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Hi,
Just my pennyworth on this discussion.
1. The specs of the FGM-3 specifies a field RANGE of +- 50 µT. This is NOT the resolution, it is just able to measure in a rather linear way a field RANGING from -50000nT to +50000nT.
2. I read everywhere the importance of the sensor alignment and voltage regulation in a gradiometer configuration and it is asolutely TRUE. However, these are not the only conditions to get a maximized precision. These sensors are also sensitive to temperature, i.e. the frequency changes not only with field value of a preferred vector, it also changes with temperature. This fact is not well documented in the specs of the FGM-3, it just says that they have a 'low temperature sensitivity'.
See the literal text in this application note:
"However for applications such as earth field magnetometry or where extremely small field variations are being studied, supply voltage variation needs to be reduced to a level which permits the temperature coefficient of the device to be the limiting performance factor."
This means that the two sensors HAVE to be at exactly the same temperature to get the best precision of field gradient. Thus, the tube containing the two sensors should be very well insulated from external temperature changes AND the two sensors should be contained inside the SAME air volume. Some people (like Jim Koehler) have tried temperature compensation circuits but with no much success.
3. External feed-back coils are, in my opinion, just able to OFFSET the range of field to be measured if a constant field is applied. They do not directly improve the sensitivity. They just help to put the field range of values in the middle of the linear part of the frequency measurement curve by applying an appropriate constant field on the sensors. Both sensors of a gradiometer would have to be equipped in order to get the best out of them.
A real dynamic feed-back system could indeed be designed but this is rather delicate to design and test.
4. Given the physical principles of flux-gate sensors, we could be happy to SEE field gradients of 5 to 10nT with very well designed HOME-MADE gradiometers. This is quite enough to detect ferro-magnetic artifacts or firs pits or ceramic/bronze/iron cooking ovens but surely too coarse to detect underground archaeological structures like wall foundations, wells or cellars.
5. The fact that a measurement storage system is designed to record field gradients with a resolution of 1nT (<EPE fluxgate gradiometer design logs the data down to 1nT resolution> ) DOES NOT mean that the data capture sub-system is able to catch that resolution.
Willy
Just my pennyworth on this discussion.
1. The specs of the FGM-3 specifies a field RANGE of +- 50 µT. This is NOT the resolution, it is just able to measure in a rather linear way a field RANGING from -50000nT to +50000nT.
2. I read everywhere the importance of the sensor alignment and voltage regulation in a gradiometer configuration and it is asolutely TRUE. However, these are not the only conditions to get a maximized precision. These sensors are also sensitive to temperature, i.e. the frequency changes not only with field value of a preferred vector, it also changes with temperature. This fact is not well documented in the specs of the FGM-3, it just says that they have a 'low temperature sensitivity'.
See the literal text in this application note:
"However for applications such as earth field magnetometry or where extremely small field variations are being studied, supply voltage variation needs to be reduced to a level which permits the temperature coefficient of the device to be the limiting performance factor."
This means that the two sensors HAVE to be at exactly the same temperature to get the best precision of field gradient. Thus, the tube containing the two sensors should be very well insulated from external temperature changes AND the two sensors should be contained inside the SAME air volume. Some people (like Jim Koehler) have tried temperature compensation circuits but with no much success.
3. External feed-back coils are, in my opinion, just able to OFFSET the range of field to be measured if a constant field is applied. They do not directly improve the sensitivity. They just help to put the field range of values in the middle of the linear part of the frequency measurement curve by applying an appropriate constant field on the sensors. Both sensors of a gradiometer would have to be equipped in order to get the best out of them.
A real dynamic feed-back system could indeed be designed but this is rather delicate to design and test.
4. Given the physical principles of flux-gate sensors, we could be happy to SEE field gradients of 5 to 10nT with very well designed HOME-MADE gradiometers. This is quite enough to detect ferro-magnetic artifacts or firs pits or ceramic/bronze/iron cooking ovens but surely too coarse to detect underground archaeological structures like wall foundations, wells or cellars.
5. The fact that a measurement storage system is designed to record field gradients with a resolution of 1nT (<EPE fluxgate gradiometer design logs the data down to 1nT resolution> ) DOES NOT mean that the data capture sub-system is able to catch that resolution.
Willy
.
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