Question about depth

Just got a Prizm III couple weeks ago and am still learning this machine. I'm finding the depth doesn't go as far down as I expected. What is everyone's experience with this? I did a test on a quarter sitting on top of the ground and I got about 7 inches and the signal became "iffy". Is there another coil to purchase for the prizm III that can detect deeper coins??? The soil I have around here is very soft and the "better" coins are probably at least 8-10 inches down.