The colours are not specific types of ground. I.e. its not colour coded to ground type.

What the sounder does is show red when its a strong return, orange when its medium, and blue when theres almost none.

So red indicates nearly all the sonar waves reflecting. The coral means most is, but not quite all. Typically coral will be much higher height on the sounder, while hard sand will be a thin red line (because it all bounces off the top of the sand, while with coral, all the hard bits are over say a meter or two perhaps).

Yellow, green and blue are weaker signals. Some small fish or seaweed in the water column might be yellow as theres not a strong signal pinging back.

The second sounding is the sound reflecting off the bottom, then off the hull/ocean surface, then to the sand, then back to your transducer, which gives the ghost image. Some people find it uselful as its like a lower powered beam (can show more detail that got "redded out" in the first ping).

There are lots of threads on here that you can search on the left that will give you heaps of info (buckle up for some reading). There are also magasines that tell you how to use/tweak/understand it (one came up recently, so try the search function).

Thanks mate...... I'm slowly getting it sussed out......maybe I will try paying more attention to my second reading!!

What about chard speed?...... If it's to fast for the speed I'm going will I miss anything on the bottom??(small lumps and bumps etc?)

dunno about others , but I match my chart speed to boat speed.

it runs at slow speed while sounding around , but if you jet off at 25 knots and the chart speed stays slow , you can miss things.