Development of a 2D adaptive finite element inversion algorithm for the marine Differential Electrical Dipole method
Final Report Abstract
In the framework of this project, an adaptive 2D finite-element inversion package was developed to interpret time-domain controlled source electromagnetic data. We implemented the most commonly applied electromagnetic source fields that were previously interpreted using simplified 1D resistivity models into a 2.5 finite element framework. The newly developed code allows for medium to large scale data sets to run on high-performance computing clusters in reasonable time fames (several days for the inversion to converge). The resulting resistivity models have substantially improved timedomain CSEM interpretations as they allow for lateral resistivity variations, which was previously not possible using a 1D inversion approach. Applications to measured marine data from New Zealand and Malta showed that this 2D inversion approach drastically improves interpretation capabilities, as data fits are improved and a higher coherency is achievable with concurrent geophysical/geological data. Using the 2D inversion approach along measured transects, a quantitative hydrogeological model is derivable from resistivity models, provided robust information about the seafloor porosity is available. Of course, this applies only when bathymetry is not highly 3D and obscures measured CSEM data. The developed code was applied to three marine data sets from the Helmholtz Centre for Ocean Research GEOMAR Kiel and the Institute of Geophysics and Meteorology, University of Cologne and one land-based LOTEM data set from the University of Cologne. In all cases, the code ran successfully providing a comprehensive sub-surface resistivity model that allowed for robust geological interpretation. Through various collaborations with other research projects, we managed to establish time-domain CSEM as a key technology for mapping offshore freshened groundwater, which has led to three further proposals – two of which have been granted and one ship time proposal that is still pending. We plan to make this software package freely accessible to the research community. Further CSEM research conducted in the framework of this project revealed that resolution differences exist between step-on and step-off current excitations. These novel findings apply mainly in the marine domain where conductive seawater surrounds the submerged transmitter/receiver system. As a result, time-domain CSEM systems may require systematic developments to optimize its sensitivity towards detecting freshened groundwater occupying the pore space of the present-day seafloor. Here, further research is required to investigate how CSEM measurement systems can be applied efficiently and increase their overall flexibility in the future.
Publications
- (2018). Calculating time-domain controlled source electromagnetic signals with MARE2DEM, Conference Proceedings: 3rd Applied Shallow Marine Geophysics Conference, September 2018, Porto, Portugal
Haroon, A., Hölz, S., Weymer, B. A., Tezkan, B. and Jegen, M.
(See online at https://doi.org/10.3997/2214-4609.201802663) - (2018). First application of the marine differential electric dipole for groundwater investigations: A case study from Bat Yam, Israel, Geophysics 83(2), pg. B59–B76
Haroon A., Lippert, K., Mogilatov, V., and Tezkan., B.
(See online at https://doi.org/10.1190/geo2017-0162.1) - (2020). 3D characterisation and quantification of an offshore freshened groundwater system in the Canterbury Bight, Nature Communications, 11, 1372 (2020)
Micallef, A., Person, M., Haroon, A., Weymer, B. A., Schwalenberg, K., Faghih, Z., Duan, S., Cohen, D., Mountjoy, J. J., Woelz, S., Gable, C. W., Averes, T., and Tiwari, A. K.
(See online at https://doi.org/10.1038/s41467-020-14770-7) - (2020). Seafloor massive sulphide exploration using deep-towed controlled source electromagnetics: Navigational uncertainties, Geophysical Journal International, 220(2), pg. 1215-1227
Gehrmann,R.A.S,. Haroon, A., Morton, M., Djanni, A.T., & Minshull, T.
(See online at https://doi.org/10.1093/gji/ggz513) - (2020). Step-on versus step-off signals in time-domain controlled source electromagnetic methods using a grounded electric dipole, Geophysical Prospecting, 68, 9, pg. 2825-2844
Haroon, A., Swidinsky, A., Hölz, S., Jegen, M., and Tezkan, B.
(See online at https://doi.org/10.1111/1365-2478.13016)
