I have the X Terra 70, so maybe the 705 handles combination of magnetic and conductive soils better than the 70...I am just speaking of my own experiences, but I will also quote from the 70 Manual, p 40.
"In Ground Balance (Beach) the X Terra 70 can Ground Balance any combination of magnetic and conductive soil(e.g. a mixture of sand, soil and salt water). However, ferrous and some low conductivity targets can be balanced out. For this reason in Ground Balance (Beach), Tracking should be used only when targets are far apart."
I take the point that this quote refers to Ground Balance (Beach), but on page 42, in reference to Adjusting Ground Balance, the manual says
"Tracking Ground Balance follows the changes in ground conditions and constantly adjusts the Ground Balance while detecting. As Tracking Ground Balance constantly updates the Ground Balance automatically, repeated passes over a target may result in the detector balancing to the target instead of the ground, diminishing the target signal. It is recommended that Tracking Ground Balance is toggled off when a target is detected".
As I said, I have and still used the 70....if the 705 has ironed out this situation, then I stand corrected on what I said about the 705. If there is little difference between the detectors on this aspect, then I guess there's a chance I could be correct.
