Exploration Report

Solution for seismic source registration

Statoil overcomes the seismic source deviation problem for 4D.

Victor Schmidt, Drilling Engineering Editor

INTRODUCTION

Statoil has developed and proven a novel solution to the source registration problem in marine time-lapse seismic (4D) surveys. By towing additional airguns, the baseline source position can be repeated, using those portions of the wider source array that best overlay the initial 3D survey’s source position.

REGISTRATION

Shotpoint registration has always been a problem with 4D surveys. At each firing of the source, the positions of the source and receivers are fixed in the record. To produce a cleaner difference volume for fluid-movement studies, these source-receiver positions need to be repeated with each subsequent 3D survey. This repetition focuses the source energy at the same reservoir positions and allows one-to-one correspondence of survey receiver points eliminating interpolation between receivers. It also minimizes processing artifacts.

Geophysicists expected streamer steering to overcome the registration issue, but found that problems still occur. Streamer steering helps to a large degree because it reduces a major source of variation from streamer movement. Present steering technology can correct for up to 3° of streamer feather. Geophysicists will need an additional 2 – 3° of steering correction to produce a zero-feather survey 90% of the time in seas with strong currents, such as those in the North Sea off Norway.¹

However, streamer steering is only half of the problem. The other part of the registration problem is the movement of the source array. Since the array is closer to the towing vessel, it is more closely tied to the vessel’s position, but is still affected by the same wave and current forces that move the streamer. Source point deviations of 15 – 20 m are common on crosslines.² Any reduction in this deviation will produce a better difference volume for the production geophysicist and reservoir engineer to track fluid movement.

At the recent EAGE convention, Ole Naess of Statoil presented the company’s Self-Overlapping System (SOS) for source registration. The system uses a group of identical sub-arrays in odd-numbered groups (3, 5, 7). As the sub-arrays drift side-to-side, a GPS-linked airgun firing-control system chooses the sub-arrays that will pass over the next source registration point. These guns are then fired, recreating the earlier survey source point. The system can be designed to handle either single- or dual-source shooting.

Statoil conducted a field test over Norne field in the North Sea using five sub-arrays. WesternGeco shot the survey using their Q-marine technology. The company towed the SOS over five lines, once the standard survey was completed. The source was kept within 5 m of the reference survey’s source position and greatly reduced the crossline deviation, eliminating the need to reshoot lines. This can save days of vessel time in normal operations.

POTENTIAL PROBLEMS

Fig. 1. As currents push the multiple source sub-arrays the airgun firing-control system fires only the sub-arrays that will pass over the registration point.

A few problems are still possible with this new approach, including hydrodynamic drag, tight turns and cold airguns. Hydrodynamic drag increases with the addition of sub-arrays to the source package. This adds to the energy overhead of the towing operation and must be weighed against the improved survey imaging.

Tight-radius turns can create problems in the sub-arrays similar to problems with uncontrolled streamers. In tight turns, the inward equipment slows relative to the vessel and drops deeper in the water. This can lead to equipment problems in shallower waters.

The most likely problem could be power and frequency variation introduced by cold guns. If currents are moderate and produce minor feathering, then the need for correction will be small and the outer SOS arrays needed for drift compensation will become cold from limited use. Their timing and source signature will be different than the guns that fire routinely and will need to be adjusted in processing. It is possible that this can be done with a far-field correction.

The most likely problem could be power and frequency variation introduced by cold guns. If currents are moderate and produce minor feathering, then the need for correction will be small and the outer SOS arrays needed for drift compensation will become cold from limited use. Their timing and source signature will be different than the guns that fire routinely and will need adjustment in processing. It is possible that this can be done with a far-field correction.

The Society of Exploration Geophysicists’ standards committee recommends the use of a far-field hydrophone to collect the airgun signal as the pulse decays away from the source. This can be done in a static test or in a dynamic tow. Three constraints on the hydrophone placement are the distance between the hydrophone and the bottom, the distance between the hydrophone and source and the hydrophone’s water depth. The hydrophone must be positioned to receive the direct arrival pulse and the surface ghost before the arrival of the water bottom ghost. The combination of the direct arrival pulse and the surface ghost provides the far-field signature.

Newman developed a method of deriving the far-field signature from the near-field recording using a reference airgun. He used a deterministic filter to correct for the surface ghost and derived the far-field signature. His method can be applied to source arrays of any configuration.

CONCLUSION

Statoil’s new SOS brings a new measure of control to marine seismic acquisition. The goal of a 4D seismic zero-feather survey is getting closer, first for the 3D baseline survey and then for succeeding surveys.³ 

LITERATURE CITED

¹  Eiken, Ola, “Improvements in 4D seismic acquisition,” World Oil, pp. 23 – 27, September 2003.

2  Naess, Ola, “The SOS source: a method and arrangement for a 4D seismic source;” B008, EAGE 67th Conference and Exhibition, Madrid, Spain, June 13 – 16, 2005.

3  Newman, P., 1985, Continuous calibration of marine seismic sources, Geophysical Prospecting, 33, 224 – 232.