In open seas, the sound pulse from the Doppler transducer may not reach the bottom, but get totally internally reflected from a layer of water in between. This is known as the echo from the ‘Water Track”.
When the sound is bounced off a water layer, called a water track, speed indicated is the ‘Speed through Water”. or from a layer of water and the echo is at a higher frequency.
The frequency of the echo from the water track will follow the same Doppler principles as the echo from the bottom track. However, the speed measured from the ‘water Track’ will not be ‘Speed over the Ground’, but it will be ‘Speed through Water’.
Speed over ground & Speed over water
Generally, Doppler log can receive echoes from seabed only up to depths of 200 meters or so. However, beyond these depths, a weaker echo is available from a layer between 10 to 30 metres below the keel. The speed of the vessel can be determined as earlier but in this case, it will be with respect to this layer and not with respect to ground. This is called “speed over water” similar to that obtained from other types of logs mentioned earlier. In this case, the effect of current has to be allowed for to get the speed made good but the reflections from a layer deep down eliminate the effect of disturbances caused by the vessel itself and the effect of surface currents caused by wind and wave motion.
Errors of the Doppler log
- Error in transducer orientation: The transducers should make a perfect angle of 60° with respect to the keel or else the speed indicated will be inaccurate.
- Errors in oscillator frequency: The frequency generated by the oscillator must be accurate and constant, any deviation in the frequency will result in the speed indicated being in error.
- Error in propagation velocity of the acoustic wave: The velocity of the acoustic wave at the temperature of 16°c and salinity of 3.4% is 1505m/sec, but generally it is taken as 1500m/sec for calculation. This velocity changes with temperature, salinity or pressure.
- Errors due to ship’s motion: during the interval between transmission and reception, the ship may marginally roll or pitch and thereby the angle of transmission and reception can change and for a two degree difference between the angle of transmission and reception, the net effect wiH be an error of 0.10% of the indicated speed which is marginal and can be neglected.
- Errors due to the effect of rolling and pitching: The effect of pitching will cause an error in the forward speed, but it has no effect on the athwartship speed. Similarly, rolling will cause an error in athwartship, but not in forward speed.
- Errors due to inaccuracy in the measurement of comparison frequency: The difference in the frequencies received by the forward and aft transducers must be measured accurately as any error in this will be directly reflected in the speed of the vessel.
- Error due to sidelobe: When the side lobe reception dominates over the main beam reception, there will be an error in the speed indicated. This error is more pronounced on a sloppy bottom, where the side lobe will be reflected at a more favourable angle and will have path length less than the main beam.