Universität zu Köln

Cai, Ji (2020). Development of 1D and 2D Joint Inversion Algorithms for Semi-Airborne and LOTEM Data: A Data Application from Eastern Thuringia, Germany. PhD thesis, Universität zu Köln.

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Abstract

By combining the resolution advantages of different electromagnetic (EM) methods, joint inversion can result in a better resolution of subsurface structures than the individual inversions of each single method. To reduce the ambiguities and parameter uncertainty in the results, joint inversion algorithms are developed to couple spatially dense sampled data from semi-airborne frequency-domain electromagnetic measurements and horizontal electric fields measured using the long-offset transient electromagnetic (LOTEM) method. The novel semi-airborne frequency-domain electromagnetic system was developed and tested successfully within the DESMEX project funded by the BMBF (German Ministry for Science and Education). The method takes advantages of both ground and airborne techniques by combining ground-based high power sources with large scale and spatially dense covered data. However, the method usually has a reduced signal-to-noise ratio compared to the ground-based method. For example, compared to LOTEM, the semi-airborne technique has a smaller depth of investigation (DOI) due to the reduced data quality and offset limitations. On the other hand, compared to helicopter-borne EM (HEM), semi-airborne indicates reduced resolution for shallow subsurface structures. Aiming at combining and validating the advantages of all these methods using joint inversion, resolution studies are performed for the HEM, LOTEM and semi-airborne data. Based on the insights of their different resolution properties, the joint inversion algorithm was developed to combine the advantages of each method effectively. For selected field data cases, the 1D joint inversion could improve the inversion results significantly. However, due to 2D effects in the field data, the 1D joint inversion faces convergence problems. Additional synthetic modeling studies are conducted to investigate and effects of 2D structures on the 1D interpretation using joint inversion. Because of multidimensional effects in the field data, a 2D joint inversion algorithm was further developed for the frequency-domain semi-airborne EM data and the time-domain LOTEM electric field data. 2D synthetic modeling studies were performed to gain insights regarding the resolution differences between the two data-sets. The 2D synthetic modeling studies show that the different field components observed in each method are the key factor that leads to different resolution properties. However, the influence of the different measurement configurations is also significant. For the inversion of the field data, characteristic structures seen in both individual inversion results can also be found in the 2D joint inversion result. Due to possible 3D, anisotropic, or even induced polarization (IP) effects in data, the applied 2D joint inversion cannot fully explain all the discrepancies observed between the single method inversions. The influence of 3D structures on 2D inversions of semi-airborne data is investigated.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Cai, Ji 03/09/20 jcai6@uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-121028
Date:	3 September 2020
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology
Subjects:	Natural sciences and mathematics Earth sciences Technology (Applied sciences)
Uncontrolled Keywords:	Keywords Language electromagnetic methods UNSPECIFIED geophysical exploration UNSPECIFIED joint inversion UNSPECIFIED
Date of oral exam:	29 May 2020
Referee:	Name Academic Title Tezkan, Bülent Prof. Dr. Becken, Michael Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/12102