Large coil for depth in light trash, small coil for target separation in heavy trash, medium coil for the middle ground between those two site conditions.
Larger coils simply gulp a larger bucket of ground mineralization below the coil. If your soil is mineral free and there's no EMI then it doesn't matter noise/sensitivity wise, all coils will be about equal in stability. But in mild to heavy mineralization the larger the coil the larger the gulp of mineralization they bite off and the noisier they will be, and may require backing off the sensitivity.
Right now its bone dry here, when the rains return and soak that iron and mineralization its going to be worse, a lot worse for the WOT. You lose some depth in bone dry soil but the iron and mineralization also quiets down significantly.
Next consider the physical size of the magnetic field vs the density of the magnetic field a given coil produces. Large coils produce large (but less dense) magnetic fields that extend further into the soil than small coils, but the field lines are not as packed together. So they go deeper, but suffer some performance fall off on smaller targets as there are less field lines to strike the target. On the flip side a small coil produces a higher density magnetic field with many more field lines to strike a target, but a smaller size field that doesn't extend into the soil as far. Good for getting hits on small to tiny targets, but not really any deeper than the small coil goes. When you wind a coil for an Explorer you can't just add more magnet wire winds to a larger coil, the length of wire to remain within coil specs is about the same, so larger coil fewer loops of wire, small coil more loops of wire.
The actual size of the magnetic field a WOT produces is huge, you can detect your car door from about 4 foot away. The issue is there are not that many magnetic field lines to strike a target 4 foot from the coil. If the target is big enough like a car door then yes but a coin never going to happen. The further away from the coil the fewer number of field lines there are.
With the WOT you have to consider the actual shape of the magnetic field it produces. The curvature of the WOT coils field is such that its particularly good at curving at around the 8-10 inches depth to strike a coin that's tilted about a 45 degree angle in the ground. Versus the stock coil. They can be difficult to pinpoint because the angle of the coin tends to shoot out its signal half sideways. Ideally you want the field lines to strike the target at 90 degrees to the largest surface of the target, for example field lines traveling straight down striking a coin laying flat produces the strongest signal. For whatever reason the WOT tends to curve just right to strike the flat of a coin that's oriented at a 45 degree angle at that particular depth. I dug many with a WOT, typically knew what I was dealing with by the signal shooting out sideways so I'd break the clod apart and check for that. With the stock coil I pretty much never dug those, with the coin angled like that in the ground at that depth not enough field lines were striking the target.
Trivia: I once mounted the 15 inch WOT coil on 4 wheels and pushed it around a field, true story lol.
