IMPACT SUBSEA
INNOVATIVE UNDERWATER PRODUCTS
www.impactsubsea.com
Document No:
0000.1991 |
Version No:
1.6 | 6
th
October 2020
30
6.2.2 Transmission Loss (TL)
As the acoustic pulse propagates through the water, it experiences spreading. This causes
the energy of the signal to be dispersed over an ever-increasing area, diminishing the
energy at any specific point as distance increases.
The acoustic pulse will also experience absorption by the water. The rate at which the
acoustic pulse is absorbed is directly related to the pulse frequency. The higher the
frequency, the higher the absorption rate.
However, typically the higher the frequency, the higher the acoustic resolution can be
achieved. Thus, another trade-off must be made: to use the highest frequency possible,
while achieving the desired range capability.
6.2.3 Noise Level (NL)
Noise level is environment specific; which can often be the reason for acoustic systems
experiencing different levels of performance in different locations, or even when operating
at different times.
There are numerous sources that contribute to the background noise level underwater. All
of which make the detection of the return acoustic signal increasingly more difficult.
From an environmental perspective, marine life such as snapping shrimp can cause a
reasonable level of noise. Also, wind and rain can be a factor if operating close to the water
surface.
Man-made sources of noise include those from machinery – such as vessel noise (thrusters
and props), and also noise from ROVs and AUVs.
Multipath effects can also add to the background noise. If operating acoustic equipment in
an enclosed area/close to a structure, the acoustic signals tend to 'bounce around' which
can cause sporadic operation of acoustic equipment.
In the case of imaging sonar, multipath can sometimes cause faint mirrored images of
targets.