Patent ID: 8116994

Claim:
A method comprising the steps of: (a) generating a prescribed acoustic wave wherein a frequency f of said acoustic wave matches at least one resonant mode of an underground device including a cavity for at least a time equal to 2/f; (b) acoustically injecting said acoustic wave into said underground device; (c) establishing a first time reference in a first clock, wherein said first clock governs the generation of said acoustic wave; (d) electronically detecting said acoustic wave, at a remote location in acoustic communication with said underground device through an earth path to produce a measurement of said acoustic wave; (e) establishing a second time reference in a second clock, wherein said second time reference of said second clock is synchronized with said first time reference of said first clock, and said second clock governs said measurement; (f) digitizing and storing a sequential series of measurements of said acoustic wave; (g) performing digital processing on said series of measurements, wherein said digital processing comprises cross correlating said series of measurements with a priori amplitude values of said prescribed acoustic wave to produce a result related to the intensity of said acoustic wave at said remote location; and (h) communicating said result to an operator.