Using near and far field ULF and ELF interferometry synthetic aperture radar for subsurface imaging
This invention relates to devices and processes for geophysical prospecting, subsurface fluid monitoring and, more particular, to the use of interferometric techniques using Control Source Electromagnetic (“CSEM”) and Magnetoturelic (“MT”) signals to create images of sub-surface structures and fluids.
1. A method of subsurface monitoring via interferometric techniques using control source electromagnetic signals, the method comprising:
positioning a first transmitter on a surface at an angle less than a critical angle with respect to a normal of a target subsurface structure;
positioning a first receiver on the surface, the first transmitter and the first receiver phase coherent, time coherent, or both with a low drift synchronizing clock;
transmitting a signal containing a plurality of transmission frequencies towards the target subsurface structure, the signal comprising one or more designed waveforms;
recording data received by the first receiver in response to the designed waveforms, the data containing small phase shifts and amplitude changes in the signal; and
creating an interferometric image of the target structure from the data using data framing that creates data samples that are phase coherent and time coherent with the low drift synchronizing clock.
2. The method of claim 1 , further comprising phase focusing by introducing a specific phase shift resulting in an angular deviation from a phase center between the first receiver and a second receiver.
3. The method of claim 1 , further comprising phase steering by introducing a phase shift to the first transmitter and a second transmitter, wherein the phase shift is the same increment for the first and second transmitters.
4. The method of claim 1 , further comprising phase steering by introducing a phase shift to the first receiver and a second receiver, wherein the phase shift is the same increment for each of the first and second receivers.
5. The method of claim 1 , further comprising producing conductivity values and corresponding phase velocities from data collected from pseudo random binary code transmissions.
6. The method of claim 5 , further comprising producing depth information by performing radar sounding timing with respect to near field phase and intermediate field velocities.
7. The method of claim 6 , wherein the subsurface monitoring is conducted during a hydraulic fracturing or liquid petroleum gas sub-surface fracturing operation.
8. The method of claim 5 , wherein the frequencies are greater than 0.25 Hz.
9. The method of claim 8 , further comprising changing the plurality of frequencies at discrete intervals in time.
10. The method of claim 8 , further comprising continuously varying the plurality of frequencies are continuously over time.
11. The method of claim 1 , creating a stack of at least two interferometric images.
12. The method of claim 11 , performing three-dimensional imaging of the target structure using the stack of at least two interferometric images.
13. The method of claim 11 , performing four-dimensional imaging using the stack of at least two interferometric images, wherein the four-dimensional imaging is three-dimensional imaging taken over one or more time intervals.
14. The method of claim 1 , further comprising: receiving, by the first receiver, one or more of:
a signal indicating a reflection;
a signal indicating a null; or
a signal returned as a result of seeing through a formation, the formation closer to the surface than the target structure.