During the three years that Ive been writing The Weekend Prospector, Ive noticed a growing interest in two-box detectors. Most treasure hunters think of them as a means of finding caches and relics buried beyond the range of conventional beepers. This is their big appeal; the depth capabilities of two-box detectors are measured in feet rather than in inches, but it doesnt
Geologists and prospectors use them to find veins of metallic ore and likely locations for placer deposits. Bottle collectors find them valuable in locating former dump sites and even old privy holes. Additionally, they are used widely in industry, mostly for finding and tracing buried pipes and electric cables.
Two-box detectors get their name from their appearance. Like modern conventional detectors, their operation is TR (transmit-receive). The difference is that the conventional instruments combine their transmitting and receiving antenna in the same search coil, which is attached to the lower end of the stem and is almost always round.
The two-box detectors, on the other hand, have separate transmitting and receiving antennas, which are always rectangular and most often box-like. In the usual treasure-hunting configuration, they are mounted to the ends of a handle, or stem, about four feet long which, while searching, is carried parallel to the ground. The two boxes are on the ends of a pole.
Two-box detectors have been around a long time. Fishers new Gemini 3 is the third in a series that traces its lineage back to the original two-box patent, granted to the late
Mainly, it is a much more sensitive and stable instrument, thanks to new crystal-controlled circuity.
It has 10-20% more depth and will trace ore veins or pipes at least 50% farther than Gemini 2 under the same conditions. A VCO (voltage-controlled oscifiator), which Gemini 2 didnt have, produces a variable audio response (it increases in both volume and pitch as the target is approached), that makes pinpointing much easier. And it does all this at a weight more than 20% less than that of its predecessor. Dr. Gerhard Fisher, the founder of FisherLabs, more than50 years ago. He called it a Metallscope. This is the origin of the term M-scope, which is used in the companys trademark today.
Prior to the advent of transistors in the 1950s, two-box detectors were large and unwieldy pieces of apparatus. Ive seen pictures of one monster (not made by Fisher) that weighed 70 pounds, a good part of which must have been accounted for by the big batteries necessary to keep its vacuum tubes aglow. By contrast, the Gemini weighs only seven pounds complete and uses AA penlight batteries.
Gemini 3 is a worthy successor to Gemini 2. This is saying a lot be-cause Gemini 2 was introduced in 1979 and over the years acquired an enviable reputation with treasure searchers and industrial users alike.
The single cell sometimes hard-to-find nine-volt battery that powered each of Gemini 2s boxes has been replaced by eight AA batteries in a battery pack. Finally, each box is wired for on-board recharging of optional ni-cads.
In common with all other two-box detectors, the Gemini 3 is not just a coinshooter but will also ignore other small objects, including nails, pull tabs and individual tin cans. Since most junk targets are small in size, there was simply no point in adding a discriminator. In conventional detector terms, all searching is done is an all metals, non-motion mode.
But the detector does have four operating modes or methods. Whichever one is being used depends upon the purpose of the search. These are:
Narrow Scan Inductive Search, Wide Scan Inductive Search, Inductive Trace. and Conductive Trace.
The two trace modes have to do with pipe finding and would be of little interest to a treasure hunter who doesnt also work for a building contractor or a public utility company.
Wide Scan Inductive Search can be used to determine the extent of ore bodies. The handle isnt used. One box is placed on the ground and the other moved around at a distance of a 30 feet away to prevent air coupling, where the receiver box is picking up the transmitters signal directly through the air instead of through a buried conductor.
In another version of this mode, two searchers each hold one box and walk parallel to each other, also at least 30 feet apart to prevent air coupling. This allows a lot of ground to be covered in a relatively short time.
Wide Scan Search is also a very specialized application and, because of the air coupling factor, is adaptable only to relatively large areas.
By far the greatest amount of treasure hunting is done in the Narrow Scan Inductive mode. Both boxes are attached to the handle and the detector is carried slowly over the area being searched with the handle parallel to the ground. Lets concentrate on that.
The detector I field tested was sent directly from the factory. It consisted of the two rectangular antenna boxes, both colored a bright red on the outside and latched together, a three-piece handle, and a carrying strap. Both boxes are made of ABS high impact plastic and each has its own carrying strap, for carrying the boxes when they are latched together and individually in the wide scan and trace modes.
Following the well written instruction book, the first thing I did was to assemble the three-piece handle by inserting the two end sections into the center section and tightening the two knobs supplied
for that purpose finger-tight until there was no wobble. Theres only one way to do it.
