Coping with Mineralisation
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Filed under: Articles, Ground Types — Norfolk Wolf
When I am detecting I always keep a watch out for non-metallic surface finds, and at the same time make a mental note of the make-up of the soil on my various sites. Sometimes it has been heavy clay, at others chalky; on occasions it can be easy to dig, at other times very hard. But whatever the case there was one thing I knew for sure: it was mineralised. For years I have accepted that fact and just got on with it. But then I asked myself the question “Why are some of my fields more mineralised than others?” It couldn’t just be because of human habitation or activity alone, for no two fields - even those with no sites - are the same.
I therefore paid a visit to my local library on the off chance I could find some reference work with a mention of soil mineralisation. The printed information available turned out to be a real surprise - there was masses of the stuff. I already had the basic knowledge that my fields contained what is known as ironstone, so I initially limited my time reading about ferrous minerals. In simple terms, ironstone is a metallic element occurring naturally as magnetite, hematite etc. these being the main minerals that come under the heading of oxides and hydroxides. They are both iron oxides originating from the Earth’s core as lava. Magnetite is found in sedimentary rock, hematite in igneous rock. Over a period of millions of years these rocks have suffered from the different agencies of wind, tide, and chemical erosion. Eventually a layer of soil, stones and mud etc have covered the rocks. The other word I came across was “limotite”, which is, apparently, a term often applied to any iron hydroxide that cannot be defined without elaborate tests.
Having found out exactly what constitutes “mineralisation” my next step was to find ways of coping with it. One of the first tests I give a detector is to determine its recovery speed, especially on sites contaminated with nails. The method is to find a rusty nail about 3- 4in long from one of your sites. Place it on a clear piece of ground and set down a hammered penny about 2-4in away. Sweep at your normal speed starting about 2ft from the nail, at 4-5in off the ground, to a foot past the coin. Keep moving the coin further away from the nail until you get a signal. Mark the spot and start again using a slower sweep speed. With most detectors you should notice quite a bit of difference. The above test, of course, does not take into account the “halo effect” produced by rusting when the nail was in the ground. The longer it has been in the ground the greater the effect, and the further away a hammered coin must be to produce a good signal. This is why you sometimes have a nail that has registered a “goodish” signal, but once it is on top of the soil your detector fails to respond to it.
The situation gets worse when the ground is heavily mineralised, and you can get an “iffy” signal from a perfectly good small coin. What all this comes down to is the slower your sweep speed the more chance you have of giving the detector time to recover. Let’s take an example of a friend who I took detecting. On reaching what I knew to be a nail-infested area, rather than slowing down he kept going at the same pace. “My machine’s spitting a bit”, was his comment. The first thing he did was to increase his discrimination setting. Eventually, finding that this did not work, he turned the sensitivity down a notch although it required all his will power to do so. Even so he was not confident with the situation and decided that the sooner he was out of this patch of the field the better. He therefore increased his sweep speed! The detector just didn’t stand a chance.
We have all been guilty in the past of trying to run a detector with too high sensitivity in conditions where it simply couldn’t cope; but some people just never learn. I have heard such comments as: “I know my machine and if it beeps then I dig. The salesman showed me how deep it was in the shop”. Sure, we have all seen the tricks that detectors can be put through in the showrooms. But I’m talking about real ground conditions, with real mineralisation, and real nail infestation. I experiment with “in air” tests myself, but only to estimate how much depth loss there is when the sensitivity is gradually turned back. I’ve yet to find a coin “in air”.
Mineralised ground seems to be par for the course in Norfolk, so finds aren’t going to be deep; you can leave your big spades at home if you come detecting here. I purchased my latest machine from Coinshooters in Aylesbury and when I saw the size of the spades he was selling I thought they were for grave digging. When I used a Toltec with a large head in all metal at the end of the sweep towards my left hand it sometimes picked up my digger. This was about the same size as a Black Ada foot assisted trowel, which has nowhere near the surface area of those monsters; in which position are they carried?
