Echo Sounders

Question 1

In what depth are Low frequency and High Frequency soundings used

Answer 1

Low is deep water
High is for shallow water

Question 2

Calculation of depth through sounder Formula

Answer 2

Speed x time
Divide by 2

Question 3

Make up of Echo sounder

Answer 3

CPU
Time Base Unit
Transducer
Display

Question 4

Variables to consider when siting Transducers

Answer 4

1/3 from the bow

Away from the stern

Away from noise pollution and internal sources of interference

Away from discharge/intakes

On the centreline ideally, below the Waterline

Access for maintenance

Low down on Keel

Large vessels may carry a multiple array

Question 5

What is the function of the Gain Control

Answer 5

Like radar, Sensitivity Setting.
If it has an auto, then the gain is determined by the range setting

Question 6

How does the nature of the seabed affect returns (causes of spurious returnd)

Answer 6

Will effect returns, soft mud may not appear above rock for instance.

Sand and rock are strong

Mud and seaweed are weak

Steep curve in shelf will send reflect away much of the pulse and result in weak returns

Question 7

Causes of Aeration for a transducer

Answer 7

High speed, running astern, shallow water, air bubbles from other vessels manoeuvres all affect the reliability of echo sounders

Question 8

Multiple echoes

Answer 8

Same as in Radar.
Shallow water may result in the echo bouncing off the bottom twice before the receiver accepts them.

Resulting in a series of weaker and weaker returns equally spaced
Countered by reducing the gain

Question 9

Side echoes

Answer 9

In a narrow channel, side lobe of the echo may send back a shallower reading than what is actually beneath the vessel

Question 10

Spurious returns

Answer 10

Fish

Water layers
(Distinct changes in temperature forming THERMOCLINES)

Freshwater springs
(Sound travels faster through freshwater)
In FW, the indicated depth will be much greater than the actual depth unless countered for with gain

Kelp or weed

Turbulence by tidal streams in the open sea or junction of a tributary in a river

Question 11

Comparison with chart

Answer 11

Always remember to correct with Draught of vessel before comparing with Chart!!!!

Question 12

How to Setup an echo sounder for use

Answer 12

1. Check draft/ HOT
2. Turn on and select a suitable range
3. Adjust gain
4. Verify the depth being displayed
5. Set grounding / anti grounding alarms
6. Check the printer if connected

Question 13

What is the known speed of Acoustic echos through the water? both for SW and FW

Answer 13

1500 m/s in SW
1460 m/s in FW

Question 14

What publication gives information on understanding errors/setting up Echo sounders?

Answer 14

NP100

Question 15

When would we set up a transducer for :

Below the waterline
Below the keel?

Answer 15

Below the Waterline: when position fixing

Below the Keel: when underway and making way, particularly in shallow waters

Question 16

When must we check an echo sounder?

Answer 16

When any part of the equipment is changed

On completion of a refit

Annually

If there is any doubt in its accuracy