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NUMERICAL STUDY OF ELECTROMAGNETIC WAVES GENERATED BY A PROTOTYPE DIELECTRIC LOGGING TOOL
Citation:
Ellefsen, K. J., J. D. Abraham, D. L. Wright, AND A T. Mazzella. NUMERICAL STUDY OF ELECTROMAGNETIC WAVES GENERATED BY A PROTOTYPE DIELECTRIC LOGGING TOOL. GEOPHYSICS 69(1):64-77, (2004).
Impact/Purpose:
Research is being conducted to improve and evaluate the resolution of the CR, EM, seismic, and GPR methods over complex geological formations (such as fractured geologies) and to evaluate the capability of these geophysical methods to delineate subsurface organic contaminants.
Description:
To understand the electromagnetic waves generated by a prototype dielectric logging tool, a
numerical study was conducted using both the finite-difference, time-domain method and a frequency- wavenumber method. When the propagation velocity in the borehole was greater than that in the formation (e.g., an air-filled borehole in the unsaturated zone), only a guided wave propagated along the borehole. As the frequency decreased, both the phase and the group velocities of the guided wave asymptotically approached the phase velocity of a plane wave in the formation. The guided wave radiated electromagnetic energy into the formation, causing its amplitude to decrease. When the propagation velocity in the borehole was less than that in the formation (e.g., a water-filled borehole in the saturated zone), both a refracted wave and a guided wave propagated along the borehole. The velocity of the refracted wave equaled the phase velocity of a plane wave in the formation, and the refracted wave preceded the guided wave. As the frequency decreased, both the phase and the group velocities of the guided wave asymptotically approached the phase velocity of a plane wave in the formation. The guided wave did not radiate electromagneticenergy into the formation. To analyze traces recorded by the prototype tool during laboratory test, they were compared to traces calculated with the finite differences method. The first parts of both the recorded and the calculated traces were similar, indicating that guided and refracted waves indeed propagated along the prototype tool.