Persistent Link:
http://hdl.handle.net/10150/288769
Title:
A numerical electromagnetic study of shallow geophysical targets
Author:
Debroux, Patrick Serge, 1957-
Issue Date:
1997
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
Prediction of the response of high-frequency induction survey tools to 3-dimensional targets is needed to aid in tool and survey design, in the interpretation of data, and to analyze the interaction of the individual field components with the target of interest. To this end, two numerical algorithms (TSAR and NEC) were imported and adapted to solve geophysical electromagnetic problems. A third algorithm (EM1DSH) was used to quantitatively analyze the role of current channeling on the response of shallow targets, and to verify that the TSAR and NEC algorithms include the important effect of current channeling in their solution. TSAR (a finite difference time-domain algorithm) proved successful in modeling the ellipticity response of a vertical magnetic dipole placed over a homogenous and layered lossy dielectric as compared to published data in the 500 kHz to 30 MHz range. Cell-size versus accuracy analyses show that little accuracy gains are made with a reduction of cell-size past the one-tenth effective wavelength modeling guideline. NEC (a method-of-moments algorithm) shows substantial but limited success in modeling the response of small loop antennas to perfectly and near-perfectly conducting geophysical targets (conductivity and permeability) in the 6.4 kHz to 8 MHz range. Comparison of NEC results are made with analytic results, fields data, and other numerical algorithms. NEC shows substantial numerical error at lower frequencies due to the effective lengths (in wavelengths) of the wire segments used. Also, the Green's function look-up table used to interpolate the effect of half-space on target response is not optimized for the geophysical problem which can lead to substantial solution error at lower (kHz) frequencies. An integral equation solution (EM1DSH) analysis shows that the quantitative effect of increasing background conductivity (which affects both current channeling and target response) on the secondary field response of a buried thin-sheet can be greater than 120 percent in the geophysical induction range. Target parameter changes show current channeling to be greatest for targets that are shallow, that are horizontal, and have a large dimensional aspect ratio. Target and survey parameter sensitivity analyses help to understand the relationship of these parameters to current channeling.
Type:
text; Dissertation-Reproduction (electronic)
Keywords:
Geophysics.; Engineering, Electronics and Electrical.; Environmental Sciences.
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Mining and Geological Engineering
Degree Grantor:
University of Arizona
Advisor:
Sternberg, Ben K.

Full metadata record

DC FieldValue Language
dc.language.isoen_USen_US
dc.titleA numerical electromagnetic study of shallow geophysical targetsen_US
dc.creatorDebroux, Patrick Serge, 1957-en_US
dc.contributor.authorDebroux, Patrick Serge, 1957-en_US
dc.date.issued1997en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractPrediction of the response of high-frequency induction survey tools to 3-dimensional targets is needed to aid in tool and survey design, in the interpretation of data, and to analyze the interaction of the individual field components with the target of interest. To this end, two numerical algorithms (TSAR and NEC) were imported and adapted to solve geophysical electromagnetic problems. A third algorithm (EM1DSH) was used to quantitatively analyze the role of current channeling on the response of shallow targets, and to verify that the TSAR and NEC algorithms include the important effect of current channeling in their solution. TSAR (a finite difference time-domain algorithm) proved successful in modeling the ellipticity response of a vertical magnetic dipole placed over a homogenous and layered lossy dielectric as compared to published data in the 500 kHz to 30 MHz range. Cell-size versus accuracy analyses show that little accuracy gains are made with a reduction of cell-size past the one-tenth effective wavelength modeling guideline. NEC (a method-of-moments algorithm) shows substantial but limited success in modeling the response of small loop antennas to perfectly and near-perfectly conducting geophysical targets (conductivity and permeability) in the 6.4 kHz to 8 MHz range. Comparison of NEC results are made with analytic results, fields data, and other numerical algorithms. NEC shows substantial numerical error at lower frequencies due to the effective lengths (in wavelengths) of the wire segments used. Also, the Green's function look-up table used to interpolate the effect of half-space on target response is not optimized for the geophysical problem which can lead to substantial solution error at lower (kHz) frequencies. An integral equation solution (EM1DSH) analysis shows that the quantitative effect of increasing background conductivity (which affects both current channeling and target response) on the secondary field response of a buried thin-sheet can be greater than 120 percent in the geophysical induction range. Target parameter changes show current channeling to be greatest for targets that are shallow, that are horizontal, and have a large dimensional aspect ratio. Target and survey parameter sensitivity analyses help to understand the relationship of these parameters to current channeling.en_US
dc.typetexten_US
dc.typeDissertation-Reproduction (electronic)en_US
dc.subjectGeophysics.en_US
dc.subjectEngineering, Electronics and Electrical.en_US
dc.subjectEnvironmental Sciences.en_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplineMining and Geological Engineeringen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorSternberg, Ben K.en_US
dc.identifier.proquest9814455en_US
dc.identifier.bibrecord.b37745013en_US
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