DD Coils Update on Depth

It's been few years now since the later GP and GPX units have allowed the use of mono coils and it seems most of us have put the DD coils away, yes it's true the monos put more power into the ground in relation to coil size and are more sensitive for shallow smaller gold that the DDs had trouble picking up, but what about the larger deeper gold.
It has been said before by knowledgeable members in past posts that simply to apply more power isn't the answer to depth problems alone and I remember it being said there comes a point of [more becomes less] and the example was, if that was the case minelab would simply do this on each new model ie double the voltage double the coil size and double the depth
It seems to me the older units with DD coils did a very good job on the large deep gold especially in noisy ground and looking back at all the reports and stories in G&T a hell of a lot had been found, there is nowhere near the same reports now so the newer units with mono coils arnt finding all the big gold that your think would have been left in the ground and out of range of the DDs especially with the older units.
Its true there are still big nuggets being found but I think that has more to do with no one passing a coil over that piece of ground before, I refer to nuggets like the golden boomerang in WA poking partly out of the ground.
I still occasionally use my big DD but that has more to do with it's anti-interferance qualities.
Anyway whos still using DDs and are any of the GPX users giving them a go and have they compared depth on large targets against monos in noisy ground.
Cheers Mark

Mark

Good question posed, cant wait for some responses






Pete in WA

Hello Mark, this subject has been chomped on a fair bit recently but I'll try to answer you succinctly. All the gold that has been found on old patches by the "Smooth Class" of timings since the release of the GPX-4000 is gold that was missed by DD coils, this is pretty obvious because once people realised what the timings were capable off they immediately returned to noisy areas to take advantage.

The moment you put on a DD coil you lose depth due to coil size and the way they transmit and receive, this is across the board except for targets close to the coil, on the other hand a Mono coil in a "Smooth" type timing has full depth (compared to normal timings) on a lot of targets and equal or better depth on the vast majority of other targets compared to a DD coil.

At the moment there are still plenty of sub gram nuggets in shallow areas that are worthwhile chasing due to the high gold price, generally shallow ground will be the noisiest so the new timings will be of most benefit there, but if the signal created by gold is greater than the background ground signal then the DD coils would have already taken those nuggets out so only nuggets that have a signal less than the ground signal will remain (relative to timings and coil used), usually this is smaller surface nuggets or mid sized gold within the first 12 to 14 inches of the surface. If the nugget is very large and the ground reasonably benign then a DD coil in Normal or even Sharp timings would have a chance but as most of us know those nuggets are few and far between now days.

Hope this helps,

JP

Thanks for the reply Jonathan
I take it your saying- the mineralisation layers that caused problems with the monos before but the DDs could still penetrate to find those larger nuggets at depth now has been corrected by better electronics by the units themselves and therefore DDs are no longer required , I know the units are quieter now in this ground with monos but beleive this sacrificed depth on large targets which doen't help much with the deeper big gold.
Surely as gold is dense, bigger gold should be much deeper than small gold unless sitting on bedrock or in geological terms a recent shed or placement .
There must still be a quantity of bigger multi oz pieces left out there, that the older units with DD coils missed if the newer units are better with monos in this area and they should be finding them in quantity if that was the case, but it seems to me the newer units with monos arnt showing this .
I understand there is a lot more small gold to big out there and it seems to me finding this has been the focus of both operators and manufactures of late.
My testing with DD coils on large targets in noisy ground at depth against big monos gave the DDs an edge in this area, I don't have the newest and latest GPX unit but I also don't see the quantities of big gold you would expect others to be finding if they were beating the older units with DD coils in this area.
Anyway this is just my way of thinking and wanted to throw that out there to see what others think.
It would be good to know for sure what the best setup is to target this area and to be honest I don't think the Gpx units with a big mono is it.
I'm thinking a SD 2200 /DD coil with a voltage booster but it could be a DD coil with a GPX which no one seems to be trying
Cheers Mark

What about an SD 2200 with a large mono? Even though it would be noisier to use, would the mono "see" the larger and deeper nuggets better than the dd??
Chris

The idea of using a DD coil is to tame the ground signal, by using a DD you restrict the size of the transmit and also the receive as well as ignoring a lot of the ground response (as well as target signal) due to the proximity of the second winding, that is why they run quiet.

Even if you're using Normal timings then you run into the problem of ground noise (Normal type timings will generate a louder response and in some case the only response compared to Smooth type timings if you know the target is there (test beds etc)), the trade off with the new timings against outright performance is obvious going by the amount of gold that's been found using the Smooth type timings with monoloop coils. The ground signal is the biggest hurdle for outright depth, when using a DD coil in Normal timings the operator is going to be getting more information for sure but a lot of that info is just plain clutter that will be masking good targets. To best deal with variable noisy ground (which is why you're using a DD coil in the first place) you will also most likely have to use Auto Ground Balance or increase your fitness levels for all the false target signals you'll have to dig, Auto GB is a trade off in outright depth but is a necessity in noisy ground with Normal type timings.

As I said above the ground signal is the hurdle, use traditional timings and the target has to be louder than the ground signal to be heard, use Smooth type timings with a Monoloop and there is some trade off on outright depth but the ground signal is nearly non-existent. I have found pieces up to 7 1/2 ounces at depth with the Smooth type timings and monoloop coils, the vast majority of those nuggets were missed due to using conventional timings with Monoloop or DD coils.

JP