Simrad EK15 Reference Manual page 275

One method for minimizing the "bottom slopes" phenomenon is to use a transducer with
narrow beam, or to increase the pulse duration.
Another way to minimize this phenomenon is to open the
then change the setting for
→ Bottom Detector dialog box on page 218
A The peak of the bottom pulse
B Default bottom backstep level
C Approximate bottom backstep level for flatfish
detection
D Approximate bottom backstep level for seagrass
detection
The bottom pulse basically identifies the bottom depth
just prior to the peak of the pulse (A). However, this
may not be the true bottom. For example, if the bottom
pulse is generated by a rock bottom under a thick layer
of mud, the actual depth is slightly shallower. For this
reason, the EK15 is by default set up to give you a depth reading a few milliseconds
before the peak of the pulse. This is done by setting the bottom backstep level to a
default value of -50 dB (B).
By further decreasing the level (make it more negative) the bottom detector will become
more "sensitive", and the bottom will be detected earlier.
On the echogram the white line will then "climb" up the slope. Make sure that you do
not increase the sensitivity too much. This will have an effect on the fish detection
on a flat bottom, and the biomass values will be wrong. Our experience show that an
approximately -75 dB bottom backstep value can be used safely.
By increasing the bottom backstep value (make it more positive) the bottom will be
detected later, and it will appear to be deeper.
Tip
Rule of thumb:
• Reduce the bottom backstep level (make it more negative) to detect the bottom earlier,
and thus increase bottom detection "sensitivity".
• Increase the bottom backstep level (make it more positive) to detect the bottom later,
and thus increase "penetration".
351718/C
. This function can also be put to other use.
Bottom Backstep
Echo sounder theory
dialog box, and
Bottom Detector
The Bottom Backstep
principle
(CD010217-003)
-30 dB
-50 dB
-70 dB
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