Simrad EQ60 - Manual (Page 21)

System description
9
857-164342 / Rev.C
Observation range
Absorption increases dramatically with frequency in salt water.
For maximum observation range you should select a low
operating frequency, a large transducer and the maximum
transmit power.
Figure 3 Observation range
Typical observation ranges are shown in the figure. Using the
Simrad 27-26/21 transducer (27 kHz, 10x13 degrees, 3000 W)
you can observe a 60 centimeter cod down to 800 meters, and
bottom detection works down to 3800 meters. However, with
the Simrad 200-7F transducer (200 kHz, 7x7 degrees, 1000 W)
you can only observe that same cod down to 260 meters, and
bottom detection becomes unreliable below 500 meters.
These range calculations assume a normal sea water salinity
(3.5%) and temperature (+10 degC), an average bottom (surface
backscattering strength = -20 dB) and a noise level typical for a
moving vessel.