Why is ground wave transmission of signals restricted to a frequency of 1500 kHz ?

Explanation

In ground wave propagation a signal glides over
the surface of earth. While progressing along
the surface of the earth, the energy of ground
wave is gradually absorbed by the earth due to
induced current set up in the ground and bending
of ground wave round the corner of the object.
These energy loses increases with the increase
in frequency beyond 1510 kHz and higher
frequencies are highly damped.