Depth measurements

The classical method of measuring the depth of the sea was by means of sounding weights and lines. The weight was lowered from the vessel until it struck bottom and the length of line measured, usually by means of a meter attached to a sheave

through which the line passed. This method has been superseded by sonic sounding.

Echo sounders and sonar systems

Sonar stands for 'sound and ranging' and is the detection of objects under water using sound. There are many applications including depth measurement, seabed profiling and location of objects such as wrecks.

Depth measurements are now predominantly made by echo-sounding. The depth of water is measured by timing the interval between the emission of a sound impulse at the surface and its return to the surface as an echo reflected from the sea-bed. The speed at which sound travels through water is known to be about 1500 m/s and so time can be converted to depth. The speed of sound through seawater varies slightly with temperature, salinity and pressure and so for really accurate sonic sounding, the actual speed needs to be measured. This can be estimated from hydrographic data where available, or direct measurements can be made by sensors lowered from a ship. These instruments contain solid-state circuitry generating acoustic pulses. The pulses are transmitted through the water to a reflector, usually a distance of 0.05 m, and back. When the reflected signal is picked up by the transducer this initiates another pulse. Thus the number of pulses generated in a standard time interval is a measure of the speed of sound in water.

The frequencies emitted by modern echo sounders lie above the audible range, usually between 15 and 50 kHz. The use of ultrasonic frequencies has several advantages. They can be focused into fairly narrow directional beams, giving a more precise echo than is obtainable from the audible part of the sound spectrum, and enabling a more detailed picture of the bottom profile to be drawn. There is also less interference from natural sounds.

A great variety of echo-sounders are now available to suit all types of vessels from inflatable boats to supertankers. One of the latest is a small hand-held version shaped like a torch. It can be used over the side of a small boat but as it is waterproof and pressure resistant, it can also be carried by a diver. The diver can thus measure the depth of seabed features below his operational limits.

Although the design varies, echo-sounders all work on the same principle. An acoustic transducer fitted on the underpart of the ship's hull or towed at a known depth, gives out short pulses of sound at a given frequency. The transducer also receives the reflected sound pulses on their return from the sea-bed. These transducers make use of the piezoelectric properties of quartz or the magnetostriction properties of nickel. In either case, rapid dimensional changes are produced by electrical excitation, causing brief pulses of vibration to be emitted into the water. The returning echo vibrates the transducer and sets up electrical signals which can be amplified and recorded as a trace on a cathode-ray tube or as a line drawn on a paper chart.

In the paper-recording instrument, which is not greatly used today, a strip of sensitized paper bearing a printed scale is drawn slowly through the recorder. A moving stylus in contact with the paper scans to and fro across the scale at a speed which can be adjusted in relation to the depth of water. A short pulse of sound is emitted from the ship's hull in a downward-directed beam as the stylus passes the zero mark of the scale. The stylus continues to move across the scale, and the echo signals are amplified and applied as an electric current to the stylus, marking the sensitized paper electrochemically. The position of this mark on the scale indicates the depth from which the echo is received. As the paper moves through the instrument, the repeated scanning of the stylus produces a series of marks which build up a line on the paper corresponding with the profile of the sea-bed (Figure 3.9).

Sonic techniques have further applications in marine biology. Fish shoals may be detected in this way, and different species may to some extent be identified by their characteristic echoes. The sonic scattering layers (see page 140) were discovered in the course of investigations with sonic equipment.

Side-scan sonar

Standard echo-sounders can only make measurements directly beneath the ship. This is adequate when depth is the main information required but where

Figure 3.9 A typical sonic sounding trace over a widely varying sea-bed.

information on the type of sea-bed is also needed, the use of side-scan sonar is more appropriate. The technique of side-scan sonar makes use of a horizontally directed beam of sound to build up a series of images or sonographs of the sea-bed. Side-scan sonars are now extensively used for surveying broad swathes of the sea-floor. Different types of substrate reflect different patterns of echo trace and sonographs can provide information on the texture, orientation and composition of features. However, interpretation of complex sonographs requires considerable experience and expertise.

An instrument called GLORIA (Geological Long Range Inclined Asdic), is a sophisticated side-scan sonar about 8 m long developed at the UK Institute of Oceanographic Sciences. It is towed behind the vessel at a speed of 8-10 knots and at a depth of 40-80 m. In water 5000 m deep, it can survey a strip of sea-bed about 40-60 km wide. Features such as volcanoes, canyons, mud slides and nodule fields can be observed by overlapping several strips. Over the last 20 years or so, GLORIA has surveyed about 6 per cent of the ocean floor.

Side-scan sonars are now being put on to deep-towed instrument packages for carrying out fine-scale surveys of the sea-bed. These instruments are now revealing extraordinarily complex structures on the abyssal plains resulting from catastrophic failures of continental slopes. Structures such as meandering channels with levies and 'flood'-plains created by turbidity currents have been

Figure 3.10 GLORIA 6.5 kHz side-scan sonar.

revealed, originating from events happening hundreds or even thousands of kilometres away on the continental margin.

A further use of side-scan sonar is in the detection of shipwrecks, and many archaeological finds and valuable cargoes have been recovered in this way. It is also being increasingly used for fish location. Echoes may be returned by objects such as fish, floating or swimming in the water in the path of the sound beam. Modern instruments are capable of detecting fish shoals, individual fish and very often, the species. The efficiency of this system has undoubtedly contributed to over-exploitation of some fish stocks (see Chapter 9). Sonar systems are becoming ever more sophisticated allowing us to 'see' the sea-bed in a way early oceanographers would have thought impossible (Riddy and Masson, 1996).

'Roxann' ultrasonic signal processor (USP)

Standard echo-sounders can provide only limited information concerning the nature of the sea-bed. This type of information is usually gained by use of side-scan sonar systems (see above). However, a new system for processing echo-sounder signals has recently been developed in the UK (Chivers etal., 1990). It is designed to process information from a straightforward echo-sounder to provide simultaneous information on the nature of the sea-bed. The system is based on the electronic analysis of the first and second echoes visualized on the echo-sounder display. Straightforward desk-top computers and software are used to analyse and display the data.

The advantage of this system is its relative cheapness and its ability to interface easily with a wide variety of echo-sounders. In the UK it is finding application in initial low-cost 'look-see' surveys where mapping of bottom types and habitats is required in shallow water.

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