Echo Sounder

Question 1

Simple Echo sounder system

Answer 1

Question 2

Errors in Echo sounders

Answer 2

Question 3

Spurious Returns

Answer 3

Question 4

How a depth sounder looks in comparison with a chart

Answer 4

Question 5

Where should they be positioned? (4 factors to consider)

Answer 5

• 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

Question 6

When would they show a depth less than what it is below the keel?

Answer 6

• 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

Question 7

How would the following affect an echo sounder. How do they look and how do you correct them?

Answer 7

• Multiple 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
• Echoes from plankton, fish or seaweed
  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
• Turbulence and aeration
  Would show up as interference near the surface of the echo sounder, potentially giving corrupt readings. Could increase the frequency as higher frequency (200KHz) is less affected by aeration. If you have two transducers you could change to the more forward of the two.

Question 8

With the aid of a diagram explain parallax

Answer 8

• Error in measurement due to offset of transducer from where depth is meant to be measured from

Question 9

What are 2 standard frequencies and what would be better for shallow and deep water.

Answer 9

• Deep water, lower frequency (50KHz)
• Shallow water, higher frequency (200KHz)

Question 10

(a) Explain the principle of how the echo sounder determines the depth.

Answer 10

a) Sound pulse transmitted from transducer. Sound reflected from seabed back
to trans. Time measured from transmission to reception. Speed of
propagation known (1500). Formula:
Depth = Time x Speed / 2 (10)

Question 11

(b) Describe the causes of multiple echoes.

Answer 11

b) Sound pulse reflected several times between seabed and own ship hull or
water interface (2)

Question 12

(c) State how the OOW is able to identify multiple echoes on an echo sounder
display.

Answer 12

c) Multiple echoes appear deeper than true depth. Multiple echoes at 2x, 3x etc
the true depth. Multiple echoes diminish in strength due to increased
attenuation. (4)