Easy Way to Identifying Bottom Type
2014 Oct 18
Bottom type is very important when it comes to fishing, especially freshwater inland waters, but that doesn’t mean that it isn’t just as important for offshore saltwater anglers.
Using sonar to look for large rock piles, reef pinnacles and wrecks is quite easy as the bottom profile changes dramatically.
When looking for hard slate, clay or gravel on dams or large flat limestone or coral reef with small holes in it out at sea, it becomes a lot more challenging if you don’t know what to look for.
Your user manual discusses ‘Bottom Hardness’ under the ‘Settings – Colorline’ section. Very often it is stated as simply as “Narrow bottom line is soft bottom and thick bottom line is hard”. They are certainly not wrong in saying that, but it is certainly a lot more complex.
How does sonar determine if the bottom is hard or soft?
This part is actually very simple:- the softer the bottom, the smaller the amplitude (strength) of the echo, and the harder the bottom the greater the amplitude of the echo.
Frequency also plays a big part because the larger the cone angle, the more ‘averaging’ takes place and therefore the peak strength is lost. So the smaller the cone angle the more accurate the true amplitude of the echo is going to be. The most common transducer today is the 83/200kHz which typically has a 20° angle on the 200kHz and 60° on the 83kHz, therefore the 200kHz is going to be the best frequency for determining bottom type. The Airmar P66 for example has a 9° cone angle on the 200kHz, thus making it a very good choice in transducers when it comes to bottom type. If you are using CHIRP, you will need to select High CHIRP.
There still remains one very important question to ask - “What is regarded as ‘thin’, and what is regarded as ‘thick’? Keeping in mind that depending on colour palette and colorline setting, you can make the bottom appear thinner or thicker by a simple adjustment. The solution is trending, or comparison at a constant depth range and Sensitivity/Colorline setting.
Make sure your transducer is installed correctly. If you are getting equal length tails on your fish arches on 83kHz or Medium CHIRP then your transducer is running true.
Make sure your sonar has the correct transducer selected from the menu, and select correct frequency – 200kHz or High CHIRP.
Set palette, sensitivity and colorline to default (factory setting), and make very slight adjustments if you absolutely must.
Once target depth is reached, turn Auto sensitivity – OFF (very important). Each time you step to go deeper or shallower you will have to turn auto back on for a second or two, then immediately OFF again.
Run parallel lines to depth contours on manual range at least double what the actual depth is. This will give you a view of the second echo line which will go a long way in helping you recognize the hard areas. If you are shallow enough, you could even go 3 times the depth so you can see the third echo line.
If you stick to these guidelines you will find it very easy to identify even the slightest change in bottom hardness.