My deepest finds have been Celtic and Bronze Age hoards and a Romano-British cauldron that was about 3.5ft down. However, the majority of my small finds come from no more than 5- 6in at the very most. Nowhere on my land have I been able to “wind-up” my machines. The very worst field I have encountered is covered with sheet iron-stone with electric pylons running over it. No machine has so far been able to touch it. Place a coin on the ground with the coil scraping it and the iron-stone just blanks it out. This is very frustrating, as I have found good surface items just eyes-only. Even the nearby stream is orange coloured from the ironstone.
On another field I dug an “iffy” signal that turned out to be a brass cartridge in good condition. The mystifying fact was that it wasn’t very deeply buried. Making sure that there was no iron around, I put my find back on the ground and swept over it with my recently bought Shadow X5. A coil height of 4in above the target was the best at which I could register a signal. I rechecked the machine to make sure it was ground balanced correctly and tried it in different modes with the same results.
Only 50yd away the mineralisation had altered and it was a different story. Admittedly, this is one of my worse fields and I was using the standard 9in coil before I took delivery of the 7-inch one, which obviously helps with the mineralisation. There have been a number of times throughout the years when I have tried a larger coil to gain a little bit more depth, only to find that it was giving less depth than normal together with a lot of false signals. Large coils may provide more depth in other parts of the country, but not on my sites in Norfolk. For working highly mineralised ground small coils definitely give better results.
Precise ground balancing is an absolute must for good results, and it should be set positive rather than negative. Set it up spot on, and then increase the positive balance by about a quarter of a turn. Mineralisation can vary from one part of the field to the next, and it always pays to check - every so often that your detector is balanced correctly - I have been caught out a number of times in the past by not following this simple rule.
Under certain conditions I have found that it hasn’t been possible to manually ground balance and keep the detector to a steady threshold, or there are sites where ground conditions vary within a few yards. In such circumstances I use fixed ground balance and adjust the detector’s sensitivity accordingly. At other times I work in fixed mode if I’m on a new field or am running short of time. Threshold also plays an important part in making finds. If it is too quiet you will miss items at the extreme range of your detector’s capabilities, when the targets are deep or simply very small. The machine still registers the targets - but you just don’t hear them.
Sensitivity can only be set as high as ground conditions will allow. The simile regarding driving in fog is very apt: you can see further ahead on low beam that you can on high. If you don’t turn the sensitivity down to the level when your detector is stable, then it will not be able to “see” through all the clutter and depth and finds will be lost. On certain fields I have had to turn the sensitivity setting of my detector down to as low as “3” or “4” but it still pulled out finds. If the settings were any higher than this then the detector just couldn’t cope with the conditions.
You can’t overcome mineralisation, but you can help your detector to work more efficiently in these conditions. The first and most obvious way is to use a widescan coil (D.D.). The second is to go down in coil size (still D.D.), for some this might go against the grain, but this is no different than if you were over a lot of ferrous. Depth you ain’t going to get, but you want every bit that is going and still have a stable detector. The footprint of a smaller coil sees less and therefore is less affected, plus the lines of the flux pattern of the electro-magnetic field will be tighter, thereby enhancing your chances.
If you must stick with a big coil, try sweeping it a couple of inches off the ground; this lessens the effect of the mineralisation. This is also a good practice to follow when the ground is wet from the surface down, when using a standard coil. The best achievable results when detecting, are when the top couple of inches are dry and the ground below this is damp. The dampness actually enhances the responses from the target, but it also enhances mineralisation, but because the ground surface is dry, this lessens that effect when the coil is swept on or near to the surface.
Finally, as with any adverse conditions met when detecting, slow it down and give your machine a chance to perform.
I am sure that if you take a bit more care and trouble in setting up your machine - and check the settings regularly during your search - your finds rate will improve.
John Lynn