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L.L.W. van Kerkhoven
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Vector and total field data inversions for magnetometer UXO surveys in the North Sea
An alternative to the industry standard
As an alternative to the industry standard processing of total field data, a Basin-hopping inversion can be used. Another alternative is the use of vector field magnetometers, which can be processed with a Gauss-Newton inversion. Both the Basin-hopping and Guass-Newton inversion consist of a two step inversion. First, an inversion to estimate the centre location and magnetic dipole moment of a point source is carried out, the so called point-source inversion. Second, the retrieved magnetic dipole moment can be used to reconstruct the magnetic moment with a prolate spheroid model, which is called the prolate-spheroid inversion.
First, both the TMI and vector data inversion are tested on 3000 noise-free synthetic datasets located in the North Sea where a prolate spheroid with varying shape, orientation and magnetic susceptibility creates an anomaly field. The largest global root mean square (GNRMS) error for the point-source inversion of the Gauss-Newton vector data inversion has a value in the order 10−9 whereas the Basin-hopping inversion for the TMI data returns for only 61% of the cases a GNRMS error in the order 10−7 or lower.
The result of the point-source inversion serves as a plateau for the prolate spheroid inversion; when the point-source inversion does not reach the set level of accuracy, it cannot be determined if the prolate spheroid model describes the true object well. For both the TMI and vector data, the UXO was correctly classified in 72% of the 3000 datasets.
Second, comparing the current industry standard to the Basin-hopping point-source inversion only gave inconclusive results. The estimated centre location of both methods are within a 0.15 m radius but the disagreement in magnetic moment leaves an advice on which technique would work better indecipherable. ...
First, both the TMI and vector data inversion are tested on 3000 noise-free synthetic datasets located in the North Sea where a prolate spheroid with varying shape, orientation and magnetic susceptibility creates an anomaly field. The largest global root mean square (GNRMS) error for the point-source inversion of the Gauss-Newton vector data inversion has a value in the order 10−9 whereas the Basin-hopping inversion for the TMI data returns for only 61% of the cases a GNRMS error in the order 10−7 or lower.
The result of the point-source inversion serves as a plateau for the prolate spheroid inversion; when the point-source inversion does not reach the set level of accuracy, it cannot be determined if the prolate spheroid model describes the true object well. For both the TMI and vector data, the UXO was correctly classified in 72% of the 3000 datasets.
Second, comparing the current industry standard to the Basin-hopping point-source inversion only gave inconclusive results. The estimated centre location of both methods are within a 0.15 m radius but the disagreement in magnetic moment leaves an advice on which technique would work better indecipherable. ...
As an alternative to the industry standard processing of total field data, a Basin-hopping inversion can be used. Another alternative is the use of vector field magnetometers, which can be processed with a Gauss-Newton inversion. Both the Basin-hopping and Guass-Newton inversion consist of a two step inversion. First, an inversion to estimate the centre location and magnetic dipole moment of a point source is carried out, the so called point-source inversion. Second, the retrieved magnetic dipole moment can be used to reconstruct the magnetic moment with a prolate spheroid model, which is called the prolate-spheroid inversion.
First, both the TMI and vector data inversion are tested on 3000 noise-free synthetic datasets located in the North Sea where a prolate spheroid with varying shape, orientation and magnetic susceptibility creates an anomaly field. The largest global root mean square (GNRMS) error for the point-source inversion of the Gauss-Newton vector data inversion has a value in the order 10−9 whereas the Basin-hopping inversion for the TMI data returns for only 61% of the cases a GNRMS error in the order 10−7 or lower.
The result of the point-source inversion serves as a plateau for the prolate spheroid inversion; when the point-source inversion does not reach the set level of accuracy, it cannot be determined if the prolate spheroid model describes the true object well. For both the TMI and vector data, the UXO was correctly classified in 72% of the 3000 datasets.
Second, comparing the current industry standard to the Basin-hopping point-source inversion only gave inconclusive results. The estimated centre location of both methods are within a 0.15 m radius but the disagreement in magnetic moment leaves an advice on which technique would work better indecipherable.
First, both the TMI and vector data inversion are tested on 3000 noise-free synthetic datasets located in the North Sea where a prolate spheroid with varying shape, orientation and magnetic susceptibility creates an anomaly field. The largest global root mean square (GNRMS) error for the point-source inversion of the Gauss-Newton vector data inversion has a value in the order 10−9 whereas the Basin-hopping inversion for the TMI data returns for only 61% of the cases a GNRMS error in the order 10−7 or lower.
The result of the point-source inversion serves as a plateau for the prolate spheroid inversion; when the point-source inversion does not reach the set level of accuracy, it cannot be determined if the prolate spheroid model describes the true object well. For both the TMI and vector data, the UXO was correctly classified in 72% of the 3000 datasets.
Second, comparing the current industry standard to the Basin-hopping point-source inversion only gave inconclusive results. The estimated centre location of both methods are within a 0.15 m radius but the disagreement in magnetic moment leaves an advice on which technique would work better indecipherable.
The aim of this study is to look into the effect of water content in clays on the electrical conductivity as obtained from measurements of an frequency domain electromagnetic induction survey and a laboratory resistivity test. The laboratory test results are compared to theoretical conductivity models. The focus is hereby on determining the relationship between decreasing water content in clay and the electrical conductivity. The research has been performed in collaboration with the land geophysics department at Fugro. I would like to thank Veerle Steenhuisen for giving me the opportunity to carry out this research at Fugro and Serkan Elgun for supervising my thesis on behalf of Fugro. I also want to thank Evert Slob and Dominique Ngan-Tillard for the help on academic level and supervising my thesis.
...
The aim of this study is to look into the effect of water content in clays on the electrical conductivity as obtained from measurements of an frequency domain electromagnetic induction survey and a laboratory resistivity test. The laboratory test results are compared to theoretical conductivity models. The focus is hereby on determining the relationship between decreasing water content in clay and the electrical conductivity. The research has been performed in collaboration with the land geophysics department at Fugro. I would like to thank Veerle Steenhuisen for giving me the opportunity to carry out this research at Fugro and Serkan Elgun for supervising my thesis on behalf of Fugro. I also want to thank Evert Slob and Dominique Ngan-Tillard for the help on academic level and supervising my thesis.