Tow Coils / Drag Coils

[Reg Wilson]

Responding to Peter in SA.

Back in the mid 1980s I had been involved in the introduction of the Minelab GS15000 metal detector at Wedderburn in central Victoria. The early Minelab crew were a bunch of clever people from Adelaide university who got together to produce an Australian made metal detector which could challenge the big American companies. Wedderburn was chosen for testing and development due to its reputation of having very mineralized, difficult soil conditions. A number of Australian built detectors had failed to live up to expectations, defeated mainly by the hot soil conditions in Western Australia and central Victoria. The GS15000 proved to be a better machine for these conditions than its competitors and soon gained acceptance throughout the gold detecting fraternity.

Craig Hughes, who was part of that early team came up with the idea of a coil towed behind an ATV. An area near the famous 'potato patch' just out of Weddurburn was chosen for testing, using a Honda three wheel ATV (horrible unstable beast of a machine). The coil was fairly small, but I can't recall the exact size. A GS15000 provided the electronics, and although noisy, did work with a small bit of gold recovered. (a few grams) Even though the manual ground balance made for uncomfortable detecting, the concept had been proven, much to our surprise.

In 1987 I was involved with the testing of the GT16000, which was the first ever automatic ground tracking. This machine was a major breakthrough and really put Minelab on the map as a serious contender for the title of the world's best gold detector. It dawned on me that the feasibility of tow detecting was a reality now that a detector could stay balanced automatically, and I contacted Don McCoy, one of the original Minelab team and asked him to build me a coil suitable for towing. The result was a rectangular coil about 3' x 2' which was very stable and sensitive. I purchased a Yamaha 4 wheel ATV as a tow vehicle and with the addition of a suppressor managed to keep the EMI to a minimum. The first day of testing yielded a 6oz lump, which was a big surprise, and paid for the ATV in one hit. I sent Don some nice specimens as payment for the coil. I later sold that coil through Miners Den in Melbourne when the SD2200 was introduced.

The SD2200 was of course the first auto ground tracking pulse induction detector, and naturally I soon made plans to adapt this concept to 'sledding'. John Hider-Smith, Ian Jaques and myself had been involved in prototype testing of Minelabs first pulse induction detector, and Bruce Candy had taught john how to wind mono coils. One of Johns coils was used in the first PI tow coil testing where we discovered that a 4 stroke engine was not feasible due to the high susceptibility of PI to EMI. With experimentation we discovered that diesel was the way to go as no spark was required for the engine to run, and an isolation switch for the alternator solved any EMI problems. We later used a GPX4000 which worked even better. 

I have to close now due to having to shut down internet but will add further details later.

[PeterInSa]

Reg, With the set up I saw years ago, 2 guys rode on the quad bike, 1 drove the other had a scab-id of spears, when they had a signal, the non driver thru a spear in the ground to show the site of the signal, when they ran out of spears they got out their detectors to detect near the spear sites. Is that what you did? ( If this is illegal in Oz don't comment)

[Reg Wilson]

The original PI sled was all plastic construction with height adjustment and replaceable black plastic poly pipe skid liners. Mounted at the rear of the sled was a plastic tank and spray nozzle. A small windscreen washer motor was wired to a button on the handlebars, and a spray of agricultural dye marked the approximate target site when a signal was received. At the end of a session of gridding hand held detecting pin pointed the target site, and detected objects recovered.

This sled worked well in Victoria on pastured paddocks, however the abrasive conditions in Western Australia soon destroyed it and a different concept was required. A visit to a mining scrap yard resulted in a short length of conveyor belt forming the basic skid requirements with a section of split poly pipe transversely snapped over the leading edge of the belt to assist passage over rocks or other obstacles. The box containing the 38" coil was then mounted on the belt with large nylon nuts and bolts which were countersunk into the rubber belt. A simple rope was then used to tow the contraption. This set up proved to be simple but highly effective. Geo marker flags were then used to mark targets. This involved stopping the vehicle and marking the target site by hand each time a signal was received. Not a huge inconvenience where targets were few and far between.

