Pitfalls of 1D inversion of small-loop electromagnetic data for detecting man-made objects |
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| Institution: | 1. Hanyang University, Department of Natural Resources and Geoenvironmental Engineering, 222 Wangsimni-ro, Seongdong-gu, Seoul, Republic of Korea;2. University of British Columbia, Department of Earth and Ocean Sciences, Geophysical Inversion Facility, Vancouver, BC, Canada;1. Institute of Geophysics, ETH, Sonneggstr. 5, 8092 Zurich, Switzerland;2. Uppsala University, Department of Earth Sciences, Villavägen 16 B, 752 36 Uppsala, Sweden;3. Institute of Environmental Engineering, Wolfgang-Pauli-Str. 15, 8093 Zürich, Switzerland;4. Institute for Geoscience, Aarhus University, C F Moellers Allé 4, DK-8000 Aarhus C, Denmark;5. Department of Geological Survey, Private Bag 14, Lobatse, Botswana;6. Department of Geology, University of Botswana, Private Bag UB00704, Gaborone, Botswana;1. BioSP, INRA, Avignon 84000, France;2. INRA Laboratoire d’étude des Interactions Sol Agrosystème Hydrosystème (LISAH), Campus de la Gaillarde, 2 place Viala, Montpellier 34060, France;3. IRD Laboratoire d’étude des Interactions Sol Agrosystème Hydrosystème (LISAH), Campus de la Gaillarde, 2 place Viala, Montpellier 34060, France;1. Instituto Geológico y Minero de España (IGME), Ríos Rosas 23, 28003 Madrid, Spain;2. Departamento de Geodinámica, Facultad de Ciencias, Universidad de Granada, Avenida Fuentenueva s/n, 18071 Granada, Spain;3. Unidad del IGME en Granada, Urbanización Alcázar del Genil, 4, 18006 Granada, Spain |
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| Abstract: | In order to interpret field data from small-loop electromagnetic (EM) instruments with fixed source–receiver separation, 1D inversion method is commonly used due to its efficiency with regard to computation costs. This application of 1D inversion is based on the assumption that small-offset broadband EM signals are insensitive to lateral resistivity variation. However, this assumption can be false when isolated conductive bodies such as man-made objects are embedded in the earth. Thus, we need to clarify the applicability of the 1D inversion method for small-loop EM data. In order to systematically analyze this conventional inversion approach, we developed a 2D EM inversion algorithm and verified this algorithm with a synthetic EM data set. 1D and 2D inversions were applied to synthetic and field EM data sets. The comparison of these inversion results shows that the resistivity distribution of the subsurface constructed by the 1D inversion approach can be distorted when the earth contains man-made objects, because they induce drastic variation of the resistivity distribution. By analyzing the integrated sensitivity of the small-loop EM method, we found that this pitfall of 1D inversion may be caused by the considerable sensitivity of the small-loop EM responses to lateral resistivity variation. However, the application of our 2D inversion algorithm to synthetic and field EM data sets demonstrate that the pitfall of 1D inversion due to man-made objects can be successfully alleviated. Thus, 2D EM inversion is strongly recommended for detecting conductive isolated bodies, such as man-made objects, whereas this approach may not always be essential for interpreting the EM field data. |
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