One of the most interesting and closely watched developments in seismic technology in recent years has been the application of towed streamer wide-azimuth and multi-azimuth 3-D data acquisition techniques as viable tools for improving seismic imaging in areas of

Figure 1. (left) Conventional 3-D streamer spread: Single source with eight parallel 19,686-ft (6,000-m) receiver cables. (right) Rose diagram showing the resulting offset and azimuth distribution for a single narrow-azimuth shot. (Images courtesy of CGGVeritas)
complex geology. In conventional 3-D marine surveys the seismic source is positioned at the head of an array of seismic receivers contained in a parallel set of streamer cables. In this configuration, the angles between the source position and the receiver positions (i.e., azimuth angles) are constrained to a very narrow sector directly behind the survey vessel except for the nearest offsets (Figure 1). In terms of subsurface illumination, this narrow azimuth sector is similar to shining a flashlight with a very narrow beam of light into a dark room — you will only see objects that fall within the narrow light cone and only see the side of the objects that are facing the light source. In areas of complex three-dimensional geology, this type of narrow azimuth seismic “illumination” is not sufficiently adequate to allow for the generation of seismic images with enough clarity to guide oil and gas companies in the exploration, appraisal and development of potential hydrocarbon reservoirs.

What is required are data acquisition methods for seismically “illuminating” the subsurface from a number of different directions. One way of doing this is to make multiple passes with the narrow azimuth geometry from a number of different directions; this technique has been called “multi-azimuth” (Figure 2). Another way is akin to widening the beam of the flashlight and then viewing from various directions so that more of the dark area, including the sides and bottoms of the structures, are illuminated from a single light source. This is what the wide-azimuth seismic techniques try to accomplish — the energy from a single seismic source is recorded into a wide array of receivers surrounding the source location. Reflected seismic energy is then received from all directions around the source location (Figure 3).

Commercial success
The first commercial wide-azimuth marine survey was acquired by Veritas for BP in the Green Canyon area of the Gulf of Mexico in fall/winter season of 2004 and 2005. Shortly thereafter, a “rich” azimuth survey (a combination of the multi-azimuth and wide-azimuth techniques) was acquired by another seismic company for BHP-Billiton over an area near the BP survey.

Both these surveys were designed to densely sample the subsurface from a grid of surface shot locations that would generate a set of seismic data traces with a wide range of offsets
Figure 2. Multi-azimuth surveys are acquired with a conventional 3-D streamer spread shot in a number of different directions.
and azimuths. These surveys were targeting specific subsalt reservoirs with known seismic imaging challenges. As the preliminary results from these early non-conventional towed streamer acquisition efforts were presented at various technical meetings and workshops, it quickly became apparent that the wide-azimuth and multi-azimuth data were providing superior subsalt images compared to conventional narrow-azimuth towed streamer data acquired over the same areas. Structure that was invisible historically could now be seen.

These clear benefits do come at a higher cost than traditional surveys as they require the use of multiple seismic vessels over a period of several months in order to provide images of relatively small subsurface areas on the order of a few hundred square kilometers. The deepwater areas of the Gulf of Mexico that suffer from similar imaging challenges amount to tens of thousands of square kilometers.

Ramping up
As more and more exploration and production (E&P) companies decide that the benefits of these types of non-conventional acquisition techniques outweigh the increased costs of acquisition and bring significant added value to their exploration and development plans, the seismic contractors will have to gear up to provide the acquisition, data processing and imaging services in a timely and cost-effective manner. With that in mind, a number of contractors have looked for ways to reduce the considerable acquisition efforts applied on the field-specific programs while maintaining the main imaging benefits of the wide and multi-azimuth techniques. The motivating objective for designing technically viable, cost-effective, “sparse” wide and/or multi-azimuth towed streamer acquisition methods is the ability to apply these techniques over large areas as a valuable tool in the exploration and development of deepwater subsalt prospects, not just in the Gulf of Mexico but worldwide. Careful design of the acquisition configuration can lead to a continuum of data acquisition cycles through the life of an oil field. The concept is that data can be successively appended to increase the subsurface sampling from the initial exploration effort as fields are discovered, appraised, and developed and managed.

As mentioned above, multi-azimuth streamer surveys, as apposed to wide-azimuth, require the use of conventionally equipped streamer crews. Survey areas are covered multiple times in multiple directions. The time it takes to complete a multi-azimuth survey is directly related to the number of required directions. Therefore, a streamer vessel will be committed to a multi-azimuth survey for significantly longer periods of time than a conventional survey.
In the case of towed streamer wide-azimuth and rich-azimuth surveys, the acquisition configurations require multiple vessels. In the simplest case a wide-azimuth streamer survey can be acquired with a single streamer vessel and an additional source vessel. But the time to acquire the survey with this minimal vessel configuration would be too long to meet decision-making timeframes for most exploration and development programs. So contractors are using multiple source and streamer vessels to shorten the acquisition time for wide-azimuth towed streamer surveys.

The use of a single streamer vessel on a multi-azimuth survey and the use of multiple source and streamer vessels on wide-azimuth surveys will change the global availability of marine
Figure 3. (left) Wide-azimuth surveys require energy from a single shot to be recorded into a wide array of receivers located around the shot location. (right) Rose diagram showing the resulting offset and azimuth distribution for a single wide-azimuth shot.
surface seismic resources if the contractors do not increase the currently available capacity. At this stage in the development of these techniques the big question for the future is, “Will these types of surveys produce a large enough market to warrant the capital expenditure for a continued and significant increase in marine seismic resources?” The answer to that question will have a direct impact on the costs of such surveys, and the costs will determine how E&P companies will evaluate the value of these types of data.

At the time of this writing, several of the major seismic contractors have embarked on large wide-azimuth data library projects in the Gulf of Mexico with underwriting from a variety of E&P companies. There have also been tenders and inquiries for proprietary towed-streamer multi-azimuth and wide-azimuth projects in other parts of the world. It appears that the business models of the seismic contractors and the perceived value of these types of surveys by the E&P companies are currently aligned and suggest a bright future for finding new reserves in complex geologic settings using these new towed-streamer acquisition techniques for acquiring marine 3-D seismic data.