DETECTING MINERALISED & BLACK SAND BEACHES
Published in the May 2015 issue of Australian Gold Gem & Treasure magazine by Nenad Lonic.
It is no secret that when it comes to detecting wet sand, P.I. or Pulse Induction units are the kings in terms of stability and depth. However, early P.I. units had no additional ground balance control, which meant that they could still run into trouble when ground conditions got quite tough. Not only that, target discrimination is either non-existent or very limited. Even with the discrimination hurdle, the stability and depth of P.I. machines is a very attractive proposition to the serious beach comber, and most metal detector manufacturers still offer at least one P.I. beach machine in their product line-up.
When Minelab released the first BBS (Broad Band Spectrum) detector, the Sovereign, it was a bit of a revolution as it offered perfectly stable operation over wet salt as well as full depth discrimination. However, where the BBS & later FBS units would suffer is very mineralised/magnetic soils such as goldfields and black sand beaches. Black sand beaches are difficult to work because they consist of both positive (magnetic) and negative (conductive) mineralisation, so a traditional detector would need two opposing forms of ground balance to work simultaneously. While all models of BBS and FBS detectors such as the Safari, E-TRAC and Excalibur II work brilliantly on most salty beaches, the addition of positively mineralised rocks, pebbles or black sand can be too much to handle.
In 2014 I visited Arnhem Land as part of the Past Masters archaeology team, and we were fortunate enough to detect on several beach sites on Elcho Island (after being granted the correct permissions of course). Most of the beaches there are very iron rich, often being made up of more lateritic pebbles than actual sand granules. Also back in 2013 on a trip to Darwin Harbour to train the Past Masters team on the finer points of metal detecting, the town beaches were extremely mineralised and I had to pull out all the tricks to get the units to perform smoothly while still detecting effectively. Some of the banks of a nearby river resembled the goldfield soils of Kalgoorlie, and in some of the rock pools at the mouth of the river there was an accumulation of these pebbles, which was enough to send the X-TERRA 705 into Overload! In this spot, the GPX 5000 with the 11” DD coil in Salt Gold timing worked like a gem, and the CTX 3030 also thrived. These observations got me thinking about which detectors are the best for these conditions, so here is a summary of my experiences with various detectors and how to get the best out of different units on black sand and highly mineralised beaches.
CTX 3030 – best all round
The CTX 3030 ticks all the boxes. Excellent discrimination, great saltwater handling ability, and a ground balance designed for magnetic soils. The FBS-2 technology will deal with the conductive nature of the salt, and by enabling and performing a ground balance will optimize the detector to deal with the magnetic aspect of the black sands. While detecting the mineralized rock pools in Darwin Harbour, the CTX 3030 worked great with the Ground Balance enabled and Seawater setting ON. There is also the Ground-Coin Target Separation option that I’d recommend using in such an environment. For best performance on a saltwater beach, Manual Sensitivity is usually recommended, but throw in tonnes of laterite pebbles and black sand, and you will find that Auto Sensitivity works a treat.
Sovereign – simple yet effective
As a long-time Sovereign user myself, it was sad to see the Sovereign series discontinued. I think in ten years it will really become a classic detector, already adding to its band of loyal followers around the globe due to its low cost, power, aggressive audio and punching power – particularly on ocean beaches. For black sand & mineralised beaches, the two best versions were the original Minelab Sovereign and the last version the Sovereign GT, as they were both equipped with an all-metal ground tracking search mode which still works well in mineralised conditions. The pin-point/all-metal mode on other Sovereigns and the Excalibur also works well, but the modulated audio response available on the original Sovereign and Sovereign GT provides superior performance in these difficult conditions in my opinion. Of course, no discrimination is available while operating in All-Metal, but on these highly mineralised remote beaches, you’ll be happy to dig everything due to the severe lack of targets of any sort.
SDC 2300 – the most sensitive
The new SDC 2300 works very well on ocean beaches and can tackle almost any mineralisation you can throw at it. The unit is ridiculously sensitive in any of the “normal” settings, but if tiny bits of iron, staples and other junk become troublesome, a switch into Salt 1 or 2 normally solves the problem. It must be pointed out that the SDC 2300 is purely all-metal so is not for the faint hearted at a normal trashy beach – you will be digging A LOT of targets! However, on the more remote, mineralized and junk free beaches as I experienced in the Northern Territory, the SDC will do a fabulous job. The fact that it is waterproof to 10ft is just the icing on the cake.
GPX 5000 – the deepest
For mineralised beaches, the deepest detector by far is the GPX 5000. The GPX 5000 has the Salt-Gold timing option, which works brilliantly in soils that are both salty and iron rich, allowing you to maximize depth and sensitivity by using a monoloop coil. But if iron targets are an issue, by using the supplied 11” DD coil, you can activate the Iron Reject and get some ferrous discrimination as well, or switch into Cancel to deal with any high levels of EMI. The GPX is not waterproof, so I’d only use it up on the dry flats, well away from any water. Waterproof coil options are available.
Other worthy mentions
While not ideal, other models that can work in these conditions are the Excalibur II and E-TRAC. Depending on the concentration of black sand, the Excalibur II may lose a lot of sensitivity if the sand is very magnetic. For best results in these conditions, use Auto Sensitivity and All-Metal mode, but this may not be possible in very trashy sites. E-TRAC users wishing to detect mineralised beaches may find that Auto Sensitivity will give superior results, as well as Fast – ON, and Difficult Ground setting. Keep an eye on your Sensitivity as it is likely to drop to quite a low level, but you will still pick up targets if they are within range. A low Sensitivity setting on a FBS unit seems to be more forgiving than on a VLF detector, meaning you won’t lose as much depth as you might think.
The Garrett ATX is also a highly sensitive beach detector, but as I haven’t used it on a mineralised beach at the time of writing this article, I can only speculate on how it may operate. There may be other units out there that would be worthy contenders for mineralised beaches, possibly Fisher’s CZ series, Whites TDI and others, but I have only had limited experience with them.
How do you know if a beach is highly mineralized?
This is a very good question, and basically the simple answer is you first need to be familiar with your detector and know how it performs in normal conditions. Then when you hit a highly mineralised spot the detector will let you know about it, either through erratic signals, “moaning” of the threshold, or in the case of E-TRAC and CTX, a very low Suggested Sensitivity will be displayed. I was detecting a beach recently and targets were few and far between, and as the tide went out, I noticed a lovely gutter that had ripped about half a meter of top sand off. As I approached it the detector went a little “silly”, where normally it behaves brilliantly. I was experiencing excessive nulling and disappearing signals, and noticed that the Suggested Sensitivity had dropped to 12, much lower than my manually selected level of 21.
After digging a few bottle caps, I noticed that the sand underneath was a black sludge, and at this point I decided to Enable the ground balance, and I also switched to Auto Sensitivity. Very quickly the Sensitivity settled on 16, and the detector was running extremely smoothly and I started to dig several targets: coins, lead, small brass items, and I was lucky enough to find a gold ring. So it pays to be observant to what your detector is telling you.
Ocean beaches that are also high in mineralisation other than salt have always been very difficult to detect, but we should consider ourselves lucky that several detectors are now available that are more than up to the challenge. Hopefully I’ve given you a few tips that will help you improve your success rate.
By Nenad Lonic (Phase Technical)