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Fisher Gemini 3 For Prospecting

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Just curious if anyone has had an experience using a Fisher Gemini 3 or similar detector in prospecting trying to find an ore vein.

Fisher Gemini-3 Two Box metal detector

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I have not but I am familiar with the Gemini-3. To bring others up to speed I added a link to your post for more info on the Gemini-3.

These "two box detectors" can be used as an integrated unit connected by a rod. These detectors are in effect a very large coil and can only detect objects about the size of your fist or larger, but at great depths depending on ground mineralization. These are induction balance detectors and a large coil pulse induction model may work better in severe soils, especially on objects that are small enough to be borderline targets for this type of detector.

However, you can also use the transmitter and receiver separately to create a larger search field, that can be used to look for large hidden bodies like large veins or ore deposits. This is what Fisher calls a Wide Scan Inductive Search on page 12 of the Fisher Gemini-3 Operating Manual:

Wide Scan Inductive Search

This mode of operation is the preferred method for searching a large area quickly. Two operators are required and the handle assembly is not used.

The Wide Scan Inductive Search mode is practical only when searching for ore veins, pipes or cable 40 feet long or more. If the  operators are too close together, “direct air coupling” will result, meaning the receiver is detecting the Transmitter signal directly through the air instead of through a buried conductor.

Figure 7 - Wide Scan Inductive Search

Set Up

Two operators line up at least 20 feet apart, one with the receiver and one with the transmitter, parallel to the assumed direction of the buried conductor. The transmitter and receiver should be in line with each other, facing the same direction.


1. Set the transmitter: Power: ON

2. Set the receiver: Power: ON Sensitivity: ZERO

3. Slowly increase the SENSITIVITY control until the audio signal is heard, then reduce SENSITIVITY until the audio just disappears. This is the point at which “air coupling” has been eliminated.


1. Keeping the receiver and transmitter sections in line, the two operators can now walk their predetermined search pattern. The SENSITIVITY control should be checked periodically to ensure that the receiver is tuned just below the “air coupling” threshold.

2. If both operators cross the same conductive body (pipe, cable, ore vein, etc.) at approximately the same time, the receiver tone and meter will rise to indicate its presence. The receiver operator should alert the transmitter operator that they have detected a conductive object.

3. The receiver operator should then hold his position while the transmitter operator moves back and forth for the strongest receiver response. At this point, the transmitter operator should stop and place the instrument on the ground with the handle grip on top.

4. The receiver operator can then pinpoint the buried object by moving the receiver back and forth in line with the transmitter. The object should be directly beneath the point of maximum response.

5. The receiver operator may then trace the signal along the length of the unseen object as described in the Inductive Trace section (page 12).

The methodology above describes looking for a pipe or buried fuel tank. You can however find a very detailed expansion of the subject as regards prospecting on pages 46 - 52 of the Handbook of Geophysical Prospecting Methods for the Alaskan Prospector under the heading "Electromagnetic Methods". This publication is an older one but this type of detecting is even older, dating back to the original two box Fisher Metalloscope of the 1930's. The booklet is written for the practical layman prospector and so describes several geophysical prospecting methods in a relatively understandable manner.

See also the discussion starting on page 4 of Geophysics For Mineral Exploration - A Manual For Prospectors for more up-to-date information on the subject.

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Interesting topic and thanks for starting it and Steve's reply,i own and use a Fisher TW-5 which is basically the same machine as the Gemini-3 model,only use it as has been suggested on the supplied rod that is suspended from the shoulder by the provided strap.

Alas as we dont in 'theory' have gold in its raw state here in the UK i cannot comment on how it would fair for gold nugget hunting or gold detecting in general,how i use mine is for locating the deep container ie hoard or deep artifact/s after i have located what i suspect is a potential scattered hoard ie the top of a container that has been clipped by a piece of farm machinery usually deep ploughing,this then drags up some coins from the top of the pot/container and scatters the coin in a dragging fashion by further ploughing.Once i have located such a coin scatter then its time to use a twin box in this case the Fishers.

My twin box does not come out too play all that often but is the ideal detecting machine for the job in hand,of course its totally useless for locating single coin size items,but i am not after those as i am after the 'holy grail' the pot itself.In the new year hope to be back on a potential celtic gold stater hoard site,we have located about 20 full staters all in a very tight radius and all the same style,so it ticks all the boxes for a possible container location rather than say a purse drop.

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We used to use my Gemini 2 box with a high sens setting to find black sand concentrations along creeks. We would set up our dredge up there with fairly good success.  Should work better for high banking. rh0dium

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