I may have mis-written something above, so to clear it up, here is the bulk of it: and I mean B-U-L-K.
Early 70's detectors could get a whopping 4" (or so) in light soil or woodchips, but in medium-to-bad soil ground they got that chopped roughly in half. By mid-70's though there were a couple of Fishers (400 and 500 series) and the old original Garrett Deepseekers that could get more than twice that depth in all-metal, but not in their TR discrimination modes. Compass and A.H. Pro had the best and deepest TR's on the market back then. .
The earlier TR's were incapable of penetrating bad ground well at all. In fact the BFO's and TR's of those days both got about the same depth. However, even the TR's and BFO's inproved somewhat later on- and for a short while the BFO's could find a gold pipe in a wrecking yard of of iron using advanced discrimination meters. The TR's though could not achieve that due to the signals being overdriven into the negative. It was a matter-of-fact, and nothing could be done to improve the designs any more. Hence, the invention of a lower frequency detector and later on the phase-shift technology to go deeper into the bad ground, which sidestepped the need for having to tend to the ground control so much in order to achieve more depth.
By the late 70's the A.H. Pro company developed a concept [invented by] Westinghouse Corp - known as "Ground Exclusion" or "Ground Balance". This term was created to suggest that (any) iron was of a certain conductive frequency that would enable the detector - to balance the positive effect of the opposite (+) iron materials (FE) by means of a potentiometer covering all reaches of both a positive and negative parameter, thereby creating a balance between the two parameters. Electronically, this means that al though a coil emits positive on top of the coil and negative on the bottom that the opposite effect is needed to counteract the effects of the material (or object) being balanced against. In theory and in fact it looks like this:
+ - + - + - + - when all the atoms are lined up magnetically (electronically) and physically.
What this means is that the question comes up "whch came first, the chicken, or the egg"?
Or, does the circuit start with a "positive" or a "negative".
In fact neither is true, but it works to say one way or another, just for giggles and fun.
By the late 70's most detectors being used had a vlf/tr mode in use. Except for the last 1978-1979 years there was little to do to remedy the poor depth TR's had, save for the invention of "fast retune", a design done by Compass Electronics. This helped minimally, but did help with smoothing the things down a bit in really tough (high Fe) ground. And yes, in MY soil 2" was still the max depth and lesser iron soils it was around the 4 or in some extreme cases even 5 inches depth, at best. Remember that by the early 80's even the best of the best in detectors had a rough time even getting 6" in air, until Keith Wills created his changes and improvements to the Compass line of detectors and the Red Baron was invented. As to this day my old Silver Saber and Golden Saber both running in prime condition only get 7-8" ait test on a US penny even with my own power modifications. Originally they got only 6" when brand new, or sometimes 7" (air) when conditions were ideal.
After the Red Baron was invented (by George Payne) the White's 6 and 6000 series pushed more gain into their vlf modes and a 10-11" all-metal White's mode was created. Garrett and Fisher though had White's by about 2" more back then.
But these detectors today seldom get better depth than they did in the early 80's. A couple of cz's, a Tejon, Nauties and of course a Nexus do better sometimes, but that about ends it.
LuckyLarry
