You can tell if your soil is "bad ground" or "highly mineralized" by pulling all the iron filings (with a magnet) out of it and measuring the filing's volume. More than a thimblefull and you will start to have problems with the noise. High gain detectors have an even rougher time with it too.
www.tesoro.com/info/faq/generaldetecting/
Here is the info from Tesoro Electronics to explain why the Tejon (or other high gain detectors) can become hard to balance. Please pay close attention to the sections called;
#1 "What is Ground Balancing", and especially the last sentence about reading small changes in the ground (AKA "noise", or "interference").
#2 "What is the Difference Between Preset, Manual, and automatic Ground Balance"?
#3 "What is the best type of Ground Balance"?
And the section about high gain or high output, or high amplification, mentioned midway in..
#4 "What is high output Technology", and that it will cause the signal to become "unstable" (AKA "noisy").
In this article it is strongly suggested that unless we tune and watch our manual GB like a hawk we are also losing depth, good discrimination, AND good overall performance. See section #3 "What is the best type of ground balance", and read the first sentence. Unless you are highly skilled and very attentive you will likely benefit more from automatic GB than manual, the words of the very Company who makes Tesoros. Don't take my world for it, take theirs, they know more about it than any of us do.
HH folks.
LL