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Perhaps someone has some empirical formulas to share that relate coil field coupling loss to distance and/or target sensitiivty.
The best I can figure, the field strength decreases along the inverse cube rule one-way, and the inverse sixth power round-trip. If correct, a target depth of 12" compared to 36" would be 1/(3)^6 times weaker (1/729 as strong) round-trip.
Likewise, the sensitivity to smaller targets as related to coil size follows the inverse cube rule. Thus, a 6" to 12" coil change (ATBE) would result in an 8x decrease in the sensitivity to a small target, (1/(2)^3 or 1/8th as strong).
The same rule applies to a changing target size with a constant coil size. Double the target size and the coil is 8x more sensitive (that is, a 2x smaller coil would detect the larger target size as if it were the half-sized target).
There are other interesting effects. Soil attenuation as it relates to primary frequency changes, the greater the freq the greater the coupling. For instance, changing from 10 Khz to 1,000 kHz would, in theory, increase coil-to-target inductive coupling by 10,000 fold (1000/10)^2). But soil absorption at the higher freqs would, as near as I can tell, increase at the square-root of op freq. So, soil absorption would be approximately 10x stronger sqrt(1000/10) at 1 MHz vs 10 KHz.
Anyway, I'm hoping there are other people with empiricle rules to share. Information is scarce and not always accurate.
Thanks,
Jackalope
The best I can figure, the field strength decreases along the inverse cube rule one-way, and the inverse sixth power round-trip. If correct, a target depth of 12" compared to 36" would be 1/(3)^6 times weaker (1/729 as strong) round-trip.
Likewise, the sensitivity to smaller targets as related to coil size follows the inverse cube rule. Thus, a 6" to 12" coil change (ATBE) would result in an 8x decrease in the sensitivity to a small target, (1/(2)^3 or 1/8th as strong).
The same rule applies to a changing target size with a constant coil size. Double the target size and the coil is 8x more sensitive (that is, a 2x smaller coil would detect the larger target size as if it were the half-sized target).
There are other interesting effects. Soil attenuation as it relates to primary frequency changes, the greater the freq the greater the coupling. For instance, changing from 10 Khz to 1,000 kHz would, in theory, increase coil-to-target inductive coupling by 10,000 fold (1000/10)^2). But soil absorption at the higher freqs would, as near as I can tell, increase at the square-root of op freq. So, soil absorption would be approximately 10x stronger sqrt(1000/10) at 1 MHz vs 10 KHz.
Anyway, I'm hoping there are other people with empiricle rules to share. Information is scarce and not always accurate.
Thanks,
Jackalope