First, air testing is one thing but I tend to look for in-ground targets . . . not trying to sound like a wise guy but ground does make a difference.
As far as silver dollars hitting in the 97+ range, a few things to consider. 1) Coin hunting, especially if you are looking for big silver in 18 KHZ is going to result in you digging a lot of tiny pieces of junk and not getting the depth you would at 8 KHZ or 4 KHZ (which will be a real contender in V4.0), 2) You need to play the odds unless you have all the time in the world to hunt - if you are hunting a site that has the rusted iron causing the wrap around, you need to think how many silver dollars there are vs how much time you have to dig all the iron . . . your call but I tend to go for keepers and likelihood of missing something over digging it all. If you want to be sure you are not missing those silver dollars, drop the frequency, get better depth and better target separation.
The reason the silver gets pressed into the wrap around iron area at 18 KHZ is that all readings get pressed up. We have a few slides in the Bootcamp where we cover this but for UK hunters (and other non-US hunters) looking for the tiny lower conductive coins, the higher frequency works better - they do not use TIDs. Until V4.0 is released which will offer some adjustability not found on V3.2, use 8 KHZ or 12 KHZ for US coin hunting with 8 being the one for high iron when you have the potential for silver dollars to be present. The 97 - 99 notch will eliminate much of the wrap around issue.
Andy