Echo Sounder Flashcards
What is the Echo Ranging Formula?
Range = Speed x Time / 2
Speed of sound in salt water?
1500m/sec
Speed of sound in fresh water?
1460m/sec
What is dual frequency?
When two pulses with different frequencies (typically around 50kHZ & 200kHz) are transmitted at once. The two return signals don’t interfere with each other.
What are the advantages of dual frequency?
To identify a vegetation layer or a layer of soft mud on top of a rock.
What are the four main parts of an echo sounder?
CPU (pulse generator), Time base unit, Transducer, Display
How does an echo sounder work?
The CPU (pulse generator) generates an electrical pulse and starts the clock in the Time base unit. The electrical pulse causes the transducer to transmit a acoustic pulse which is returned from the sea bed and causes the transducer to send an electric pulse back to the CPU which calculates the time taken and therefore the depth and sends this to the Display.
Where do you preferably place a echo sounder? (4 factors to consider)
- Away from the front 1/3 of the ship to avoid aeration
- Not too close to the aft to avoid vibration from the props
- At least 2/3m away from the speed log
- Accessible for maintenance
- Beneath the waterline
- Away from thrusters
- Away from internal sources of interference
- Ideally on the centreline
How would you set up an echo sounder? (4 factors)
- Check the depth and verify if it corresponds
- Select a suitable range scale
- Adjust gain for optimum picture
- Set shallow water alarms
(5.) Check printer
Name some echo sounder errors
Aeration
Going high speed, pounding, shallow water, going astern, altering course and interruptions in water flow causes air bubbles to hit the transducer and since sound travels differently in air, it gives a wrong reading.
Index error
Due to faulty offset correction.
Parallax (Separation) error
Receiver and transducer fitted, the horizontal distance between them causes an error in depth. (Often you only have one transducer and this won’t occur)
Second trace returns
In deep water.
Echo from last transmission received after the next transmission has already been sent therefore indicated depth will be shallower than the true depth.
Multiple (multi-path) echoes
Occur in shallow waters and show echoes at multiples of the actual depth and are caused by the pulse energy being reflected a number of times between the seabed and bottom of the boat. To resolve you could increase the frequency (200KHz) as this is better for shallower water
Propagation error
The speed that sound travels through water changes with it’s temperature and saline content (travels faster in cold and more saline waters), could display 5% - 10% less than actual depth
Name some spurious returns
Side echoes
The edge of a narrow channel, mooring piles, channel bouys in example may give an echo from the side lobes of the sound wave. “Like a side lobe error”
Fish / kelp or weed
Large amounts could show up as lines across the echo sounder, smaller amounts could show up as blobs. You can adjust the frequency to try and minimise the affect or zoom into the seabed to avoid having the above on the display
Fresh water springs
Interference
When operating in high density traffic, own vessels Echo sounder may return other vessels sound waves. This may completely block the signal or give wrong returns.
Characteristics of the seabed
Water layers
Different layers of temperature can affect speed of sound wave and give false readings
What are 2 standard frequencies and what would be better for shallow and deep water?
50 KHz- Deep water, lower frequency (Cone is wider and results in less defined image
200 KHz - Shallow water, higher frequency
When would an echo sounder show a depth less than what it is below the keel?
From second trace echoes.
The first pulse is received after the second pulse therefore the timing unit has reset making the echo sounder think the depth is shallower than what it really is.
When the temperature or salinity of the water causes the sound wave to travel faster than calculated.
How can an operator reduce spurious returns?
- Check and verify depth on chart
- Change the scale and therefore the pulse length
- Changing the gain can eliminate some of the effects
