methodology of towed search
It has been asked why not stop? The answer is that, aside from the practical considerations of a tow, a stationary detector provides no useful information. The 90 sec interval gives an idea of the wreckage being on a line perpendicular to the tow route corresponding to the middle of the 90 sec interval.
The technique would be to circle wide through 270 degrees and survey along the line just mentioned to hopefully detect over an interval thus producing another line. Approach this crossing point from a third direction between the other two to create a third line. The result of the three lines is known as a cocked hat and gives an idea of the position of the source. If the signal strength is measurably variable then the maximum strength gives a position for the perpendicular line.
The signals can be distorted by sea bed features and thermal/ salinity features in the water column.