Laterally constrained inversion of helicopter-borne frequency-domain electromagnetic data |
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| Authors: | Bernhard Siemon Esben Auken Anders Vest Christiansen |
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| Institution: | 1. Research Group Soil Spatial Inventory Techniques, Department of Soil Management, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium;2. Renard Centre of Marine Geology, Department of Geology and Soil Science, Ghent University, Krijgslaan 281, B-9000 Ghent, Belgium;3. Flanders Heritage Agency, Koning Albert II-laan 19, B-1210 Brussels, Belgium;1. Key Lab of Submarine Geosciences and Prospecting Techniques, MOE and College of Marine Geosciences, Ocean University of China, Qingdao 266100, PR China;2. Evaluation and Detection Technology Laboratory of Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, China;3. Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, PR China;1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. Chinese Academy of Sciences University, 100049 Beijing, China;3. Institute of Earth Science, Chinese Academy of Sciences, Beijing 100029, China;4. School of Computer Science and Telecommunication Engineering, Jiangsu University, Zhenjiang 212013, China |
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| Abstract: | Helicopter-borne frequency-domain electromagnetic (HEM) surveys are used for fast high-resolution, three-dimensional resistivity mapping. Standard interpretation tools are often based on layered earth inversion procedures which, in general, explain the HEM data sufficiently. As a HEM system is moved while measuring, noise on the data is a common problem. Generally, noisy data will be smoothed prior to inversion using appropriate low-pass filters and consequently information may be lost.For the first time the laterally constrained inversion (LCI) technique has been applied to HEM data combined with the automatic generation of dynamic starting models. The latter is important because it takes the penetration depth of the electromagnetic fields, which can heavily vary in survey areas with different geological settings, into account. The LCI technique, which has been applied to diverse airborne and ground geophysical data sets, has proven to be able to improve the HEM inversion results of layered earth structures. Although single-site 1-D inversion is generally faster and — in case of strong lateral resistivity variations — more flexible, LCI produces resistivity — depth sections which are nearly identical to those derived from noise-free data.The LCI results are compared with standard single-site Marquardt–Levenberg inversion procedures on the basis of synthetic data as well as field data. The model chosen for the generation of synthetic data represents a layered earth structure having an inhomogeneous top layer in order to study the influence of shallow resistivity variations on the resolution of deep horizontal conductors in one-dimensional inversion results. The field data example comprises a wide resistivity range in a sedimentary as well as hard-rock environment.If a sufficient resistivity contrast between air and subsurface exists, the LCI technique is also very useful in correcting for incorrect system altitude measurements by using the altitude as a constrained inversion parameter. |
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