Oculus M-Series Sonar
User Manual
14
By combining the known Velocity-Of-Sound (VOS
–
‘
c
’) in water with the time the echoes were received (‘
t
’), the
sonar can calculate the distance the sound has travelled (‘
d
’).
As sound must travel from the Sonar to the target object and then back
again, the range (‘
r
’) between the Sonar
and target is half of the total distance travelled.
𝑑 = 𝑐 × 𝑡
𝑟 =
𝑑
2
𝑟 =
1
2
× 𝑐 × 𝑡
Typically, the VOS is about 1500ms
-1
, but this can vary depending on the water temperature, water salinity and
sonar operating depth. To calculate this, Oculus contains a sensor that continuously measures the operating
depth and water temperature, and combines this with an appropriate Salinity value for the body of water that it
is operating in, entered by the user in the control software.
In the real-world, the Sonar will see more than one target in front of it. The transmitted pulse of sound will reflect
off each target at a different time, but the receiver will be recording all the echoes over a period equivalent to
twice the maximum operating range (for sound travelling to and from the target).
The received echoes are combined into a single received signal where their position in time directly relates to their
range, while the strength of each echo depends on how reflective the target is.
Targets with densities very different from water (such as gas/air or rock/concrete) will be very reflective and have
strong echoes, while echoes from materials like mud, silt, sand and plants will have weaker echoes.
The strength of the echo is
displayed on the divers display as
the brightness of a colour palette.
Typically, darker colours in the
sonar palette are used for weak
echoes,
while
bright-white
colours are used for strong
echoes.
The sonar image opposite shows
how a mid-water steel cable
appears brighter than the muddy
seabed behind it.
