Surface waves propagating around the spherical Earth are scattered by medium heterogeneities and topographical variations. Wave trains appearing between the multiple arrivals of surface waves are interpreted as scattered waves. Mean-Squared (MS) envelopes of vertical-component seismograms in long periods can be explained by the single isotropic scattering process of Rayleigh waves of the fundamental mode based on the radiative transfer theory on the spherical Earth. For the case of circular radiation from a point source on the sphere, we can analytically solve the single scattering approximation of the radiative transfer equation. The resultant MS envelope is written by using an elliptic integral of the first kind.