Next, I laid the transmitter on its back and inserted the end of the handle with the single knob into a V shaped slot and tightened the knob. Finally, I aligned the two threaded knobs at the other end of the handle with the holes in the transmitter panel and tightened the lower knob finger-tight.
The upper knob, which is marked with a white arrow, is also the balance knob and is spring-loaded. This has to be turned clockwise (opposite from the direction of the arrow) until the spring comes under tension; it takes about 10 turns. The two knobs used to assemble the handle are threaded into it and the three used to attach the boxes are captive, thus minimizing the possibility of loss in use.
As I became more familiar with the Gemini 3, I found it easier to attach the transmitter and receiver to their respective sections of the handle and then to assemble the handle; but this is a matter of preference. Either way, assembly was done in a few minutes and without tools.
Both the transmitter and receiver have a black control panel with gold lettering and the function of each component and control is clearly marked.
Both also have condensed operating instructions lettered on each panel, a push-pull off and on switch, a battery test button and an input for the optional ni-cad charger together with an indicator light that shows when it is on.
One thoughtful little touch is that when the two boxes are to be latched together when the Gemini 3 isnt in use, the two push-pull power switches abut and so turn each other off when the latches are closed, avoiding battery drain by either unit being left on accidentally.
The receiver control panel also has an analog (needle) signal strength meter calibrated from 0 to 100, an audio speaker, a head phone jack that shuts off the speaker when a standard phone plug is inserted, and sensitivity control consisting of a two-position normal and high range switch and a fine-tuning knob calibrated from 0 to 10.
Both boxes have identical battery compartments located at about the center of the back. Each holds eight AA batteries or optional ni-cads in an easily removable battery pack. The transmitter and receiver will work for from 30 - 40 hours with good quality carbon batteries and substantially longer with alkalines. Ni-cads are good for about 20 hours per charge.
Alkaline and carbon batteries may be quickly charged by-using a large screwdriver or a coin to loosen a
screw that holds each access plate. The screws are captive to prevent accidental loss. Ni-cads in both boxes can be charged simultaneously by plugging in a special charger to the receptacles on the control boards. Charging time is 16-24 hours.
The battery condition of each box can be checked with or without the power being on by pressing and holding the battery test button on the box being checked. The transmitters battery voltage reads on as mail meter thats there for that purpose only. The receivers voltage reads on the larger signal strength meter, which also has a battery condition section marked on its scale.
if the batteries are o.k., both units may be turned on. First the transmitter, then the receiver. The Gemini 3 can now be balanced. This should
always be done at least 15 feet away from any appreciable amount of metal, such as a chain-link fence or an automobile.
Searching can be done by holding the detector at arms length so that the handle is parallel to the ground or by suspending it from the strap that comes with each detector by clipping it to the handle and adjusting it so that the detector is as close to the ground as possible; using the strap will get about another foot of depth, but may make use difficult in rocky terrain. Balancing should be done in whichever search position has been chosen.
The sensitivity control on the receiver should be set to 7 and the range switch to normal. There should be a strong audio tone and a high meter reading at this point The balance knob is now turned counterclockwise (in the direction of the arrow) until there is sharp nul, both in the audio and on the meter.
Continuing to turn the knob should restore the audio and meter indication in between 1/8 - 1/4 of a turn. If its less than 1/8 of a turn, sensitivity should be lowered and the procedure repeated. If its more than 1/4 of a turn, the sensitivity can raised.
Failure to produce a nul at the lower searching position generally means that theres too much mineralization to use the Gemini 3 that close to the ground. If it wont nul in either position, theres some unknown metal closeby, and it will be necessary to move to a new position before trying to balance again.
When searching, balance should be checked periodically to be sure that the nul remains within the 1/8 1/4 turn limitation as it is affected by a changing intensity of ground mineralization.
After the first time the Gemini has been balanced, its a good idea to practice by spreading some metal objects on the ground. I used a set of socket wrenches in a metal box, a three-foot length of two-foot pipe and a wire coat hanger. I began searching at the nul and as I approached each object from several angles, I began getting a response from both the audio and the meter.
They both peaked when 1 was directly over them and then began to fade as I continued walking. In this way, I quickly learned to recognize the sound of the audio and was able to observe the interplay of the audio and visual responses under various observable conditions.
