Method and apparatus for transmitting electromagnetic signals into the earth at frequencies below 500 KHz from a capacitor emplaced on the surface of the earth or raised aloft in an aircraft

An apparatus for transmitting an electromagnetic signal or pulse through an earth formation and recovering the returning signal or pulse for processing to determine electrical properties at depth including,

• a capacitor assembly comprising a co-planar pair of highly conductive metallic plates with electrical input or output terminals between the plates wherein each pair of terminals connect to an input or output transformer with magnetic flux cores and wherein each pair of plates has a largest dimension L disposed either in direct contact with the earth formation to form an earth capacitor comprising the capacitor assembly and the region of the earth formation near said capacitor assembly;
• or elevated above the earth formation to form a capacitor comprising the capacitor assembly, and the earth formation separated from the capacitor assembly by the air of thickness Z,
• a means for applying a short voltage or current pulse to the capacitor assembly via the transformer with magnetic flux core adjusted for radiation <0.5 MHZ, to transmit electromagnetic radiation into the earth formation either directly or via the air of thickness Z, wherein for each frequency component of the electromagnetic radiation, the largest dimension L satisfies the relation Re(KL)<1, where K is the wave number in the earth formation of said frequency component, wherein the short voltage or current pulse as applied to earth capacitor plates via and in convolution with the transformer with magnetic flux core has frequency components in the broadband <1 MHZ and,
• a means for detecting electromagnetic radiation that has propagated through the earth formation in response to application of the short voltage or current pulse, including means for processing only frequency components of the detected electromagnetic radiation having an upper frequency less than 1 MHZ and a lower frequency very much less than 1 MHZ depending upon the depth of interest for wave penetration.

A method for transmitting electromagnetic signals through the earth formation, including the steps of:

• (a) disposing capacitor plates having a largest dimension L in direct contact with the earth formation or elevated a distance Z above the earth formation forming an earth capacitor comprising the plates and a region of the earth formation near the plates or the region of air between the plates and the earth formation whereupon the frequencies employed make the air thickness electrically small so that the plates are electrically close to the earth formation,
• (b) applying a short voltage or current pulse to transmit electromagnetic radiation into the earth formation, wherein for each frequency component of the electromagnetic radiation, the largest dimension L satisfies the relation Re(KL)<1, where K is the wave number in the earth formation of said frequency component, wherein the short voltage or current pulse applied through the transformer has radiation field components in a broad band below 1 MHZ with the additional requirement that KoZ<1 thus enforcing the further desirable restriction of using much lower frequencies below 500 KHZ,
• (c) detecting electromagnetic radiation that has propagated through the earth formation in response to application of the short voltage or current pulse:
• and
• (d) processing only frequency components of the detected electromagnetic radiation in a frequency window having an upper frequency less than 0.5 MHZ which have propagated into the formation to a penetration depth very much greater than the largest dimension L.

An apparatus for transmitting an electromagnetic signal through the earth formation to very great depths relative to a restrictive size, R, of the capacitor plates, including:

• a capacitor assembly having a largest dimension L<R disposed in direct contact with the earth formation or elevated above the earth formation a distance Z to form an earth capacitor comprising the capacitor assembly, the space between the capacitor assembly and the earth formation if Z>0, and a region of the earth formation coupled to the capacitor assembly or if Z=0, comprising the capacitor assembly and the adjacent earth formation, also including
• a means for applying a short voltage or current pulse to capacitor assembly to transmit electromagnetic radiation into the earth formation, wherein for each frequency component of the electromagnetic radiation, the largest dimension L satisfies the relation Re(KL)<1, where K is the wave number in the earth of said frequency component, provided if Z>0, KoZ<1 and a means for detecting electromagnetic radiation that has propagated through the earth formation and the space Z>0, in response to the application of a short voltage or current pulse, including means for identifying frequency components of the detected electromagnetic radiation in a frequency window which have propagated into the earth formation to a penetration depth much greater than several tens of thousand feet, wherein the means for applying the short voltage or current pulse to the capacitor assembly includes:
• a pulse generator
• a transmission line transformer or a conventional transformer both with core magnetic flux current adjusted for frequencies <0.5 MHZ, between the pulse generator and the capacitor assembly.