Wide azimuth

Wide azimuth or (WAZ) is a term describing a seismic data acquisition technique with a wide distribution of source-receiver azimuths. They usually involve multiple source-boats or Ocean Bottom seismometers.

Introduction

Comparison of subsalt imaging from a narrow-azimuth conventional marine acquisition (left) and wide-azimuth towed-streamer acquisition (right).

Accurate imaging of sediments beneath hard seafloors, salt, basalt, and carbonate layers has presented a long-standing challenge to developers of seismic technology. In deep water, towed-streamer geometries are currently the only viable solutions for the acquisition of large 3D data sets. Conventional narrow-azimuth 3D surveys, usually acquired using a single vessel, have proved their value in a wide variety of geologic circumstances. However, complex geology and highly refractive layers cause ray bending that can leave portions of the subsurface untouched by seismic waves or poorly illuminated.

The term wide-azimuth is used generically to describe any acquisition geometry that is wide compared to conventional towed streamer geometries that are narrow. When the acronym from table 1 is used it infers a specific type of geometry.

Table 1. Azimuth acquisition geometries.

Term	Meaning	Remarks
NAZ	Narrow-azimuth	One vessel towing an array of streamers and source(s).
MAZ	Multi-azimuth	Three or more coincident NAZ surveys with different survey azimuths combined in processing, dual-azimuth combines acquisition in two directions.
WAZ	Wide-azimuth	Typically two or more vessels are used simultaneously to increase the range of azimuths and offsets available for each shot gather in processing.
WATS	Wide-azimuth towed streamer	A particular flavor of WAZ pioneered by BP.
RAZ	Rich-azimuth	Typically a combination of MAZ and WAZ designed to yield the most continuous distribution of azimuths and offsets possible with towed streamers.
FAZ	Full-azimuth	Perfect azimuth and offset distribution at every point in the survey. Possible only in practice when the source and receivers can be physically decoupled from the receiver spread, such as land or OBC 3D seismic.

History

In 2000, a conventional 3D streamer survey focusing on improved acquisition parameters over the Mad Dog discovery did not deliver the needed improvements over a previous traditional 3D streamer survey shot in a different direction. This result led several companies to investigate the possible benefits of widening azimuthal coverage.

The first GoM WAZ survey was conducted by BP with the contractor CGGVeritas over the Mad Dog field in 2004-05, using one recording vessel and two source vessels. The WAZ data delivered a breakthrough in imaging and initiated a broad WAZ acquisition program to enhance the imaging of subsalt discoveries.

Wide azimuth timeline

• In 1984 William French proposed the circle shooting technique to improve imaging of circular shaped salt dome flanks and to reduce the amount of unproductive time spent on line turns. was tested with streamer data acquisition in the Gulf of Mexico and in the North Sea.

• In 1987 Texaco developed a vertical-cable acquisition and processing strategy to overcome the narrow-azimuth nature of conventional marine acquisition.

• In 1988 The first dedicated multi-azimuth streamer acquisition was dual-azimuth survey at Bullwinkle. The survey results clearly demonstrated that different areas of the survey were imaged better with one azimuth or another.

• From March 1998 through November 2002, the Subsalt Multiple Attenuation And Reduction Technology SMAART JV (BHPB, BP, Chevron-Texaco) – to explore how wide azimuth acquisition could be implemented with towed streamer acquisition.

• In 2002 Dwight Sukup proposed the four-vessel WAZ configuration.

• In 2004 BP acquired the first wide-azimuth (WATS) marine data acquisition was carried out over Mad-Dog, Gulf of Mexico.

• In 2006 a flurry of papers associated with the wide-azimuth acquisition was presented at the SEG conference.

Wide azimuth benefits

Geophysicists recognized the need for wide azimuth acquisition long before the advent of recent commercial wide-azimuth surveys , the preliminary results of the early wide-and rich-azimuth towed streamer surveys were used to evaluate the benefits of this new marine acquisition technology. The benefits and the design features are summarized in Table 2. The benefits were separated into proven and potential categories, potential being the perceived benefits of the new technology that still have to be demonstrated.

