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ACCOUSTIC CORRELATION LOG

ACCOUSTIC CORRELATION LOG. Unlike the previously described speed logs, which measures the vessel speed with respect to water only, this log measures the speed with respect to the seabed or to a suspended water mass. This log derives the vessel speed by the use of signal acoustic correlation.

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ACCOUSTIC CORRELATION LOG

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  1. ACCOUSTIC CORRELATION LOG • Unlike the previously described speed logs, which measures the vessel speed with respect to water only, this log measures the speed with respect to the seabed or to a suspended water mass. • This log derives the vessel speed by the use of signal acoustic correlation.

  2. ACCOUSTIC CORRELATION LOG • This is a way of combining the properties of sonic waves in sea water with a correlation technique. • Speed measurement is achieved by bottom – tracking to a maximum depth of 200 mtrs. • If the bottom echo becomes weak or the depth exceeds 200 mtrs the system automatically switches to water – mass tracking and will record the vessel speed with respect to a water mass approx. 12 mtr below the keel.

  3. ACCOUSTIC CORRELATION LOG • The transducer transmits pulses of energy at a frequency of 150 kHZ from 2 active elements that are arranged in the fore & aft line of the vessel. • Each element transmits in a wide lobe perpendicular to the seabed. • As with an echosounder, the transducer elements are switched to the receive mode after transmission takes place. • The seabed or water mass reflected signals possess a time delay depending upon the contour of the seabed.

  4. ACCOUSTIC CORRELATION LOG • Thus the received echo is uniquely a function of the instantaneous position of each sensor element plus the ship’s speed. • The echo signal, therefore in one channel will be identical with that in the other channel but will possess a time delay. • It should be noted that the time delay calculated is that between the two transducers elements is precisely fixed, therefore when the time has been determined the speed of the vessel can be accurately calculated.

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