I find that wheat pennies and silver dimes respond nearly the same as far as depth goes. If you're finding deep wheat pennies you should also be able to detect deep silver dimes if they're there. At a really trashy old school ball field, I found several wheat pennies around 8 + inches deep with my Vaquero. I had the discrimination set just high enough to discriminate out screw caps and zinc pennies. I had the sensitivity and threshold set as high as they would go without getting any chatter. Using Grey Ghost headphones, going really slow, I listen for soft repeatable whispers.
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tabman