The accuracy of a marine seismic streamer location system using magnetic compasses was determined under realistic, at-sea conditions by having a survey vessel make several passes through a U.S. Navy acoustic tracking range. Comparison of positioning data from the compasses with data from the acoustic tracking system showed that the compass information determined the streamer shape with cross-line errors of 5 to 10 meters. However, the lack of heading data for the portion of the streamer nearest the vessel and an error in the assumed magnetic declination caused an offset and rotation in the estimate of the streamer position, resulting in cross-line errors that grew to 30 meters at the streamer tail-end.
The practice of towing long, thin streamer hydrophone arrays for the acquisition of marine seismic data creates a number of operational and technical problems for those who must collect and process data. These problems are worsened by the fact that the personnel aboard the towing vessel have, at best, an incomplete knowledge of the location of the streamer. Even so, they must maneuver the towing vessel so that the necessary seismic data can be collected while both the vessel and the streamer avoid hazards such as shallows, offshore platforms, and other vessels. The lore of the marine seismic survey industry provides many examples to demonstrate the magnitude and extent of these operational difficulties.
The technical difficulties caused by an incomplete knowledge of the location of the streamer have increased as seismic data processing requirements have become more complex. In particular, the 3-D marine seismic survey technique requires that a geodetic position be determined for each of the hydrophone groups in the .streamer for every seismic shot point. These position determinations allow the data collected by a hydrophone receiver group to be assigned to particular cells within a geodetic survey grid. Based on the cell dimensions and the seismic source-reflector-receiver geometry, the position of a hydrophone receiver group must be known to an accuracy of typically 12–25 meters in the direction along the vessel's track-line and 25–50 meters in the cross-line direction.
Several methods have been used for streamer location; they fall into the general categories of:
assuming that the streamer follows where the ship has been;
locating a surface buoy attached to the tail of the streamer and interpolating the position of the streamer between the buoy and the point of attachment to the vessel;
locating a surface buoy attached to the tail of the streamer and interpolating the position of the streamer between the buoy and the point of attachment to the vessel; using magnetic compasses to determine the magnetic heading at several points along the streamer, deriving a streamer shape and constraining the shape solution so that it passes through the point of attachment to the vessel; and
making acoustic ranging measurements between points along the streamer and points with known geodetic locations.
