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After a few pointers - Would like some advice on the capabilities of metal detectors

Ronnie		Posted : Monday, 30 March 2015 6:37:44 PM(UTC)
Rank: Gold Flake Groups: Registered Joined: 30/03/2015(UTC) Posts: 15 Location: Christchurch Thanks: 8 times		Hi there I am looking at studying soil transport on hillslopes being grazed by cattle, and will be doing a project on it this winter, likely based in Southland. An idea that I have to measure the soil movement is to place some engraved metal balls, or perhaps nuts, in the field with a known gps location before the paddock is grazed, then return to the site with a metal detector after grazing has finished to find the balls/nuts and take their new gps point to measure the distance that they have moved over winter. Each ball/nut (I haven't decided which yet) will be engraved with a different number, allowing me to measure the distance that each specific ball has moved very precisely. I haven't had anything to do with metal detectors before, so I have a couple of questions that I was hoping people on this forum could help me with, and am also open to any other ideas or advice that anyone might have! Firstly, what is the accuracy of a metal detector? ie, is it within a couple of centimeters, or is it more likely to be 50-60 cm or a meter or something like that? Many of the balls/nuts will be beneath the soil when I come back, so I will need to know as precisely as possible where they will be to reduce the area that I need to dig up for each ball, and to measure the distance travelled as accurately as possible. Secondly, how big does a ball/nut need to be for the metal detector to be able to detect it? Will a nut to fit a 12 or 14 bolt be big enough? Thirdly, what is the range of the detector? ie, how far away from the nut/ball will I need to be before it is detected? And fourthly, will the device have the same detection capabilities for different types of metal? This may influence what type of balls/nuts I need to buy! Thanks in advance for your help!

Mudwiggle		Posted : Monday, 30 March 2015 6:55:13 PM(UTC)
Rank: Gold Ingot Groups: Registered, Moderators Joined: 22/10/2013(UTC) Posts: 742 Thanks: 182 times Was thanked: 683 time(s) in 317 post(s)		Make sure the targets are subsurface - You don't want the stock to pick them up. Accuracy of the GPS will be the biggest issue, even with GLONASS enabled, you'll still have an EPE of ±2m, depending on the satellite constellation at the time (when waypointing each mark, waypoint it again after a few hours so you have a different satellite configuration. A temporary spot of dazzle paint will help here). Detectors can pinpoint to under 2cm for a shallow target the size you're talking about, error increases only slightly with depth, within reason. however, the coil will need to pass over the target, shouldn't be an issue if your GPS marks are good. 12mm nut will ring out loud and clear, stainless will be better as if you go for a standard iron nut, you might end up chasing fence staples, wire, bits of Massey Ferguson etc... Stainless allows you to pick it out easily amongst any junk (assuming the detector is up to it, probably don't want a $100-wonder from TradeMe...) Alternatively, chunks of copper will ping down to a good depth. I pull up 1c coins from 8" easily. A nice 12mm stainless nut or chunk of copper, or even a lead fishing sinker will be easily detected several inches down - you have the advantage that your site is unlikely to be infested with bottle caps and can debris If you're looking at a brand name detector, you'll have no issues, but a "GoldMagnet 4000" off TradeMe will likely need to be tested prior to setup to make sure it actually will read your chosen target...or any target. Sounds interesting, would be keen on reading a copy of the final. Edited by user Monday, 30 March 2015 6:59:31 PM(UTC) \| Reason: Not specified

Ronnie		Posted : Monday, 30 March 2015 8:16:58 PM(UTC)
Rank: Gold Flake Groups: Registered Joined: 30/03/2015(UTC) Posts: 15 Location: Christchurch Thanks: 8 times		Thanks for all that! The crop we will be looking at will be a forage crop, i.e. brassicas, so stock picking the targets up shouldn't be an issue, as they won't be grazing hard against the ground. Good point though! And we were thinking about using a surveying transit, rather than GPS points as such. If we can get the gear and someone out to do it, then that will give us a lot more accuracy than the standard GPS, but still looking at the options. Great to hear about the level of accuracy with the detectors! I'll steer clear of a "GoldMagnet 4000", what brands would you recommend as being reliable?

