When we want to obtain a three-dimensional view of geologic horizons, we must survey a very dense grid of seismic lines. Such a grid presents a double disadvantage:
it is very costly, and
it solves only the problem of seismic horizons continuous enough to be followed without ambiguity along a complete loop. This grid does not take into account discontinuous events and those of limited extension, all information very useful for a thorough interpretation.
At sea, the efficiency of a dense grid of lines is jeopardized by the lack of accuracy of the positioning work.
The wide-line profiling (WLP) method solves these problems. It has the advantage of using the principle of continuous seismic profiling, adding to it a transverse dimel1.sion. This dimension is short enough not to burden the field operations and it allows the use of the standard seismic representation to which the_ geophysicists are accustomed. It is also wide enough for an accurate determination of the cross dips. The determination is Q, one continuously so the dip components of the seismic events are defined, however short these events are.
Let us summarize first how the WLP method is implemented in land seismic. source and receiving stations is way; however. groups of three to The layout of done the usual four source points are placed along oblique lines, with a maximum lateral offset of the order of 600 ft.
The purpose is to obtain a strip of Subsurface points spread along three or four parallel lines, each subsurface point resulting from the same coverage multiplicity, i.e., sixfold for four lines and 48-trace spread (Fig. 1).
At sea, in order to attain the same results we would need several sources, slightly spaced. For economical reasons, working with several'. boats, sailing side by side, is excluded. This would also be dangerous and the accurate navigation requirements would furthermore complicate our problem.
Using several powerful success with a sole vessel is also impractical. The usual sources, Air Gun and Vapor choc, would require the conveyance of air or steam under pressure at distances of several hundred feet and part of the power would be dissipated.
It seems that electric sources could be towed at such distances without major problems, but they are not powerful enough.
This is why we adopted a WLP field technique different from the one used on land. The results, however, can be processed in a similar way. We tow a single source close to the boat, and we use three streamers - one behind the vessel, the others at port and starboard. The lateral offset, of the order of 100 to 130 ft, can be maintained with parvenu systems (Fig. 2).
For economical reasons again, and because they are cumbersome, we cannot use very long streamers. We cut into three pieces our standard 48-trace streamer and thus operate with three 16-trace, 8oo-m long streamers. The nearest trace is 400 m from the source and the remotest trace is at 1,200 m.