Gold Bug 2 can eliminate the effect of black sand to a degree, that is the best you can get among high fre. If you cannot afford it, then the 20Khz gold detectors are perfect. If you are moneywise, then, you like the big nuggets, not the tiny ones. With high freq. Think of high and low freq.
Radio waves are low freq.
Light cannot pass though most objects and reach the target, but you can still listen to radio in a submarine. That is the defference. The problem of ground imbalance caused by non-conductive iron mineral is reduced, then, if the phase shifted signal is used for detection. It is conductive but also has magnetic properties. When the secondary inductor has no load, it has a little effect on primary, but if we add any real or imaginary load to the secondary.