Sometimes there is a need in getting response of a metal detector only to large targets. Scrap metal collecting or application of a metal detector as a mine detector. Large coil and minimum sensitivity level.
For such type of searching large Mars Tiger and Mars Goliath coils are suitable. While experimenting in practice, you can shorten the length of a connecting stem (increasing distance between the coil and the surface) and reduce sensitivity not to the minimum but only by half.