I hope this article helps answer your question about "flat coral".

As for "gravel bottom" my understanding of the term relates to a sludgy type of bottom which is usually the result of an area used by dredges to dump the material that has been recovered from another area in order to deepen the area dredged. There are a number of areas in Cockburn Sound which are referred to as "gravel bottom" which are the result of the dredging of the Parmelia Channel.

I hope that that makes a bit of sense, it is not to my knowledge "gravel" as we identify on the land (as in gravel roads). How it got the name gravel bottom I haven't a clue.

https://en.wikipedia.org/wiki/Fringing_reef

 Sounder you are useing. I use jrc and I can 100 percent tell the difference. Between coral , gravel , sand and reef. Guys with furuno sounders even better. pretty sure the Lawrance sounders would do the same. I always use bottom lock. And watch the depth( thickening of bottom). (Density of the bottom.) Flat hard coral goes very thick. Reef is also thick but it isn't constant thickness. Sand thin . gravel thin and patches of thickening. Hard to explain but pretty simple and basic to read and understand sounder if correctly set up.

If you look at youtube for wa lobster or reef divers you will often see flat coral and coral/rock lumps that are between 1 and 3 metres tall with the crays under the ledges. Often when people show dhufish from the gopro they are over large flat areas of rock/coral/light weed covering. Dhufish use caves for protection but a cave can be a simple overhang as well.

'Gravel' is often the broken flats of old coral structures that have broken down into small chunks and kind of look like gravel.  Usually these are adjacent to still living coral areas but can also be found in deeper water where the coral lumps have died out over the years.

Either way, for demersals you are looking for hard bottom, some kind of structure and/or feature that will hold the fish as there is shelter or food or both.

Gravel had me very confused as i was thinking of gravel as we identify on land

So thanks for the answers on that  

As for flat coral, i think of coral being attached to reef so flat coral seemed unlikely.

Normaly coral areas would be anything but flat. 

Understand about fringing reefs etc that Magladon refered to.

Thanks for the tips on sounders , i have to confess i am bloody useless at reading mine 

I caught a dhuie on Friday so all good - never stop learning though

Probably im lucky that i cut my teeth with fishing before sounders were invented  

 Buy John Adam's book from the guys at Oceanside. That will enlighten you no end.

 i got the book as s Xmas present and have read it twice 

best value for money in the tackle store IMO

I have been diving out of bunbury for over 40 years from the beach to 20 miles out and I am still looking for gravel. This includes areas that show as gravel on old maps of bunbury. The only thing I have seen that resembles gravel is areas that have a lot of what looks like crushed up shells and coral. We see this mainly in areas of low limestone country that can be quite extensive but barren as far as fish and crays. They seem to like edges and sand. Everyone seems to categorise the bottom into sand or coral or weed etc but most of the time you can see all of these things in the one spot. Like already mentioned have a look at some youtube footage. I will keep my eyes open tomorrow and see if I can spot some gravel.

 Nice to know im not the only one confused by the gravel thing.

 Good to read your comments, especially as you're a diver

 Ill check out you tube   

 This is probably a better functional description. It will be sandy or muddy with lots of scattered small bits of limestone or coral, with odd larger pieces. Habitat for very small creatures, and  good hunting ground for the larger species that target them. Excellent for spangled emperor, particularly during a heavy swell or late in the afternoon. Pinkies also scatter over it, particularly as the light fades, at least up our way. Baldchin will certainly hunt it.

 I know for reds, being able to distinguish that flat, hard or rubble bottom is key. I'm not a diver but with John's book and a Furuno 585 with 1kw transducer it was easy!