rather than a lack of appreciation of the benefits. The development of variable-depth streamer, staggered-vessel acquisition provides the means to obtain the benefits of both broad bandwidths and full azimuths to produce better images in a timely fashion to reduce E&P risk in the most challenging environments.