TY - JOUR A1 - Ley-Cooper, Alan Yusen A1 - Viezzoli, Andrea A1 - Guillemoteau, Julien A1 - Vignoli, Giulio A1 - Macnae, James A1 - Cox, Leif A1 - Munday, Tim T1 - Airborne electromagnetic modelling options and their consequences in target definition JF - Exploration geophysics : the bulletin of the Australian Society of Exploration Geophysicists N2 - Given the range of geological conditions under which airborne EM surveys are conducted, there is an expectation that the 2D and 3D methods used to extract models that are geologically meaningful would be favoured over ID inversion and transforms. We do after all deal with an Earth that constantly undergoes, faulting, intrusions, and erosive processes that yield a subsurface morphology, which is, for most parts, dissimilar to a horizontal layered earth. We analyse data from a survey collected in the Musgrave province, South Australia. It is of particular interest since it has been used for mineral prospecting and for a regional hydro-geological assessment. The survey comprises abrupt lateral variations, more-subtle lateral continuous sedimentary sequences and filled palaeovalleys. As consequence, we deal with several geophysical targets of contrasting conductivities, varying geometries and at different depths. We invert the observations by using several algorithms characterised by the different dimensionality of the forward operator. Inversion of airborne EM data is known to be an ill-posed problem. We can generate a variety of models that numerically adequately fit the measured data, which makes the solution non-unique. The application of different deterministic inversion codes or transforms to the same dataset can give dissimilar results, as shown in this paper. This ambiguity suggests the choice of processes and algorithms used to interpret AEM data cannot be resolved as a matter of personal choice and preference. The degree to which models generated by a ID algorithm replicate/or not measured data, can be an indicator of the data's dimensionality, which perse does not imply that data that can be fitted with a 1D model cannot be multidimensional. On the other hand, it is crucial that codes that can generate 2D and 3D models do reproduce the measured data in order for them to be considered as a plausible solution. In the absence of ancillary information, it could be argued that the simplest model with the simplest physics might be preferred. KW - airborne KW - electromagnetics KW - exploration KW - inversion KW - target Y1 - 2015 U6 - https://doi.org/10.1071/EG14045 SN - 0812-3985 SN - 1834-7533 VL - 46 IS - 1 SP - 74 EP - 84 PB - CSIRO CY - Clayton ER - TY - JOUR A1 - Klose, Tim A1 - Guillemoteau, Julien A1 - Vignoli, Giulio A1 - Tronicke, Jens T1 - Laterally constrained inversion (LCI) of multi-configuration EMI data with tunable sharpness JF - Journal of applied geophysics N2 - Frequency-domain electromagnetic (FDEM) data are commonly inverted to characterize subsurface geoelectrical properties using smoothness constraints in 1D inversion schemes assuming a layered medium. Smoothness constraints are suitable for imaging gradual transitions of subsurface geoelectrical properties caused, for example, by varying sand, clay, or fluid content. However, such inversion approaches are limited in characterizing sharp interfaces. Alternative regularizations based on the minimum gradient support (MGS) stabilizers can, instead, be used to promote results with different levels of smoothness/sharpness selected by simply acting on the so-called focusing parameter. The MGS regularization has been implemented for different kinds of geophysical data inversion strategies. However, concerning FDEM data, the MGS regularization has only been implemented for vertically constrained inversion (VCI) approaches but not for laterally constrained inversion (LCI) approaches. We present a novel LCI approach for FDEM data using the MGS regularization for the vertical and lateral direction. Using synthetic and field data examples, we demonstrate that our approach can efficiently and automatically provide a set of model solutions characterized by different levels of sharpness and variable lateral consistencies. In terms of data misfit, the obtained set of solutions contains equivalent models allowing us also to investigate the non-uniqueness of FDEM data inversion. KW - frequency-domain electromagnetics KW - laterally constrained inversion KW - minimum gradient support regularization KW - peat characterization Y1 - 2022 U6 - https://doi.org/10.1016/j.jappgeo.2021.104519 SN - 0926-9851 VL - 196 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Guillemoteau, Julien A1 - Vignoli, Giulio A1 - Barreto, Jeniffer A1 - Sauvin, Guillaume T1 - Sparse laterally constrained inversion of surface-wave dispersion curves via minimum gradient support regularization JF - Geophysics N2 - We have developed a 1D laterally constrained inversion of surface-wave dispersion curves based on the minimum gradient support regularization, which allows solutions with tunable sharpness in the vertical and horizontal directions. The forward modeling consists of a finite-elements approach incorporated in a flexible nonparametric gradient-based inversion scheme, which has already demonstrated good stability and convergence capabilities when tested on other kinds of data. Our deterministic inversion procedure is performed in the shear-wave velocity log space as we noticed that the associated Jacobian indicates a reduced model dependency, and this, in turn, decreases the risks of local nonconvexity. We show several synthetics and one field example to demonstrate the effectiveness and the applicability of the proposed approach. KW - surface wave, inversion, near surface Y1 - 2022 U6 - https://doi.org/10.1190/GEO2021-0247.1 SN - 0016-8033 SN - 1942-2156 VL - 87 IS - 3 SP - R281 EP - R289 PB - Society of Exploration Geophysicists CY - Tulsa, Okla. ER - TY - JOUR A1 - Klose, Tim A1 - Guillemoteau, Julien A1 - Vignoli, Giulio A1 - Walter, Judith A1 - Herrmann, Andreas A1 - Tronicke, Jens T1 - Structurally constrained inversion by means of a Minimum Gradient Support regularizer: examples of FD-EMI data inversion constrained by GPR reflection data JF - Geophysical journal international N2 - Many geophysical inverse problems are known to be ill-posed and, thus, requiring some kind of regularization in order to provide a unique and stable solution. A possible approach to overcome the inversion ill-posedness consists in constraining the position of the model interfaces. For a grid-based parameterization, such a structurally constrained inversion can be implemented by adopting the usual smooth regularization scheme in which the local weight of the regularization is reduced where an interface is expected. By doing so, sharp contrasts are promoted at interface locations while standard smoothness constraints keep affecting the other regions of the model. In this work, we present a structurally constrained approach and test it on the inversion of frequency-domain electromagnetic induction (FD-EMI) data using a regularization approach based on the Minimum Gradient Support stabilizer, which is capable to promote sharp transitions everywhere in the model, i.e., also in areas where no structural a prioriinformation is available. Using 1D and 2D synthetic data examples, we compare the proposed approach to a structurally constrained smooth inversion as well as to more standard (i.e., not structurally constrained) smooth and sharp inversions. Our results demonstrate that the proposed approach helps in finding a better and more reliable reconstruction of the subsurface electrical conductivity distribution, including its structural characteristics. Furthermore, we demonstrate that it allows to promote sharp parameter variations in areas where no structural information are available. Lastly, we apply our structurally constrained scheme to FD-EMI field data collected at a field site in Eastern Germany to image the thickness of peat deposits along two selected profiles. In this field example, we use collocated constant offset ground-penetrating radar (GPR) data to derive structural a priori information to constrain the inversion of the FD-EMI data. The results of this case study demonstrate the effectiveness and flexibility of the proposed approach. KW - Controlled source electromagnetics (CSEM) KW - Inverse theory KW - Electrical properties KW - Ground penetrating radar KW - Frequency Domain Electromagnetics KW - Inversion Y1 - 2023 U6 - https://doi.org/10.1093/gji/ggad041 SN - 0956-540X SN - 1365-246X VL - 233 IS - 3 SP - 1938 EP - 1949 PB - Oxford Univ. Press CY - Oxford ER -