In my cellar, I stacked a box of clay cat litter on top of a bag of clay cat litter. (I have a cat). The stack measures 7.5" high from the top of the brown cardboard box. Harder to detect coins through the cat litter, meter numbers are a lot lower than air test at 7.5". A silver quarter is under the bag and another quarter is on the brown box in the upper right corner for air trest comparison. The clay might be similar to soil, don't know for sure. Most all coins under the clay give a lower meter number than the air test coin. Gotta stay home, got 6" of snow.