Abstract: A method of selectively displaying data, whereby the data is arranged in a consecutive series of data-blocks, each data-block being a matrix of information such as a Lofargram. The method comprises selecting an initial set of data-blocks from the series of data-blocks, wherein the initial set of data-blocks comprises an initial data-block and every following nth data-block in the series and then displaying this initial set. The method includes displaying different sets of data-blocks subsequent to the initial set. Each different set of data-blocks comprises an initial data-block, differing from the initial data-block of the preceeding set by an integer m, and every subsequent nth data-block in the series. The method further comprises, displaying a reference data-block adjacent to any one of the sets of data-blocks for comparing with any one of the data-blocks in the series.

Abstract: A sonar survey apparatus comprises a submersible vessel having a structure adapted to receive a towing cable. A sonar signal transmitter is mounted on the structure. At least one acoustic reflector is mounted on the structure and is shaped to produce, in a focussing plane, line foci of acoustic echo signals received from remote positions. A line array of receiver transducers are mounted on the structure in the focussing plane and extend in a line substantially at right angles to the line foci. Each receiver transducer element has an output for providing a signal representative of a focussed echo signal. A selector is coupled with the output of each of the receiver transducer elements and has a control input and an output. The selector is operable in response to the control signal to select the output of a receiver transducer element, relating to a predetermined direction, for display.