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SHALLOW SUBSURFACE MAPPING BY ELECTROMAGNETIC SOUNDING IN THE 300 KHZTO 30 MHZ RANGE: MODEL STUDIES AND PROTOTYPE SYSTEM ASSESSMENT
Citation:
Stewart, D. C., W. Anderson, T. P. Grover, AND V. F. Labson. SHALLOW SUBSURFACE MAPPING BY ELECTROMAGNETIC SOUNDING IN THE 300 KHZTO 30 MHZ RANGE: MODEL STUDIES AND PROTOTYPE SYSTEM ASSESSMENT. Geophysics 59(8):1201-1210, (1994).
Description:
A new instrument designedfor frequency-domain sounding in the depth range 0-10 m uses short coil spacings of 5 m or less and a frequency range of 300 kHz to 30 MHz. In this frequency range, both conduction currents (controlled by electrical conductibity) and displacement currents (controlled by dielectric permittivity) are important. Several surface electromagnetic survey systems commonly used (generally with frequencies less than 60 lHz) are unsuitable for detalied investigation of the upper 5 m of the earth or, as with ground-penetrating radar, are most effective in relatively resistive environments. Most computer programs writtenfor interpretatoio of data acquired with the low-frequency systems neglect displacement current, and are thus unsuited for accurate high-frequency modeling and interpretation. New forward and inverse computer programs are described that include displace,ment currents in layered-earth models. The computer programs and this new intrrument are used to evaluate the effectiveness of shallow high-freguqncy soundings based on measurement of the tilt angle and the ellipticity of magnetic fields. Forward model studies indicate that the influence of dielectric permittivity provides the ability to resolve this layers, especially if the instrument frequency range canbe extended to 50 MHz. Field tests of the wherein both the resistivity and the dielectric permittivity of layers are determiend. Althoiugh data collection and inversion are much slower thatn for low-frequency methods, additional information is obtained inasmuch as there usually is a permittivity contrast as well as a resistivity contrast at boundaries between different materials. Determination of dielectric permittivity is particularly important for hazardous waste site characterization because the presenc of some contaminants may have little effect onobserved resisitivity but a large effect on observed permittivity.