GB2083219A - Sonar system - Google Patents

Abstract

Sea bed surveying apparatus is described comprising a pair of sonar signal emitters TX each associated with a bank of receiving transducers 41, 42 etc. 51, 52 etc.. The banks of transducers are inclined at an equal an opposite angle to a line 6 connecting the emitters to a target zone. The apparatus also includes an inclinometer 15 mounted with the transducers so that the echo signals received by the transducers can be continuously compensated for roll and pitch of the vessel or log, after processing in a multiplex processor 12, by means of an electronic stabilisation unit 123, before display on a cathode ray tube 13. <IMAGE>

Description

SPECIFICATION Improved submarine survey methods and apparatus This invention relates to improvements in underwater surveying methods and apparatus, and more especially to "sonar" devices of the type utilising sonic or ultrasonic signals and analysing the reflected "echo" signal.

Conventionally, a sonic signal is emitted from beneath a ship's hull, either from a hull mounted unit or from a log streamed from the ship and the reflected echo signal sensed by receivers also mounted on the ship or the log. The echo signal is analysed to determine the reflection delay of signals returned from points at various depths, and the result displayed as a line trace which represents a profile of the seabed along a line e.g. athwart the ship, based on the parameters of angle of incidence, and echo delay.

A plurality of these traces could be built up graphically by a pen recorder, or displayed visually on a cathode ray tube, then photographed to give a printed record of the contours of the seabed target area.

The conventional electronically scanned system, distinct from a simple echo sounder that has capacity for giving range only, having a single housing for two transducer systems does not possess the capability of enabling both sides of irregularities, for instance, boulders or pipe lines to be "seen" with the consequence that the nature of an irregularity cannot accurately be ascertained the other slope being in a "shadow" area.

In orderto obtain a profile from a range-finding echo sounder a narrow beam is mechanically scanned yet it must stay in the same position for a period extending during transmisson and the time for reception of the returning echo. The velocity is relatively slow and the period of time to accomplish a full scan is correspondingly extended; each scan is undertaken with the ship temporarily riding to anchor and then the ship moved on a short distance and again brought to a halt for accomplishing successive scan.

The unambiguous scan angle depends on the spacing of the receivers, being less with greater spacing, and production difficulties attend the construction of receivers with unambiguous scan angles in the range greater than 70 - 80 . Additionally bearing accuracy is the greater, the less the scan angle. In consequence at least two transducer systems are required, operating sequentially to cover larger scan angles.

An object of this invention is to provide surveying apparatus for enabling submarine target irregularities to be surveyed within a zone of interest to both sides of a survey vessel and capable of collating signals rapidly and in such a way that movements of the vessel can be accommodated whereby a survey is obtained continuously without the necessity of stepping or moving very slowly; pitch and roll movement may be compensated for presenting successive record of the submarine features of interest with correction in relation to the vertical; both sides of the submarine target irregularities may be scanned in a predetermined interest target area.

According to the present invention sonar apparatus comprises at least one emitter and associated transducer bank having a plurality of echo receivers in sequential spatial relationship in a direction inclined relative to a line normal to the target zone and passing through the emitter and amultiplex processor for correlating the incident angles of a returned signal at the respective transducer receivers. Visual display means actuable by the processor then enable recordal of the topography over a submarine target zone.

An inclinometer is preferably coupled with the processor whereby correlation from successive emitted signals is accompanied by correction of the incident angles of echo signals with reference to the vertical. Alternatively, or additionally a mechanically pitch/roll compensated platform is not required yet may be employed e.g. where present for another installation.

At least one additional emitter is normally added, spaced from another by a distance at least corresponding to that of a target interest area and generally with an additional transducer bank having a plurality of echo receivers in sequential spatial relationship in a direction inclined, preferably at an angle corresponding to that of the first mentioned bank, relative said normal line.

The sonar scanner according to the invention uses the phase of the incoming signals to determine the direction of the target and by using several receiving transducers the spatial correlation of the received signal is obtained. This spatial correlation is employed to determine the quality of the received signal and it is used to give a varying intensity at a processor display station for the measured angle. In the case the spatial correlation is good maximum intensity is obtained and where spatial correlation is poor the display is blanked for that measurement.

This quality factor can be used for automatic detection and processing of the information, for instance if only the maximum correlation signals are used with the emitter(s) directed downwardly a profile of the seabed will be produced.

