Metal Detecting

After having dug and found what I thought was the main single I re scan the hole and I appear to get faint sounds generally using GM power, is this usual as I have tried digging a little deeper,
how deep do you dig, I feel like I'm leaving something behind.

I normally give it a spade full .. Then go over it again .. If I still get a good signal in the high 90's its normally deep iron ... If I get a good signal in the high 80's before I dig its a viccy penny