The number on the display when you ground balance is actually a numeric representation of the phase angle of the ground. That is why I refer to it as the ground phase number, not the ground balance number. Ground phase does not tell us how strong the mineralization is. But it does tell us if it is more magnetic in one area than another. (smaller ground phase numbers are indicative of more magnetic soil conditions and larger ground phase number are typical of less magnetic soil.) Regardless, when you properly set the ground balance on the 705, you are electronically adjusting what the detector interprets as the phase angle of the ground. The parameters of the ground phase adjustments on the 705 are: 0 = -1 degree .................. 90 = +9 degrees. So in your example, when you are able to ground balance your 705 with a ground phase reading on or around 20, your detector has determined that the ground at that specific location requires a "phase shift"" of approximately +1 degree, to allow your detector to "compensate" for the phase angle of that soil. If you don't properly ground balance your detector, TID numbers become skewed and you can actually end up getting an audio response from the soil itself.
In my eBook, "Understanding your X-TERRA", I talk about my "theory" on when it is best to use a DD coil, compared to a concentric coil. I believe that if you are able to properly set your ground balance with a ground phase reading of 28 or larger, when using the 9-inch concentric coil at 7.5 kHz, the soil conditions (low to moderate magnetic mineralization) allow for concentric coils to perform quite well. However, if when properly ground balancing your X-70 or 705 you get a ground phase number that is smaller than 28, your soil conditions indicate a higher level of magnetic mineralization and would be better served by using a DD coil. That does not mean that you can't use a DD coil for areas that are moderately mineralized. Nor does it mean that you can't use a concentric coil in areas with higher levels of mineralization. It simply means that, in my opinion, to get the optimum performance out of your X-TERRA, you need to equip it with a coil design that allows the electronics to best "compensate" for the mineralization and provide the most effective phase angle adjustment. And the "magic" ground phase number of 28 is what I came up with. JMHO HH Randy