Table 2. Wide- and rich-azimuth towed-streamer acquisition benefits.

Proven benefits	Why
Improved signal-to-noise-ratio of the subsalt events	Better attenuation of the multiples and other coherent noise due to: - Variability of the multiple (traveltimes) with azimuth - Variability of the coherent noise with azimuth - Variability of the coherent noise with azimuth - Continuity of the azimuths (not discrete azimuths) - Very high prestack migration fold - Improved source sampling for multiple attenuation - Improved data regularization
Improved reservoir illumination	Raypaths from different azimuths
Efficient migration based on common shot WEM	Repeatability of the shots at the same location allows grouping the shots into supershots
Potential benefits	Why
Improved velocity model for imaging	More measurements of the earth properties from different directions (measurement redundancy) It may be possible to estimate the anisotropy and incorporate it into the velocity model
P-wave fracture characterization for fractured reservoir; geomechanical studies around planned deepwater well locations	Wide azimuth allows application of amplitude variation with azimuth (AVAZ) techniques for processing and interpretation

Worldwide applications

Distribution of offshore salt basins, wide-azimuth acquisition may be applicable to all of these areas.

Wide-azimuth surveying is appropriate for any area of complex structural geology or where velocity contrasts are significant and salt causes imaging problems, the area that has seen the greatest application of wide-azimuth acquisition is in the Gulf of Mexico and since 2008 wide-azimuth surveys have been recorded in other areas; notably the Aptian salt basin of the west coast of Africa, offshore Indonesia, the Red Sea, and Brazil.

Wide-azimuth acquisition and survey design

wide-azimuth surveys are large-scale multi-vessel operations that require at least two source vessels in addition to the streamer vessel, and some may be acquired with multiple streamer vessels to improve acquisition efficiency.

BP WATS

BP WATS configuration used in the Mad Dog field trial for the first acquisition pass or Tile 1

The BP WATS geometry features one multi-streamer recording vessel in conjunction with two source vessels, each with dual arrays. In this way, acquisition can be more efficient. Each source line is shot four times while the streamer vessel sails in the same relative position to the source vessels except for a cross-line move up each time to enable a series of “tiles” to be acquired. The source line increment is a quarter the width of the spread in the cross-line direction.

WAZ

A four-vessel wide-azimuth acquisition configuration. Two recording vessels (left and right ), both equipped with an airgun source array and 10 streamers, are joined by two source vessels (center ). The source on the left-hand vessel is fired, and data are recorded by streamers on both recording vessels, providing two areas of subsurface coverage (dark tan). Sources are then fired in sequence by the other vessels, providing a wider area of subsurface coverage (light tan) and a broader range of source-receiver azimuths than can be achieved by a single vessel. Sail lines may be repeated with source vessels in different positions, providing different ranges of azimuths.

Wide AZimuth (WAZ) configuration is an alternative wide azimuth geometry to the WATS method; it has been widely used in the Gulf of Mexico. This configuration consists of four-vessels (2x4) two streamer vessels and two source vessels, all equipped with a single source, three-vessel (1x3) option exist for this geometry. In this acquisition geometry, there are four sources shooting consecutively (flip-flop-flup-flyp). The key advantage over WATS is the speed and acquisition efficiency. The geometry does however record a much lower trace density compared to WATS. Some advantages of using single sources rather than paired sources as in the WATS configuration are:

1. better positioning, centered on the center of the source array.
2. larger source arrays can be used.
3. supershot regularization should perform best.

Further reading

• Vermeer, Gijs J. O. (2012). 3D Seismic Survey Design, Second Edition. Society of Exploration Geophysicists. doi:10.1190/1.9781560803041. ISBN 978-1-56080-304-1.

External links

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