Perhaps twisso needs a more detailed explanation. A survey...

Perhaps twisso needs a more detailed explanation. A survey vessel tows an array of sonic devices called canons that each send a pulse through the ocean waters and through the earths crust. Each pulse is created by an implosion within the device. The time taken for the pulse to travel down through the crust and then back to the towed measuring instruments is measured and interpreted into a graphic representation of the substrate. In order to record in a logical format the vessel must traverse an imaginary grid pattern on the ocean surface. Each leg may be 50 - 100nm long. The grid-lines are predetermined and the vessel cannot deviate b/c it would compromising the survey. At the end of each grid line the vessel must turn and align itself with a new line drawn on a chart. It can be imagined this grid to be a paddock with defined boundaries. Perhaps now you can see how it would not be difficult to patrol this paddock without shadowing the seismic vessel doing only 4 kts?