When using a TR it may be necessary to scrub the ground to get more depth by rubbing the coil on the ground as closely as possible, but this is not true when there are no ground minerals such as saline or iron ores, etc. If there is nothing but pure silica then there is no reason to need to try that hard to get that close to the soil. Those days are gone now, hardly anybody but a few of we old collectors ever use an old TR now. VLF's are by far more successful in depth, target location and discrimination by searching 1-2 inches off the actual ground (1" of turf plus 1" of air =2"). George Payne, Erik foster, and Jack Gifford have all written scrolls about this subject explaining the hows and whys of searching 1-2" AWAY from the ground with VLF's and not scrubbing it.
ALL current VLF'x (including Minelabs, Garrett, and White's multi-frequencies) are designed to operate best when not disturbing the ground exclusion balance of the detectors. If someone scrubs the ground with a detector he or she defeats the electronically designed parameters for these detectors which operate at a -5% to a + 5% over/under the prescribed positive and negative voltages used and controlled by a capacitive discharge circuits to keep the null and self adjusting threshold at bay, and to not overload it. When you scrub a coil with a VLF you take away the very good properties built into the detectors.
I suppose that if one beats the ground with his detector or likes to wear the coil out by using his coil as a scrub brush (just for fun) that it may be better to use several layers of coil covers to overcome his compulsive behavior, but usually most people don't do that.
The fact simply put is, that current detectors of the VLF variety work better and deeper when searching 1-2" off the ground, not right on top of it. They are designed that way.
