Tailbuoy Tracking for Twin Streamer 3D Surveys: A Case History
Authors: Stigant J.P. 
Published: October 2008

3-D Seismic Surveys are a major development tool in today's oil industry. Accurate surface positioning of the source and receivers in twin cable marine surveys is crucial to the successful processing of the seismic data and ultimately to successful delineation of reservoirs and enhancement of recoverable reserves.

For years the seismic industry has estimated where the source and receivers are located behind a seismic vessel. Development and fielding of the systems described in this paper allow some of the previous estimates of locations to be replaced by actualmeasurement of locations of source and streamer.

INTRODUCTION

It is generally accepted that the relative surface accuracy requirement, for 3-D seismic. streamer surveys source and group locations, needs to be ½bin dimension (Morgan 1983) or ± 12.5 meters crossline and ± 6.5 meters inline for a 25 meter × 12.5 meter bin dimension. This requirement may have to be significantly tightened with horizon crossline dips in excess of 45 degrees to accommodate pre-stack DMO partial migration algorithms (Houston 1987). Use of magnetic compasses alone to locate seismic streamers has been insufficient to be certain of achieving these accuracies; some cases exist where uncertainty of the far group location was as great as 100-120 meters (Figure 1). Tail buoy radio location devices have been proposed often over the last 6 years (Morgan 1983, and 1986) and have been used since late 1986 (Hector/ Williams 1987, Houston 1987, Manin et al1988) to properly close the magnetic traverse represented by the use of streamer compasses (Figure 2). The need for accurate location of the far end of seismic streamers becomes more important in conjunction with multiple streamer surveys, in which position uncertainties are exacerbated by lack of knowledge of the location of the streamer first groups, offset from the centerline towing position at the stern of the vessel.

The survey under discussion in this paper presented Chevron with the opportunity to evaluate one contractor's technique to provide accurate radio positioning of the tail buoy locations. A considerable quantity of data was collected and carefully analyzed. Subsequently, it was possible to make adjustments to the streamer locations to more accurately bin the data. It is important to understand that while the results of past surveys may have been within the tolerances outlined above, there is no way of being certain without actually making appropriate measurements. The use of radio location tail buoy devices in this survey allowed the errors in the magnetic compass computation of the streamer to be measured, and highlighted additional needs. First, there remains uncertainty in the estimated positions of the source and first group. Second, to be fully effective (Le., to ensure that the field acquisition 3-D binned coverage is accurate), the results of the measurements of source and streamer locations must be integrated into the 3-D navigation and QC systems and processed on board the vessel. Third, processing in this survey allowed correction of systematic biases, but did not allow for shot by shot corrections.