Techniques for determining physical properties of underground structures using lightning

1. A method comprising:

detecting lightning induced electromagnetic pulses; and

determining, on a processor, a physical property of a displaced underground structure based on the lightning induced electromagnetic pulses,

wherein the physical property is at least two of horizontal location, horizontal extent, depth, vertical extent, electrical conductivity, or electromagnetic permittivity.

2. A method as recited in claim 1 , further comprising positioning on a surface of the Earth a plurality of very low frequency electromagnetic receivers, wherein:

the plurality of very low frequency electromagnetic receivers bracket a geographic region of interest; and

detecting lightning induced electromagnetic pulses further comprises measuring electromagnetic signals at the plurality of very low frequency electromagnetic receivers.

3. A method as recited in claim 2 , wherein each very low frequency electromagnetic receiver comprises an antenna comprising three substantively perpendicular loops of electrical conductors.

4. A method as recited in claim 3 , wherein positioning the plurality of very low frequency electromagnetic receivers further comprises orienting one loop of electrical conductor of each antenna in a substantively horizontal plane.

5. A method as recited in claim 3 , wherein positioning the plurality of very low frequency electromagnetic receivers further comprising orienting one loop of electrical conductor of each antenna in a substantively vertical plane substantively parallel to a fixed vertical plane.

6. A method as recited in claim 3 , wherein detecting lightning induced electromagnetic pulses further comprises measuring very low frequency electromagnetic signals and calibrating, on a processor, measured signals to account for inter-channel timings among signals from the loops of each very low frequency electromagnetic receiver and to convert from millivolt values at a loop of each very low frequency electromagnetic receiver to picoTesla values characterizing an incident electromagnetic signal.

7. A method as recited in claim 3 , wherein determining the physical property of an underground structure further comprises determining, on a processor, a horizontal direction of propagation of a lightning induced electromagnetic pulse based on phase differences between a lightning induced electromagnetic pulse detected on two or more of the substantively perpendicular loops of electrical conductors.

8. A method as recited in claim 7 , wherein determining the physical property of an underground structure further comprises:

determining, on a processor, an upstream receiver and downstream receiver of the plurality of very low frequency electromagnetic receivers based on the direction of propagation; and

determining, on a processor, a depth classification based on a delay in arrival of the lightning induced electromagnetic pulse at the downstream receiver compared to arrival at the upstream receiver.

9. A method as recited in claim 3 , wherein determining the physical property of an underground structure further comprises determining, on a processor, a depth classification of a lightning induced electromagnetic pulse based on phase differences between a lightning induced electromagnetic pulse detected on two or more of the substantively perpendicular loops of electrical conductors.

10. A method as recited in claim 3 , wherein determining the physical property of an underground structure further comprises determining, on a processor, a depth distribution of the physical property based on a plurality of lightning induced electromagnetic pulses in a corresponding plurality of different depth classifications.

11. A method as recited in claim 10 , wherein determining the physical property of the underground structure further comprises computer tomography and a vertically layered propagation model.

12. A method as recited in claim 3 , wherein each very low frequency electromagnetic receiver comprises a low noise amplifier that amplifies electromagnetic signals in a frequency band of interest from about 1.5 kiloHertz to about 50 kiloHertz without emitting airborne electromagnetic waves in the frequency band of interest substantively above an antenna noise threshold.

13. A method as recited in claim 2 , wherein each very low frequency electromagnetic receiver comprises a processor and transmitter positioned about 100 meters from the antenna that is configured to receive amplified signals from the low noise amplifier and to send processed data to an aggregator processor.

14. A method as recited in claim 2 , wherein determining the physical property of the underground structure further comprises determining, on a processor, the effect of local Earth surface topography on a very low frequency electromagnetic wave received on at least one of the plurality of very low frequency electromagnetic receivers.

15. A method as recited in claim 1 , wherein detecting lightning induced electromagnetic pulses further comprises measuring very low frequency electromagnetic signals and removing, on a processor, narrowband very low frequency electromagnetic signals broadcast from known transmitters.

16. A method as recited in claim 1 , wherein detecting lightning induced electromagnetic pulses further comprises measuring very low frequency electromagnetic signals and removing, on a processor, harmonics of power line transmissions at one of about 50 Hertz or about 60 Hertz.

17. A method as recited in claim 1 , wherein detecting lightning induced electromagnetic pulses further comprises measuring very low frequency electromagnetic signals and detecting, on a processor, a pulse with sferic characteristic temporal shape and duration of about 2.50 milliseconds in a frequency band of interest from about 1.5 kiloHertz to about 50 kiloHertz.

18. A method as recited in claim 1 , wherein determining the physical property of an underground structure further comprises determining, on a processor, a horizontal distribution of the physical property based on a plurality of lightning induced electromagnetic pulses in a corresponding plurality of different horizontal directions of propagation.

19. A method as recited in claim 18 , wherein determining the physical property of the underground structure further comprises computer tomography and a one-dimensional propagation model.