If you've already covered front to back with a 50% overlap, you've probably detected everything there is up to 6 or 8 inches. You've also detected about 1/2 to 1/3 of anything deeper to about 12-14 inches.
With the slower sweep, the really deep stuff has a better chance to have an effect on the threshold. You might have passed some minor changes in theshold cause you didn't notice them. Slowing down makes it more evident when a minor change occurs. So even though you might have detected it (gone over it with the coil) you may not have heard it....
If you didn't overlap by at least half, your full coverage depth is decreased from 6 to 8 inches to something less. (Original concentric coil of course).
If you've done the right overlap and you're satisfied with the speed, then the next thing is to go side to side asssuming you've already gone front to back. Using the same overlap but going side to side increases the coverage at greater depth.
e.g. If you saw 50% of 8-12 inches the first pass, you will hit an additional 20-30% of the deeper stuff. Then if you want to run diagonals, you will pick up another 5-10%.
The larger coil will give you an edge on depth, and coverage, but if you don't overlap well, and use a slow technique, chances are still good that you're missing a lot. e.g. No overlap will give you about 70% coverage to 4 or 5 inches. Almost full coverage at the surface, but not much deeper. Missing more than 30% of stuff inside 5 inches is why many detectorists don't mind following someone that is somewhat careless of their seaching technique.
Disclaimer: All numbers are rough estimates, based on lots of reading and a couple years detecting. NOT measured at all.
Visualize a cone of detection as wide as your coil and as deep as the coil diameter plus 10 to 20 %. Now, imagine moving that cone horizontally across a grassy lawn. How deep are you getting full coverage with no overlap? ONLY AT THE SURFACE! Now visualize the overlap. etc...
Hope this hasn't confused anyone, or muddied the water for you. This is only an effort to help based on my understanding of how things work....!
Good luck!
P.S. even a 95% overlap will not give full coverage at maximum depth, but it won't miss much.... <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt="
"> Most of us don't have that level of patience either.
My analogy will give you roughly 80% coverage at maximum depth and full coverage to 10-12 inches or so if you criss/cross and diagonal with 50% overlap and a SLOW sweep with good headphones.
Comments and corrections welcomed!
