Echo sounding

The sounder is a used in shipping unit for electro-acoustic measurement of water depths ( sounding ). The time between the transmission of a sound pulse (water sound ) and the arrival of the reflected sound waves from the bottom of the water is measured elapses.

History

The depth sounder was developed simultaneously and largely independently of each other in the years of the First World War in different states. In Germany, the physicist Alexander Behm made ​​a breakthrough with his kingdom Patents Nos. 310 690 of 7 January 1916, and No. 367 202 of 1 June 1920. On the economic exploitation of his invention, he founded in 1920 in Kiel, Behm sonar society.

Operation

The sonar sends a donor a sound signal down. There, the signal from the water bottom, but also wrecks or shoals of fish, reflected. The reflected pulse is received on the ship. From the travel time, the water depth can be calculated.

Most sounders using pulses with a frequency in the range between 50 and 200 kHz ( with an ultrasonic wave of 7.5 mm to 3 cm). An ultrasonic transducer (usually a piezo speaker ) in the ship's bottom or rear radiates pulses. The sound waves are reflected at the bottom of the water and received by the transducer. From the travel time of the waves and the propagation velocity, the depth is determined.

The number of pulses per second is between about 5 and 40 The pulses last only a few milliseconds, so the sonar between them has enough time to receive the reflected pulses. In deep water, the pulses must be further apart due to the longer term.

The opening angle of the measurement beam is usually between 5 ° and 20 ° (At this limit is only half of the signal power measured from the center ). This results in at the bottom, a planar measuring range, the diameter of which increases in proportion to the depth. Depending on the heeling of the ship, but also in non-vertical mounting of the encoder caused by oblique measurements large run-time values ​​, and incorrect ( too high ) depths.

In the ideal case, the first echo of the desired depth. With sloping or irregular ground or in silt and mud cause measuring errors and corrections are necessary. The surface of the ground may be determined by the use of two frequencies. The usual 100-200 kHz are already reflected at layers of low density, the deeper penetration of low frequency of 15 to 50 kHz ( wavelengths from 3 cm to 10 cm ), however, only to stronger layers.

Measuring range

The depth measurement range varies depending on the transmission power and noise. Simple device with a transmitter power between 100 and 1000 watts usually reach more than 100 meters. For greater depths more power radiation. Low frequencies, such as 50 kHz are attenuated less in the water and go deeper than high frequencies. The beam is attenuated even when the encoder is mounted inside the ship and the beam only has to penetrate the hull. A strong damping is produced by air, for example, if the fuselage is made in " sandwich construction ", or if under the donor by rough surfaces and the fast water flowing by air bubbles (cavitation ). Air bubbles are also formed by the propeller wash from passing ships.

Calculation of the sound velocity in water

The speed of sound in the water is dependent on the temperature, the salinity and the pressure; and these three variables are themselves affected by the water depth. The speed of sound is about 1500 m / s in the salt water and 1463 m / s in fresh water. In large warm salty depths (Red Sea ) it can be s to 1900 m / s.

For accurate measurements of these parameters must be determined in each case and taken into account in the translation of the term in a water depth. These are various empirical formulas.

Multibeam

For the measurement of riverbeds, estuaries and coastal flat Multibeam be used. With a beam of the survey is accurate and easy to control at multiple coverage of the measured strip.

Beam echo sounders ( Fish Finder) are increasingly being used on yachts and by anglers and fishermen.

The multibeam sonar sends successively several hundred measurement beam with an opening angle of about 1.5 °. Depending on the distribution of the beam transverse to the direction created a fan from 120 to 150 ° width. The fan opening may be the water profile, the ship's speed and the water depth adapted in modern systems. For the measurement of port facilities fan angle can be used up to 210 °. The subjects covered strip of the sea floor along the ship's course. The greater the water depth, the wider the recorded stripes. If you add multiple overlapping strips together, can be calculated from a digital terrain model and finally created a topographical map.

Side-scan sonar

Also, the side-scan sonar is used for water brine survey. In contrast to the several multibeam sonar transducers are used at a defined distance and small aperture angle here. Each transducer is operated at Ping with a different transmission frequency. Transmission frequency range, opening angle and transducer distance are determined by the measurement depth and the salt status of the measurement area and can be set or changed by the user.

Fish Finder

The fish finder shows on a color screen, a profile of the seabed. Soil, vegetation and schools of fish can be distinguished. Echoes of fish are particularly highlighted.

Apart from such sonar equipment for the maritime and commercial fishing, there are also smaller devices for the angler and amateur fishermen. The Deeper fishfinder for example, has a diameter of only 6.5 cm and weighs just 100g. In conjunction with a mobile device ( smartphone or tablet ) this sounder shows professional and amateur anglers in addition to the soil of the fishing spots also on where the fish just are.

Sediment echosounder

Sediment echo sounder used lower sound frequencies in the range between 0.5 and 6 kHz (wavelength of 3 m to 25 cm ), and thus penetrates to about 100 m in the sea floor. It provides geologists and geophysicists information on hardness and texture of the soil layers ( sediments ). This soil is also important for hydraulic engineering.

Echolocation

The echolocation is a closely related method, with which the position of objects, such as ships or fishing, can be determined. Echolocation is used for anti-submarine and search for Demining ( active sonar method). High-resolution devices are used to wreck search and persons and bodies search. Toothed whales and bats use echolocation for orientation.

Related Procedures

Chance of a similar method is also used in aviation. The Sonic altimeter is used for measuring altitude of aircraft above ground.

The autofocus system of some cameras works with an ultrasonic distance meter.

With the geo scanner soil changes are made visible.

Humans can also develop a sense for the size of rooms, if they have some reverberation, which is used especially in the dark or by the blind.

Bathymetric map

General Bathymetric Chart of the Oceans ( GEBCO ) is a bathymetric worldwide record that is based on data collected from vessel sonar. On the maps created from it you can see the exact shape of the ocean floor with his back and valleys. Google and OpenSeaMap use such cards.