Coherent transmit and receiver bi-static electromagnetic geophysical tomography
Devices and processes provide for geophysical oil, gas, or mineral prospecting and subsurface fluid monitoring, using a controlled source electromagnetic system that transmits a designed probe wave to create images of sub-surface structures and fluids either statically or while in motion.
1. A method of collecting subsurface geophysical data, comprising:
generating a probe signal by a transmitter, the probe signal comprising a continuously repeated predetermined binary waveform;
receiving a reflection of the probe signal from a subsurface layer of interest by a receiver, the receiver synchronized with the transmitter;
processing the reflection of the probe signal to generate resistance, velocity, and attenuation profiles associated with the subsurface layer of interest; and
detecting real-time movement of fracing fluids in the subsurface layer of interest based on the processed reflection of the probe signal,
wherein the subsurface layer of interest is greater than 800 m below a surface of the earth, and
wherein the transmitter and receiver are coherently timed to better than 100 nanoseconds in a time domain.
2. The method of claim 1 , wherein the predetermined binary waveform is a pseudo random noise code of a length at least 8191 chips.
3. The method of claim 1 , wherein generating resistance, velocity, and attenuation profiles comprises:
cross-correlating received binary waveforms with the predetermined binary waveform, producing a cross-correlation data.
4. The method of claim 3 , further comprising:
allocating the cross-correlation data into time bins;
removing time bins within the cross-correlation data; and
calculating resistance and velocity profiles for multiple subsurface layers, responsive to the removal of time bins.
5. The method of claim 3 , wherein detecting injection of fluids injected into the layer of interest by detecting changes in the cross-correlation data over time.
6. The method of claim 1 , wherein the probe signal comprises a sequence of predetermined binary waveforms, wherein successive elements of the sequence of predetermined binary waveforms are selected based on a sequence of different subsurface layer depths of interest.
7. The method of claim 1 , further comprising:
positioning the transmitter and receiver between 15 m and 1 km of each other; and
detecting changes in subsurface resistance resulting from an introduction of fluid into the subsurface layer of interest.