Its even more effective to practice on known buried conductive objects. I was fortunate because the sections of drain lines from several houses in my neighborhood are metal rather than ceramic where they bridge across a number of deep creek gullies and for some distance on both sides. Their locations are immediately evident, so I was able to get in a good bit of practice with minimum effort.
Pinpointing is done in somewhat the same way as with a conventional coinshooter, but the procedure is considerably different because of the larger size of the targets. When the Gemini 3 begins to respond to a target, the operator continues walking until the response has reached max and just begins to fade. A heel mark is made at this spot.
The target is then approached from the opposite direction, the same routine followed and another heel mark made. The target is then approached again in two directions 90 degrees away from the original (like X-in a coin), resulting in two more heel marks. The target will be centered between the heel marks.
Pinpointing with older Gemini models was sometimes complicated because the meter needle went on the peg at the top of its scale at the same time the audio reached maximum saturation. When this happened, it was necessary to adjust the sensitivity fine tuning knob in the 0 direction to bring the meter needle back to something less than 100 for
Not so with the Gemini 3. Thanks to the new voltage-controlled oscillator, both audio and pitch continued to increase after the meter needle had pegged. In fact, it seemed to me that the pitch continued to increase even after volume had reached saturation. This very much reduces the number of times that searching has to be interrupted to back off sensitivity for pinpointing, a very valuable improvement.
The Gemini 3 is designed to detect conductivity changes in the ground over which it is being carried, much the same as a conventional beeper. Its depth penetration depends on a number of things, which include the size and shape of the object, the mineralization of the soil, the length of time the target has been buried and, very importantly, the skill of the operator.
Although it wont sound off on individual coins, it will detect a quart jar full of them as deep as three feet. The larger the object, the deeper it will detected. A 55-gallon oil drum or a metal chest about that size will
respond from about 10 feet. Something really big, such as an automobile, can be detected as deep as 20 feet, possibly even deeper.
This additionally makes it a valuable tool for bottle hunters who are looking for old dumps. Although the Gemini 3 wont respond to small conductive objects, a great many of them on the surface will impair its performance and its helpful to clear them away before starting to search.
Small objects, such as a jar of coins, require a tight search pattern. Large objects can be found with a much looser one. sometimes its helpful to mark out a grid to keep track of whats been searched. Brightly colored golf tees or nonmetallic tenqiegs are ideal, depending upon the nature of the terrain.
High mineralization, particularly magnetic and wet conductive mineral salts greatly decreases the penetration power of the Gemini 3. Really intense mineralization can cut the depth capability to the instrument to about one-third of what it would be in neutral soil.
The other side of the coin is that this also makes possible the detection of placer gold, although the nuggets themselves would be too small for a response from the Gemini 3. Such deposits generally contain magnetite (black sand) and sometimes iron sulfide, both of which can be detected, thus revealing the possible presence of the gold.
The longer an object has been in the ground, the easier it is to detect. The halo effect is well known. An iron object will leave a detectable residue long after it has completely rusted away.
Such signals are apt to be quite weak and the use of earphones is suggested. The same is true of old privy holes, since over many years their original contents decay to detectable residue another boon to the bottle collector.
I once met a fellow who amassed
a valuable collection of bottles, some dating back to the 1860s, in this way. First, he showed an early map of the town in Nevada where he lived to his grandmother, who had been a pioneer there. He asked her to check off the locations of the houses of the people who didnt drink, which she did for him. Then, he located their old privy holes and from them dug up his bottles.
His theory was that the people who drank openly tossed their empties into the town dump, but those who drank on the sly and this was just about 100% of the rest of the population quietly dropped them into their privy holes. And he was right on the mark. Unfortunately for me, by the time I caught up with him, hed sold off all the really valuable ones to collectors and there was nothing left to photograph, and with-
out that, not much of a story.
The shape of an object and its relationship to the direction of the search are both important. Ring- or loop-shaped targets produce the best results. Flat or dish-shaped objects are also easy. Rod-shaped targets, especially when scanned on end, are the most difficult.
All in all, I found the Gemini 3 to be both reliable and easy to learn to use. The list price of $525 is, to my way of thinking, quite moderate for a top-of-the-line detector. I also feel that the Gemini will significantly increase the interest in two-box detectors that I have noticed in more than three years of corresponding directly with Lost Treasure readers.
For more information, contact
Fisher Research Laboratory, 10051
St., Los Banos, CA 93635-4398;