Gp 3500 Field Test
By Steve Herschbach
From Page 12
September, 2005 issue of Lost Treasure

When Lost Treasure contacted me about doing a field test of the new Minelab GP 3500 metal detector I had to admit I had not even tried one yet. I did want to give the new unit a spin, however, and was flattered at being asked to write a review. So I agreed, giving me an excuse to finally get out and try the new machine.

My partners and I own nearly 3,000 acres of mining claims at Moore Creek, Alaska. This ground is an ideal place for Minelab detectors, as hot rocks are bad enough to make using normal nugget detectors a difficult proposition. We needed to fly some freight up to the mine, and so I contacted my father, Bud Herschbach. He is one of the partners and our official pilot. I figured we could fly supplies to the mine, and then I could get in some detecting in with the Minelab GP 3500.

The flight was uneventful under sunny blue skies. We off-loaded the equipment, and I geared up for some nugget hunting with the GP 3500. I have owned the Minelab SD2200D, GP Extreme, and GP 3000 models, so this was old hat for me. The only difference in assembling the unit was the new push-button on the end of the grip handle and the little cable for it that goes to the control box. I must admit my first impression of the thin cable was of how small the wire is and I worried about it getting hung up in brush and pulled out or damaged. This would not keep the detector from working, however, and so it is a minor worry on my part.

When I nugget detect I usually think in terms of days, not hours, and so I did not expect to find any gold in less than two hours. I just wanted to see how the unit responded to the ground and hot rocks. There are a couple old tailing piles off the end of our runway I've eyed while taking off and landing that I had not been on before, so I wandered on down that way to see what I could find.

I fired up the GP 3500 and experimented with the new ground balancing settings and push button. There are now three ground balance speeds slow, medium, and fast. A ground tracking speed can be chosen, and pushing the new button on the grip handle will temporarily stop the ground tracking to prevent very weak targets from being tracked out by the system. Conversely, the ground balance can be set to the locked or fixed position. Pushing the button on the handle then temporarily engages the ground balance system to adjust for new conditions. When released this new tuning point is then locked in.

At Moore Creek the ground comprises most of what the detector must deal with, but the hot rocks are such that if you tune to the ground you will get positive responses that mimic gold nuggets from the rocks. There are enough hot rocks that these false positives are prohibitive. I have found balancing to the rocks themselves helps eliminate these false targets, while not causing adverse signals from the ground itself. Running the detector in the fixed mode works best as ground tracking adjusts back to the ground itself instead of the less common rocks. So putting the machine in fixed then pushing the button while sweeping over a few hot rocks allowed me to easily find and keep the best ground balance setting. When reading about this button I thought it a bit of a joke, but once I actually used it I took an immediate liking to it. It made it a lot easier to tweak the ground balance settings then fumbling for out-of-view switches.

I dug a few targets; some little steel items and a small shell casing. There was little trash so I was digging everything, the preferred method when nugget detecting. I got a nice solid signal towards the middle of the tailing pile, and was pleasantly surprised when a 6.8 pennyweight gold nugget popped out of the ground! Not the prettiest nugget in the world, but a gold nugget nonetheless. I was already feeling pretty good about the GP 3500 and it sure did not hurt to find a gold nugget so quickly with the new unit. Normally this would be the start of a very long day of nugget hunting, but time was already running late for the return trip to Anchorage. I decided to end the visit on this happy note and head on back to town, my nugget hunting portion of the Minelab GP 3500 field test completed. Many people might think of the Minelab GP detectors as nothing more than gold nugget detectors. This is a huge mistake in my opinion. These are some of the most powerful metal detectors available, and the depth of detection that can be achieved with ground balancing pulse induction (PI) technology must be experienced to be believed. As an avid detectorist of over thirty years I'm always looking for the next big thing in detector technology. I truly think that machines like the Minelab GP series hold great promise for all types of metal detecting, not just nugget hunting.

There is a freshwater beach near my home that may be the most detected piece of real estate in Alaska. The site is very popular in the summer and so it constantly replenishes with easy to dig targets. It is also an older site dating back to the 1930's. Just about everyone hits the area, and so old coins are now very hard to find. In fact, normal VLF detectors produce very few signals at all on the beach. The local detecting regulars find recently lost items quickly.

