Air test are effective in finding the max potential depth on certain size targets. The only thing that will improve depth in the soil is a halo effect, which makes the target look bigger. If your detector can't air test a dime at 8in, it won't find a dime at 8in in the soil UNLESS it has a halo. You must also take into consideration that some power is lost going through the soil, my theory is Minelabs do terrible in air test but good in the ground because they lose less power/signal in the ground. I'm not an expert, that's just my take from using various detectors.