Radio atmospheric Sferics received from about distance or greater have their frequencies slightly offset in time, producing "tweeks". When the electromagnetic energy from a sferic escapes the Earth-ionosphere waveguide and enters the magnetosphere, it becomes dispersed by the near-Earth plasma, forming a whistler signal. Because the source of the whistler is an impulse (i.e., the sferic), a whistler may be interpreted as the impulse response of the magnetosphere (for the conditions at that particular instant). A lightning channel with all its branches and its electric currents behaves like a huge antenna system from which electromagnetic waves of all frequencies are radiated. Beyond
Surface wave inversion Surface wave inversion Seismic inversion involves the set of methods which seismologists use to infer properties through physical measurements. Surface-wave inversion is the method by which elastic properties, density, and thickness of layers in the subsurface are obtained through analysis of surface-wave dispersion. The entire inversion process requires the gathering of seismic data, the creation of dispersion curves, and finally the inference of subsurface properties. Surface waves are seismic waves that travel at the surface of the earth, along the air/earth boundary. Surface waves are slower than P-waves(compressional waves) and S-waves(transverse waves). Surface waves are classified into two basic types,