Superheterodyne seismic vibrator and method

A seismic energy source, comprising:

• at least a first and a second acoustic radiator operatively coupled to a non-linear medium proximate an upper limit of formations in the Earth's subsurface, the first and second acoustic radiators configured to convert electrical energy directly into acoustic energy and to have a base or mean operating frequency near or above 10 Khz;
• and
• means for operating the first and the second acoustic radiator at respective first and second frequencies, the first and second frequencies selected near or above 10 Khz and such that substantially no acoustic energy propagates through the non-linear medium, the first and the second frequencies selected such that a difference there between is swept through a range of frequencies of seismic energy capable of propagating through the Earth's subsurface to at least one acoustic impedance boundary within the Earth's subsurface.