It was my pleasure to have had the company of the late Jim Stewart and James Beatty on that trip to WA. They had traveled over with a very large double D coil that Jim had built and was hoping to use this coil to achieve greater depth in the hot WA soils. We conducted some tests using hid double D on the sled, but it was not successful as the double D required very slow speed to be responsive. Hence we discovered that Mono is the way to go for sledding.

[Steve Herschbach]

 

 

 

 

 

[Reg Wilson]

A couple of points when sledding.

Noise suppression coils sacrifice sensitivity for quieter use.

Longer cables equal less efficiency. 

The old KISS principle is best (keep it simple, stupid)

The sled that I still have works best in conjunction with the GPX4000 running on sensitive smooth. I have tried other models but for some reason the 4000 is the best. The 38" round mono easily picks up .22 slugs at 5KPH with a clear unmissable signal.

[mn90403]

What is the optimal orientation of the coil when dragging?

[Reg Wilson]

Simon, the problem with your magnet idea is that it would soon be clogged with stones with a high iron content.

Mitchel, the beauty of using a base of conveyor belt is that the coil sits flat and parallel with the ground. this makes for good ground tracking. When used on pasture the grass is flattened by the weight of the belt keeping the coil as close to the ground as possible, but the grass stands up again once the sled has passed over it.

[mn90403]

5 hours ago, Reg Wilson said:

Mitchel, the beauty of using a base of conveyor belt is that the coil sits flat and parallel with the ground. this makes for good ground tracking. When used on pasture the grass is flattened by the weight of the belt keeping the coil as close to the ground as possible, but the grass stands up again once the sled has passed over it.

Reg,

I guess I was also interested in the 'angle' the coil was fitted on the sled.  I mean we see coils like a clock so to speak with 12 o'clock being the top.  I was always told to not push a coil but to swing it.  That would mean you want to have a 3-9 o'clock motion for the electronics to be optimal on a round coil.  This would only happen if one is standing still.

Sledding would allow for a target to come in a 3 o'clock or 2 o'clock or anywhere o'clock.  Does it make any difference to the detector?

Some coils are said to be tip sensitive if elliptical so I would think that type of coil should not be used for a sled.  You want an even field with just audio (headphones or speakers) sensors or would a digital strength screen help to identify a target outside of the noise of the tow vehicle.

Maybe I'm just making it too complicated.  

You put a round coil on a sled and when it goes over metal it sounds off and you mark the target, right?  Then you go back and see if it is a good target or not so good.

I know the meteorite men used a very large sled to find giant meteorites in a Kansas field.

Mitchel

 

 

[Norvic]

In most of the country I detect in because of grass height, you are often pushing the coil along a brumby or cattle trail, only a foot or so in width, very little swinging. Many patches have been found "linear" detecting, I suspect because these trails follow the easiest path for the brumbies/cattle to traverse they also go across most likely places for a reef to have shed. Reg Is this so with tow coils? ie. the pulling vehicle follows the easier path thus increasing the chances of gold. 

[Reg Wilson]

I have not used an oval or rectangular pulse induction coil, only round ones, and this means that the horizontal placement of the coil is irrelevant and usually decided by the position of the lead. The shorter the lead the better the performance, so the point at which the lead enters the coil should be orientated as close to the tow vehicle as possible.

With a large round coil shallow or surface targets will give a double 'beep' as the windings pass over, whereas a deep target will give a broader single signal (whuuump). Unlike swinging a coil, where a signal can be tested by multiple sweeps, sledding only gives you one opportunity to analyse that signal and decide whether or not to mark it. The occasional false signal from bumps or EMI spikes can be a nuisance, but good tight fittings will keep these to a minimum.

In the vast areas of Western Australia a sled can be used as a prospecting tool, following reef lines, patches of quartz, or tertiary gravels, whereas in Victoria it is mainly a means of eliminating planned areas by systematic gridding.

One definite advantage of tow detecting is that paddocks of crop stubble or long grass can be scanned which would be impossible with a hand held detector. The sled simply folds flat the flora without causing damage and it stands up again after passing over it.