The scanner apparatus is also stabilised in the roll axis electronically so an expensive mechanically stabilised platform is not required. The digital nature of the signals after limiting renders the multiplexing of several receivers a straight forward task and to obtain wide electronically scanned angles two or more receive/transmit systems, scanning adjacent sectors, are sequentially operated from one or both of two emitter: transducer bands to yield any desired scan angle using the one processor. The system hereinafter described has two 70" systems multiplexed nevertheless, where greater accuracy is required several adjacent narrower scan angle systems may be employed.

The transmitter can operate at up to 500 kHz i.e.

higher definition than presently available to digitally processed systems.

To overcome limitations of the phase only processing, the transmitters may be sequentially scanned by electronic or mechanical means.

The system can be either mounted to scan vertically to produce seabed profiles for map purposes where the electronic stabilisation is in the roll axis, or used to scan horizontally as a forward search sonar and the electronic stabilisation can be used to reference the display to a particular gyro heading.

The invention will be further described with reference to the annexed drawings wherein: Figure lisa block diagram of a sonar scanner apparatus according to the invention: Figure 2 shows a pair of transmitters each having an associated bank of receivers: Figure 3 is a vertically directed submarine profile plot showing a close-definition target interest area; and Figure 4 is a forward profile plot in a harbour.

Receiving equipment consists of a transducer housing 1 with preamplifiers 11,19 processing unit 12 and a CRT 13 display that may be interchanged with a computer controlled video display 10.

Each transducer configuration has transmitters P & S and receivers 2, 3 divergently directed and the receivers have a bank 4, 5 or transducers 41,42,43, 44,45 and 51,52,53,54,55 respectively aligned at an incline relative a line 6 normal to the target zone. A control unit 14 generates a short pulse which drives TxP via amplifier 16. At the same time the multiplexer 12 is switched to allow signals from 2 limiters 17 into angle digitisation station 121. The angle digitisation measures the angle between the signals from adjacent receivers of bank 2 by digitising the time between successive zero crossings. The present system has 5 transducers which gives 4 angle measurements. This number can be increased for greater accuracy.These angle measurements are fed to the correlation and digital averaging unit 122 which determines the average of the four readings and the spread of the angles. The spread of the angles is used to control the intensity of the display for that particular bearing measurement. With a greater spread angle the intensity is reduced. The average angle is fed to the electronic stabilising unit 123 which corrects the angle for roll of the transducer with reference to a stored signal from inclinometer 15 through roll measurement control 18.

This corrected angle together with the spread signal are fed to the display control board 124 which generates analogue signals to produce a P.P.I.

display on the C.R.T. monitor. In addition the digital signals are serialised for twisted pair transmission to a computer system which generates the P.P.I. on a video display system. The next sequence is to transmit on TxS, the cycle repeating employing preamplifiers 19 and 3 limiters 20 into angle digitisation station 121 through multiplexer 12, from control unit 14 pulse drive via TxS power amplifier 21 P.P.I.

display.

Description for a dual 5 transducer system scanner apparatus can be extended not only to employ more transducer receivers but additionally or alternatively several transmitter receiver units.

Examples of application of the scanner are shown in Figure 3 in the case where survey is required to detect any portion of a pipeline that is uncovered or extends out of a trench that shall otherwise ensure its being covered as and when silt fills the trench.

The nature of a hindrance to the closure of dry dock gates is illustrated in the forward directed scan of Figure 4.

Claims (6)

1. Seabed surveying aparatus comprising at least one sonar emitter with an associated bank of transducers, the bank of transducers havinvg a plurality of echo receivers in sequential spaced relationship in a direction inclined to a line extending to the target zone and passing through the emitter, a multiplex processor for correlating the incident angles of returned signals at the respective transducer receivers.

2. Apparatus according to claim 1, wherein an additional emitter with an additional bank of transducers having a plurality of echo receivers in sequential spaced relationship in a direction inclined at an equal and opposite angle to the said line, with respect to the first bank of transducers.

3. Apparatus according to claim 1 or 2, wherein visual display means actuable by the transducer are arranged to display a line trace (sensed depth versus scan direction) which enable recording of the topography sensed in a target zone.

4. Apparatus according to any preceding claim, wherein an emitter inclinometer is coupled with the processor so that correlation of successive emitted signals is accompanied by correction for variation in the incident angles of the reflected signals with reference to a datum to compensate for platform movement.

5. Apparatus according to any preceding claim wherein the or each sonar receiver uses the phase of the incoming signals to determine the direction of the target, and by using several receiving transducers the spatial correlation of the received signal is obtained.

6. Seabed surveying apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.