A
Anonymous
Guest
Hi all,
Here is the post I made on the Finders.co.au forum. I guess some of the content will be of interest here.
The Goldscan actually started as a gold prospecting detector after a visit to Australia in 1982. So long ago <IMG SRC="/forums/images/wink.gif" BORDER=0 ALT="
"> It was evident from tests that I did that the ground mineralisation was very much stronger than anything I had encountered elsewhere, and that a straight PI had almost no hope of working. I then devised a multiple sampling and filtering system to get rid of the ground signal. First there was Goldscan 1 in 1983, which was a completely shaft mounted unit. Just a few of these were made and tried in the more mineralised areas of the UK, such as in parts of Scotland. From the production point of view it was easier to make a chest mounted control unit with built in rechargeable battery pack. This also had the advantage that the coil and shaft assembly could be made very light and comfortable. The Goldscan 2 was produced from 1984 and continued until 1992. By further miniaturising the electronics and having a belt mounted battery pack, it again became feasible to have a lightweight, shaft mounted control unit and the Goldscan 4 was born, which was latterly made by Pulse Technology. What about Goldscan 3?. This was a high power unit running from built in 24V lead acid gel batteries. It certainly had performance, but only a dedicated and rugged few braved its weight and the heavy digging tools required for retrieving targets.
All of these detectors had manual ground balance. An auto-ground balance version was designed on paper but never got to production. No real magic to auto-GB; just a feedback system to correct changes in ground mineralisation. Actually, with manual GB, little change in setting is required when going from one area to another, provided the ground signal stays within the linear range of the receiver. Hot rocks have very high concentrations of iron minerals and, particularly when using a mono coil, can cause temporary overloading. This distorts the decay curve and the signal then falls outside the pattern required for the GB filter to work. Hence a false response. This is where the Goldscans fell down in Australia, but having moved on to other detector developments, mainly industrial and underwater, I did not pursue it further. There are various ways of preventing receiver overload, but one good way is the use of a DD coil. SD users will be aware that a DD is quieter on highly mineralised ground than a mono. This is because a DD has a natural cancellation of near surface mineralisation signals, due to its balanced geometry. A mono coil, on the other hand will give a very strong response to mineralised ground due to the same coil being used for both functions. A hot rock passing under the coil edge will give a very strong signal due to the tight TX-rock-RX coupling.
Another recent post caught my eye on Threshold Warbling. It is quite usual for the audio threshold tone to have some unsteadiness. This is due to electromagnetic signals that are picked up on the search coil, which acts as an antenna for, not only the wanted signals, but unwanted ones as well. Generally, it is low frequency signals that cause a problem i.e. power line to long wave, however, VHF, UHF and microwave have been known to interfere, if their level is high enough to cause non-linearity. PI detectors let in more noise than induction balance VLF type as they are broad band devices which respond to a wide range of frequencies. Manufacturers generally set the amplification of the detector receiver so that the level of warbling is acceptable in average interference conditions. Some conditions will be quieter and others, such as near an airport, noisier. I had an SD2200 for testing a couple of years ago, and inside my factory, it was quite noisy when using large mono coils. However, if I wound the coil in a noise cancelling, figure of 8 configuration, all noise virtually ceased and the threshold was as smooth and sweet as you could ever want. Even better in the field.
Eric.
Here is the post I made on the Finders.co.au forum. I guess some of the content will be of interest here.
The Goldscan actually started as a gold prospecting detector after a visit to Australia in 1982. So long ago <IMG SRC="/forums/images/wink.gif" BORDER=0 ALT="
"> It was evident from tests that I did that the ground mineralisation was very much stronger than anything I had encountered elsewhere, and that a straight PI had almost no hope of working. I then devised a multiple sampling and filtering system to get rid of the ground signal. First there was Goldscan 1 in 1983, which was a completely shaft mounted unit. Just a few of these were made and tried in the more mineralised areas of the UK, such as in parts of Scotland. From the production point of view it was easier to make a chest mounted control unit with built in rechargeable battery pack. This also had the advantage that the coil and shaft assembly could be made very light and comfortable. The Goldscan 2 was produced from 1984 and continued until 1992. By further miniaturising the electronics and having a belt mounted battery pack, it again became feasible to have a lightweight, shaft mounted control unit and the Goldscan 4 was born, which was latterly made by Pulse Technology. What about Goldscan 3?. This was a high power unit running from built in 24V lead acid gel batteries. It certainly had performance, but only a dedicated and rugged few braved its weight and the heavy digging tools required for retrieving targets.All of these detectors had manual ground balance. An auto-ground balance version was designed on paper but never got to production. No real magic to auto-GB; just a feedback system to correct changes in ground mineralisation. Actually, with manual GB, little change in setting is required when going from one area to another, provided the ground signal stays within the linear range of the receiver. Hot rocks have very high concentrations of iron minerals and, particularly when using a mono coil, can cause temporary overloading. This distorts the decay curve and the signal then falls outside the pattern required for the GB filter to work. Hence a false response. This is where the Goldscans fell down in Australia, but having moved on to other detector developments, mainly industrial and underwater, I did not pursue it further. There are various ways of preventing receiver overload, but one good way is the use of a DD coil. SD users will be aware that a DD is quieter on highly mineralised ground than a mono. This is because a DD has a natural cancellation of near surface mineralisation signals, due to its balanced geometry. A mono coil, on the other hand will give a very strong response to mineralised ground due to the same coil being used for both functions. A hot rock passing under the coil edge will give a very strong signal due to the tight TX-rock-RX coupling.
Another recent post caught my eye on Threshold Warbling. It is quite usual for the audio threshold tone to have some unsteadiness. This is due to electromagnetic signals that are picked up on the search coil, which acts as an antenna for, not only the wanted signals, but unwanted ones as well. Generally, it is low frequency signals that cause a problem i.e. power line to long wave, however, VHF, UHF and microwave have been known to interfere, if their level is high enough to cause non-linearity. PI detectors let in more noise than induction balance VLF type as they are broad band devices which respond to a wide range of frequencies. Manufacturers generally set the amplification of the detector receiver so that the level of warbling is acceptable in average interference conditions. Some conditions will be quieter and others, such as near an airport, noisier. I had an SD2200 for testing a couple of years ago, and inside my factory, it was quite noisy when using large mono coils. However, if I wound the coil in a noise cancelling, figure of 8 configuration, all noise virtually ceased and the threshold was as smooth and sweet as you could ever want. Even better in the field.
Eric.