UM,
Thanks for the link but I am already familiar with the Geotech1 site. I use the same name over there as I do here.
Now, I suspect the black sand detection problem is lack of familiarity of what has to be done to use the GB 2 in areas with heavy black sand. Actually, all VLF's suffer from this condition but the detectors with a concentric coil are affected the most and the GB 2 has a concentric as does the LST and Goldmasters prior to the GM4.
Concentric coils experience something called, by one engineer, foldback. What this means is the coil becomes overwhelmed by the strong signal and the signal will change polarity. In other words, under extreme conditions you can't perfectly ground balance but you can get close and to operate effectively, you have to operate with the coil elevated. Just how much you have to elevate the coil depends upon the black sand intensity and the machine being used, with some being worse than others. If you try to operate with the coil right on the ground or even very close to the ground, you will find it range from extremely difficult to impossible and the problem will be a constant signal.
This occurs with all VLF designs but the effects are much greater when using a concentric coil. Now, how much one might have to elevate a coil to get the detector to work right may be as much as 3" to 4" or possibly higher in certain places and with certain detectors. Another detector my work at closer to 3" or maybe less especially if they have a "Difficult" or black sand setting. Generally, such settings simply reduce the gain so the early stages of the electronics are not overwhelmed with signal.
If you look at the newer designs of gold detectors, the primary coil type is a DD design. Detectors like the Gold Bug Pro and the associated Gold Bugs from the basic unit, and the SE normally come with a DD coil White's GMT also comes equipped with a DD.
The reason is simple, DD coils handle bad ground much better than a coil of concentric design. The design of the DD is such that a percentage of ground signal is basically cancelled, thus what might overwhelm a concentric can still be hunted reasonably well with a DD.
Now, detectors with the concentric coil generally have or can have a slight sensitivity advantage to small gold but quite often the extra sensitivity to black sand will offset this sensitivity capability.
As for a trip in July, wow, it should be hot at that time. My limited hunting in NM has generally been by invitation, so my access is extremely limited. As you probably know, much of the area I mentioned before is now either private or under claim and much is behind locked gates.
I have a friend who may be going your way this fall and he actively uses the GB 2. In fact, he has mine which he used until he was able to get his sent in and now he has his recently repaired unit also. If I find that he will be going your way, I will see if he can meet up with you. As for me, well that is a 5+ hour trip and I have reached that point in my life where trips of that distance are becoming extremely seldom, if at all.
Reg