This all changes when a ground balancing PI detector is used. I rigged up the GP 3500 with a 14 round mono coil, which is an excellent coil for this particular use. I also used the Minelab accessory half-size battery attached directly to the control box so as to not be tethered to the machine via the battery cable as is normally the case. This makes it easier for to vigorously dig those deep targets.

The Minelab series has an undocumented feature that I have found extremely useful. Due to the way the ground balancing system works, targets produce either a rising high tone or lowering low tone. In general, high tones are produced by low conductive targets like gold, aluminum, lead, nickels, most zinc pennies, and small iron items. Low tones are produced by silver and clad coins, copper pennies, a few zinc pennies, and larger iron items. Large rusty bent nails seem to be a common trash low tone item.

The split is much like that of a VLF detector with a discrimination point set on zinc penny. Most items below zinc penny give a high tone; most above give a low tone. Iron is the wild card, reading both low and high depending on size and shape. This crude audio discrimination opens up vast possibilities beyond normal dig-it-all PI detecting. Depending on the mix of targets in the ground, sufficient discrimination is achieved to produce some surprising results. I almost hate to give the concept wide exposure in this article, as I'm tempted to keep it to myself as a secret weapon.

Digging only the low tones eliminates the most aluminum and small iron items. Gold jewelry will be missed but that is ok if coin hunting is the goal. For gold one would dig only high tones. Nickels and zincs are missed, but they are usually lower value targets. The only other trick is that the targets are now so thick because of the extra depths achieved that slow scanning is recommended. I found that setting the GP 3500 for slow automatic ground balancing worked best for this location and use.

One important test at this location was the overhead power lines. Each generation of Minelab seems better than the last at handling outside interference. The GP 3500 ran very smooth until I got quite close to the power lines. It holds a better, smoother threshold under adverse conditions than any of my previous Minelab detectors. This is very important for this sort of in-town use.

I scanned carefully, looking only for low signals and ignoring the highs. I'll go back someday when I have more time and dig them, but will get lots of foil and pull-tabs. Low tones are much more rare by comparison, and I immediately pulled a wheat back penny out of the worked out beach. I ended up with about 35 targets before I quit. Fourteen were coins, so just over one trash item per good item. Almost all the junk items were bent nails. There were five wheaties from the 1940s and 1950s and a 1950 dime. The more recent 1960s pennies were also deeply buried, and only two shallow coins were found. The heavy detecting at the beach is almost a plus, as there are almost no very recent items to be found.

This may not seem exceptional to many coin hunters, but you'll have to trust me when I say this place is hammered. To pull up five wheaties and a silver dime in less than a couple hours is amazing when you know that the best VLF detectors available miss them. Thousands of passes have been made over these coins over the last 30 years yet the GP 3500 was not stretching to hit them. The signals were loud and strong. To sum up, I would have to say the Minelab GP 3500 is not a leap ahead as much as a refinement of the Minelab GP 3000, which is now out of production. As such, the GP 3500 represents the state of the art in ground balancing pulse induction technology.

The machine offers depths of detection that put it in a world apart from normal detectors, and once you get addicted to this raw power it is hard to go back to standard metal detector offerings. Granted, the discrimination is crude, but I have found that with practice the GP 3500 can be used effectively for more than nugget detecting.

For those wanting more than just a tiny increase in depth, the Minelab GP 3500 may just be the answer. I would personally use it for all types of detecting, including coins, relic hunting, jewelry, and of course nugget detecting. The only serious limitation is the inability to use the GP 3500 in groomed parks and other areas where digging extremely deep holes may be an issue.

I've experienced the future of metal detecting, and I'm not going back. I'll be using the Minelab GP 3500 until something better comes along. Pulse induction technology reminds me of the early VLF detecting era, with its amazing depths but poor discrimination. I'm sure we will see continuing improvements in PI discrimination, and as that happens we may be looking back on current VLF detectors the way we now look back on BFO and TR detectors.

Gp 3500 Field Test

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