Dairyground,
I dare to disagree.
So if the disturbance caused by the aircraft reaches the ground it will be propagated at a speed greater than the local speed of sound. So a shockwave should form somewhere between the aircraft and the ground.
It seems to me like you are mixing two completely different things up here. Let me try to get this straight although it's a bit difficult to explain without a picture. A shockwave or "disturbance" effectively
is nothing else than a soundwave and can
never travel faster than the local speed of sound because the speed at which such a wave is traveling itself
defines the speed of sound. If an aeroplane flies with Mach 0,85 there is no sonic boom whatsoever, neither at altitude nor on the ground regardless of how high the groundspeed may be.
As it concerns supersonic aircraft there is a common misconception regarding the traveling speed of the wavefront along the ground. If a fighter aircraft flies with lets say Mach 3.0 then it produces a shockwave in the shape of the "Mach cone". This shockwave travels
vertically from the aircraft to the ground at sonic speed (because it actually is a soundwave, see above). The intersection of the Mach cone with the ground of course is moving
horizontally with the ground speed of the aircraft, in this case around Mach 3.0 - that's why an observer on the ground can get the impression that there is a blastwave moving along the earth's surface at supersonic speed. In fact the shockwave is hitting the observer from above not head on like a bomb blast for instance.
Hope this makes it a bit clearer.