Looks like sand to me.
Finer, silty sand on the top sending back medium signals (yellow)or perhaps weed/grass.
More dense, compacted sand underneath sending back stronger signals (red)

Yeh looks like sand !

Happy to be corrected, my thoughts though -

your gain seems very close to correct. There are dots across whole screen but not too many dots which IMO is good.

I would call the bottom to be flat or very close to it, very likely to be sandy with a uniform cover of low height weed if there is any weed at all.

Boat speed and scroll rate have a bearing as to how an image appears and how correctly the image is interpreted, swell height and period also affect the image.

There is a very good sounder book on the market which you might consider buying. Site sponsors stock it. 30km out and 30km back is about $50 of fuel, so the book is cheap in the context of your fishing.

I guess you have a standard transducer of 600W and mounted to transom skimming, so colour line setting should be some where around the 75% range. It does become a personalised thing where the colour of materials suits you the user. So when your bouncing a sinker on hard ground using braid you can fee the hard bottom through the line, check your colour line setting % and change it up and down 10% each way to see the affect. Then stop off in the 30m zone West of the five, drop a sinker down again to feel soft ground and check your sounder. Change the colour line up and down 10% again and make some decisions from the information gathered.

get the book. Good luck and tight lines.

I would think that to be rocky bottom. Maybe thats why i dont get the great catches i often see on here? And as mentioned already, boat speed and picture speed have alot to do with interpreting sounders.

And buy the book! "How to Read Your echo Sounder, By John adams".

 Exactly what I see when I'm not catching anything besides sand whiting or the odd baby flathead!

As opsrey said...try to determine the type of ground by other means...braid/sinker, or shallower on a bright day where you can see the bottom, and see what screen indicates...adjusting gain and/or colour line accordingly.

 +1 for sand.

Next time you go out drive over any known reef structure and take note of what it looks like, patchy ground is good because you can sometimes see reef and sand and back to reef in the same screen which should put you in good stead for spotting reef out deeper.

 Cheers guys... good idea about the sinker thing.

Check out an older thread of mine: fishwrecked.com/forum/whiting-sounder-shot 

My pics the same colour pallette and definitely sandy bottom given the number of whiting caught.  Almost half the depth and different screen proportions, but very similar showing to what you have.

Lots of good advice above, also have the John Adams book and would recommend.

Yeah just looks like flat sand to me.

Your settings look fine, you want a little clutter in the top 1/3 of the water colomn to show

you are running sensitivity high enough.

Need to look for more uneven looking bottom and a darker edge will signify harder bottom, showings

of fish or bait even better.

 Thanks everyone, heading out tomorrow so I'll give those things a try...

But basically all sounders show the same return. Its the colours and display that changes. A basic rule is the bottom ECHO is smaller on sand than solid. In your case its the yellow and red. Mine shows the bottom as a red line. Over sand the line is solid but not wide. Over reef the bottom line is solid but wider, indicating more returned ECHOs bounching back to the receiver.
KISS principle, a thick bottom line = solid, a thin line soft, ie sand or mud.
Everyone seem to look for lumps, but off Dampier some off the best Red ground is small rises over solid ground, maybe only 1m. So being able to tell solid from soft bottom is a must.
In this shot its barren sand/mud, and 30ks out I would almost guarantee no mud!!

 As others said, good idea to play around where you can see/feel the bottom, then look at your screen. Harder bottom will show as longer tails than soft bottom, and really broken up/cave and crevice riddled stuff will show the longest tails of all. This is due to the returned echo being broken up and re-reflected by the rugged bottom. Flat hard rock may show as a thicker echo at the top but the tails won't be there. if you have the gain up and enough screen real estate visible, you will get a third or even fourth echo as the return bounces back and forth. The third echo will usually be absent on really good bottom as the energy from the pulse gets lost among the structure. 

I spent 26 years using Furuno, JRC, JVC and Simrad sounders as my tools of the trade, and this Lowrance(HDI-7) really doesn't impress me. I'm still trying things, won't give up on it, but really miss not having bottom lock. THAT is a feature will clearly shows you good ground from just hard crap when you get out deeper. And is really useful if you are chasing red emporer on flats-they love the sponge gardens.

 the above answers, are ALL different to mine and I dont want to disagree but.. that has prompted me to go back to the "bible" and rethink this. I am wondering why there is "tails" present at the bottom of the shot in red colour. Red being a stronger echo indicating harder than the above  yellow. Just makes me think there is a layer of sand over hard rock. Then with a depth of 40mtrs the yellow is almost  2mtrs thick?

Still learning so feel free to fire away and educate. I need a night out at Oceanside when the next talk is on I thinks.

Cheers. 

Only sand showing, nothing else.
If you are unsure of what you are seeing and its too deep to view with a viewing glass or dive down, then used some duct tape ties sticky side out on a big sinker and drop it over the side, same goes for type of weed, lots of real small hooks dragged along the bottom to get a sample to look at.
Its old school stuff that the pioneers along the coast used with the update using duct tape instead of fat or dripping.

 A "layer " of colour over another doesn't mean a layer of one kind of substrate over another--that is just a product of the tranmitted signal's returned strength. This will vary with the software that particular sounder uses as well as the actaul bottom it is seeing and the amount of gain you are using. Furuno picture is different, for example, to Lowarnce, and the colour palette is different, but the principle is the same. The colour pallette thing works arse about with Lowrance than it does with Furuno--with Furuno, you just progressively remove the lighter colours (starting with light blue) to remove the weaker echoes. (with any given gain/tvg combination) IMO this is a logical way to do it, remove the fluff and clean the picture up. With Lowrance, it works backwards to that. And yellow is the strongest colour, not red.(apparently) Colours in an echo ( if you are not using white line or equivilant) will be strongest colour on top, fading down to weakest in the tails. Tails are very important, in colour and length. And like carnarvonite said, that just looks like a sandplain. I'll grab some screenshots from my HDI-7  later this week, it might help.

 Mate... Thank you!  That makes a whole lot of sense. I really appreciate the time you took to tell me your some of your wisdom. Cheers a heap.

Now i cant wait to get back on the boat and play again.

 Obviously, the depth of the water is relative to the transducer, which is tied to the boat. So as the boat goes up, the transducer gets further way from the bottom, so it looks "deeper" You've probably worked this out already. But there is another effect which affect length of tails. if the boat is rolling a bit, and small boats usually are unless it's a glass-off, the impulse gets shot out a bit to the side, and comes back in staggered returns ( best way i can explain it). This results in longer tails. You can see it in the screenshot you have supplied, notice tails are a bit longer as you lift and roll (echo goes down) Don't confuse this with the longer tails you get on harder bottom. 

Bottom lock is very useful in swell and slop, it takes all that out of it. But it isn't on the HDI, only on the HDS. ( or anything Furuno). I see you can get Amplitude Scope(A-Scope) on the HDI--as crayfishermen, we looked at that when Furuno first intro'd it in the nineties, played with it a bit, then largely dismissed it as a gimmick. Still think it is, but that's just my opinion, YMMV. Of course, our main concern was with bottom composition, not fish (mostly), so it may be different for fishfinding nowadays.

Edit--jump onto the for sale section in this forum for the Raymarine stuff at this link fishwrecked.com/forum/sale-raymarine-cp450c-clear-pulse-chirp-sounder-module

There are some sounder shots which show difference in tails really well. The top picture, right half  of the screen , it goes from sand onto good reef. Then the picture below that , right half of the screen again, he is going over broken sand and reef and some small lumps. Juicy pic.