Operation Manual
FarSounder, Inc.
F31552 (Rev. 3.8.0)
Page 56 of 69
A. Understanding Water Depth Performance
Besides operating as a true 3D sonar, creating a 3D image with a single ping, FarSounder's technology
distinguishes itself as a shallow water sonar system with its impressive Water Depth Performance and
its ability to detect objects in shallow water even beyond its water depth limit. To best understand how
FarSounder's technology is able to operate so well in shallow water it is first necessary to understand the
metrics used in shallow water sonars.
There are two metrics used when talking about the maximum range of a forward-looking sonar: Maximum
Range and Water Depth Performance. Both of these metrics are important, yet most other forward-looking
sonar products do not like to talk about their Water Depth Performance.
Maximum Range is the farthest distance at which the system can detect targets under ideal conditions. This
range is absolute and varies based on a target's acoustic reflectivity. Generally, larger objects have more
reflectivity. However some materials and shapes also effect a target's acoustic reflectivity. In some cases, a
“large” object who's material or shape does not reflect well will not be detected as well as “smaller” objects
who's material and shape do reflect well. Good reflecting targets will be detected at the full range of the
sonar. Weaker reflectors will be detected at shorter ranges.
Water Depth Performance is the ratio between the depth of water below the transducer and the range at
which the sea bottom can be mapped. As an example, if a 2 water depth sonar system is operating in 50
feet of water, that system can generate bottom maps only out to 100 feet. Water Depth Performance is
limited by various physical effects and the bottom mapping range of a sonar is primarily a function of the
water depth below the transducer.
Forward looking sonars, by definition, send much of their transmitted energy forward with varying downward
(and/or upward) angles. When the down angles are steep with much of the energy pointed towards the
seafloor, a large percentage of the energy is reflected back to the sonar's receiver, though some of the
energy is reflected forward just like light bouncing off a glass mirror. The energy bouncing back to the sonar
is aptly called "back-scatter" while the energy bouncing forward is called "forward-scatter".
As the down angle becomes more shallow, the amount of back-scatter decreases as the forward-scatter
increases. The actual ratio of back-scatter to forward-scatter for a given angle varies slightly with sea bottom
composition. If the seafloor is rough, relative to the size of the acoustic wavelength, then there are more
"edges" on the seafloor that are reflecting back towards the sonar. If the seafloor is comparatively smooth,
then the seafloor acts more like a true reflecting mirror. Hard rough sea bottom materials like rocks and
sand reflect better than soft materials like mud and silt. Figures A.1-A.3 illustrate this point.
Figure A.1. Reflections from a steep angle
