A method of subsurface monitoring via interferometric techniques using control source electromagnetic signals, the method comprising the following steps: a. positioning at least one transmitter (605, 610) on a surface (125, 200, 400) and illuminating a target area with the at least one transmitter (605, 610), such that the at least one transmitter (605, 610) illuminates this target area, beneath the surface (125, 200, 400), with source radiation at or below a critical angle using one or more phase coherent chirps having a plurality of transmission frequencies in the ULF and ELF frequency ranges, said critical angle being the largest angular displacement of the incident radiation from the normal with respect to the target area and being dependent on each transmission frequency and the conductivity ratio between the target area and the overburden layer; b. positioning at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) on said surface (125, 200, 400), said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) positioned such that said transmission(s) from said at least one transmitter (605, 610) are received by at least one of said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620), wherein said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) detect phase shifts and amplitude changes from the transmitted chirps through phase coherence; c. wherein said at least one transmitter (605, 610) and said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) are phase and/or time coherent with the same low drift synchronizing clock, wherein each of said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) comprise a Delta Sigma Analog to Digital Convertor, ADC,, wherein each Delta Sigma ADC comprises an ADC clock and executes a state machine that consists of a number of process steps that produce a data sample (520, 530, 575) at the completion of the steps, wherein the steps occur at the ADC clock rate; d. recording data received by said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) in response to said chirps using data framing, wherein said data framing produces data samples (520, 530, 575) using said state machine of each Delta Sigma ADC while, using a clock management system, delays a single step in said state machine on a regular basis after aligning the ADC clock with a phase alignment pulse output by said same low drift synchronizing clock, wherein said data samples (520, 530, 575) are phase coherent and coherent with said low drift synchronizing clock; e. repeating steps a. through d. for a plurality of said transmission frequencies, said angles, and/or positions of the at least one transmitter (605, 610) on said surface (125, 200, 400); and f. creating at least one interferometric image of said target structure from said data. a. positioning at least one transmitter (605, 610) on a surface (125, 200, 400) and illuminating a target area with the at least one transmitter (605, 610), such that the at least one transmitter (605, 610) illuminates this target area, beneath the surface (125, 200, 400), with source radiation at or below a critical angle using one or more phase coherent chirps having a plurality of transmission frequencies in the ULF and ELF frequency ranges, said critical angle being the largest angular displacement of the incident radiation from the normal with respect to the target area and being dependent on each transmission frequency and the conductivity ratio between the target area and the overburden layer; b. positioning at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) on said surface (125, 200, 400), said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) positioned such that said transmission(s) from said at least one transmitter (605, 610) are received by at least one of said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620), wherein said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) detect phase shifts and amplitude changes from the transmitted chirps through phase coherence; c. wherein said at least one transmitter (605, 610) and said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) are phase and/or time coherent with the same low drift synchronizing clock, wherein each of said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) comprise a Delta Sigma Analog to Digital Convertor, ADC,, wherein each Delta Sigma ADC comprises an ADC clock and executes a state machine that consists of a number of process steps that produce a data sample (520, 530, 575) at the completion of the steps, wherein the steps occur at the ADC clock rate; d. recording data received by said at least two receivers (120, 205, 210, 405a, 405b, 405c, 620) in response to said chirps using data framing, wherein said data framing produces data samples (520, 530, 575) using said state machine of each Delta Sigma ADC while, using a clock management system, delays a single step in said state machine on a regular basis after aligning the ADC clock with a phase alignment pulse output by said same low drift synchronizing clock, wherein said data samples (520, 530, 575) are phase coherent and coherent with said low drift synchronizing clock; e. repeating steps a. through d. for a plurality of said transmission frequencies, said angles, and/or positions of the at least one transmitter (605, 610) on said surface (125, 200, 400); and f. creating at least one interferometric image of said target structure from said data.