Mudwiggle		Posted : Monday, 30 March 2015 8:31:13 PM(UTC)
Rank: Gold Ingot Groups: Registered, Moderators Joined: 22/10/2013(UTC) Posts: 742 Thanks: 182 times Was thanked: 683 time(s) in 317 post(s)		Depending on the site, you could just run a stringline between two opposite posts, and place a target every 2m for instance. As long as you identify the posts for next visit, I'd say it'd be a fair bet your targets would be within a short distance downhill from the string at the final measurement. The crop may also contribute to target movement as it grows, not sure what sort of scale of movement you're looking for, but I monitor various sites for deformation, and some of them can move 2cm purely due to the wet-dry-day-night cycles. As for brands, even an entry level Fisher F2, or Garrett Ace 250 will do the job. Chances are you could then onsell after the trial for close to what you paid. MW Edited by user Monday, 30 March 2015 8:41:38 PM(UTC) \| Reason: Added detector info

Ronnie		Posted : Monday, 30 March 2015 8:49:44 PM(UTC)
Rank: Gold Flake Groups: Registered Joined: 30/03/2015(UTC) Posts: 15 Location: Christchurch Thanks: 8 times		Again, thanks for the good advice :) We'll be looking at soil movement on a hill slope. Am waiting to hear back where exactly the site is, but we will try to place transects on different parts of the slope (convex vs concave, different gradients etc), so we'll see how that works out with fence posts etc. I had thought of something along those lines, but it may not be possible, depending on the scale of the paddock and the shape of the slope. What time of year were you measuring those differences on a day-night scale? I am expecting there to be a bit more movement than that, with exposed soil surface, potentially waterlogged conditions, and a mob of cattle being in the paddock continuously for several weeks, and on a slope... This hasn't really been looked into before, so it's hard to guess, but I'm expecting the movement of different targets to vary, with some having not much movement at all, and others being quite substantial. But it's all guess work until we go back out there and measure them at the end of winter!

Mudwiggle		Posted : Monday, 30 March 2015 8:54:45 PM(UTC)
Rank: Gold Ingot Groups: Registered, Moderators Joined: 22/10/2013(UTC) Posts: 742 Thanks: 182 times Was thanked: 683 time(s) in 317 post(s)		We'll take it to PMs as drifting off the MD topic now.

1864hatter		Posted : Monday, 30 March 2015 9:14:24 PM(UTC)
Rank: Gold Ingot Groups: Registered Joined: 4/08/2010(UTC) Posts: 1,007 Location: Auckland.... Thanks: 49 times Was thanked: 566 time(s) in 269 post(s)		Without reading all the replies above I recon the relative density of the metal target relative to the soil will affect its movement downslope. What I mean it wont behave the way as the soil of the slope will. Sure it will move downhill but not in the same way that soil will. A point to consider.
		And now....On sandy beaches and muddy soil, rings and coins await my coil!

Ronnie		Posted : Monday, 30 March 2015 9:27:30 PM(UTC)
Rank: Gold Flake Groups: Registered Joined: 30/03/2015(UTC) Posts: 15 Location: Christchurch Thanks: 8 times		Hi there. Thanks for your input. Definitely a point that has been considered. We are working under the assumption that the average amount of movement should be fairly representative as, when the cow's hoof enters the mud and splays the soil around the hoof, there will be some soil and some targets that will move forward and some that will move backward. We are interested specifically in soil creep, which is the gradual downhill movement of soil, and we are assuming that the targets will move at the same rate as the soil around it. It is an assumption, but it is the best means possible that we have to measure this type of soil transport. If you have any other ideas, we'd love to hear them! It is soil creep in particular that we are interested in measuring, rather than sediment that is suspended in overland flow of water.