@article{GoeritzBergerGegeetal.2018, author = {G{\"o}ritz, Anna and Berger, Stella A. and Gege, Peter and Grossart, Hans-Peter and Nejstgaard, Jens C. and Riedel, Sebastian and R{\"o}ttgers, R{\"u}diger and Utschig, Christian}, title = {Retrieval of water constituents from hyperspectral in-situ measurements under variable cloud cover}, series = {Remote sensing / Molecular Diversity Preservation International (MDPI)}, volume = {10}, journal = {Remote sensing / Molecular Diversity Preservation International (MDPI)}, number = {2}, publisher = {MDPI}, address = {Basel}, issn = {2072-4292}, doi = {10.3390/rs10020181}, pages = {19}, year = {2018}, abstract = {Remote sensing and field spectroscopy of natural waters is typically performed under clear skies, low wind speeds and low solar zenith angles. Such measurements can also be made, in principle, under clouds and mixed skies using airborne or in-situ measurements; however, variable illumination conditions pose a challenge to data analysis. In the present case study, we evaluated the inversion of hyperspectral in-situ measurements for water constituent retrieval acquired under variable cloud cover. First, we studied the retrieval of Chlorophyll-a (Chl-a) concentration and colored dissolved organic matter (CDOM) absorption from in-water irradiance measurements. Then, we evaluated the errors in the retrievals of the concentration of total suspended matter (TSM), Chl-a and the absorption coefficient of CDOM from above-water reflectance measurements due to highly variable reflections at the water surface. In order to approximate cloud reflections, we extended a recent three-component surface reflectance model for cloudless atmospheres by a constant offset and compared different surface reflectance correction procedures. Our findings suggest that in-water irradiance measurements may be used for the analysis of absorbing compounds even under highly variable weather conditions. The extended surface reflectance model proved to contribute to the analysis of above-water reflectance measurements with respect to Chl-a and TSM. Results indicate the potential of this approach for all-weather monitoring.}, language = {en} } @article{MarschallSkorovZakharovetal.2020, author = {Marschall, Raphael and Skorov, Yuri and Zakharov, Vladimir and Rezac, Ladislav and Gerig, Selina-Barbara and Christou, Chariton and Dadzie, S. Kokou and Migliorini, Alessandra and Rinaldi, Giovanna and Agarwal, Jessica and Vincent, Jean-Baptiste and Kappel, David}, title = {Cometary comae-surface links the physics of gas and dust from the surface to a spacecraft}, series = {Space science reviews}, volume = {216}, journal = {Space science reviews}, number = {8}, publisher = {Springer}, address = {Dordrecht}, issn = {0038-6308}, doi = {10.1007/s11214-020-00744-0}, pages = {53}, year = {2020}, abstract = {A comet is a highly dynamic object, undergoing a permanent state of change. These changes have to be carefully classified and considered according to their intrinsic temporal and spatial scales. The Rosetta mission has, through its contiguous in-situ and remote sensing coverage of comet 67P/Churyumov-Gerasimenko (hereafter 67P) over the time span of August 2014 to September 2016, monitored the emergence, culmination, and winding down of the gas and dust comae. This provided an unprecedented data set and has spurred a large effort to connect in-situ and remote sensing measurements to the surface. In this review, we address our current understanding of cometary activity and the challenges involved when linking comae data to the surface. We give the current state of research by describing what we know about the physical processes involved from the surface to a few tens of kilometres above it with respect to the gas and dust emission from cometary nuclei. Further, we describe how complex multidimensional cometary gas and dust models have developed from the Halley encounter of 1986 to today. This includes the study of inhomogeneous outgassing and determination of the gas and dust production rates. Additionally, the different approaches used and results obtained to link coma data to the surface will be discussed. We discuss forward and inversion models and we describe the limitations of the respective approaches. The current literature suggests that there does not seem to be a single uniform process behind cometary activity. Rather, activity seems to be the consequence of a variety of erosion processes, including the sublimation of both water ice and more volatile material, but possibly also more exotic processes such as fracture and cliff erosion under thermal and mechanical stress, sub-surface heat storage, and a complex interplay of these processes. Seasons and the nucleus shape are key factors for the distribution and temporal evolution of activity and imply that the heliocentric evolution of activity can be highly individual for every comet, and generalisations can be misleading.}, language = {en} } @article{EsfahaniGholamiOhrnberger2020, author = {Esfahani, Reza Dokht Dolatabadi and Gholami, Ali and Ohrnberger, Matthias}, title = {An inexact augmented Lagrangian method for nonlinear dispersion-curve inversion using Dix-type global linear approximation}, series = {Geophysics : a journal of general and applied geophysics}, volume = {85}, journal = {Geophysics : a journal of general and applied geophysics}, number = {3}, publisher = {GeoScienceWorld}, address = {Tulsa, Okla.}, issn = {0016-8033}, doi = {10.1190/geo2019-0717.1}, pages = {EN77 -- EN85}, year = {2020}, abstract = {Dispersion-curve inversion of Rayleigh waves to infer subsurface shear-wave velocity is a long-standing problem in seismology. Due to nonlinearity and ill-posedness, sophisticated regularization techniques are required to solve the problem for a stable velocity model. We have formulated the problem as a minimization problem with nonlinear operator constraint and then solve it by using an inexact augmented Lagrangian method, taking advantage of the Haney-Tsai Dix-type relation (a global linear approximation of the nonlinear forward operator). This replaces the original regularized nonlinear problem with iterative minimization of a more tractable regularized linear problem followed by a nonlinear update of the phase velocity (data) in which the update can be performed accurately with any forward modeling engine, for example, the finite-element method. The algorithm allows discretizing the medium with thin layers (for the finite-element method) and thus omitting the layer thicknesses from the unknowns and also allows incorporating arbitrary regularizations to shape the desired velocity model. In this research, we use total variation regularization to retrieve the shear-wave velocity model. We use two synthetic and two real data examples to illustrate the performance of the inversion algorithm with total variation regularization. We find that the method is fast and stable, and it converges to the solution of the original nonlinear problem.}, language = {en} } @article{LeyCooperViezzoliGuillemoteauetal.2015, author = {Ley-Cooper, Alan Yusen and Viezzoli, Andrea and Guillemoteau, Julien and Vignoli, Giulio and Macnae, James and Cox, Leif and Munday, Tim}, title = {Airborne electromagnetic modelling options and their consequences in target definition}, series = {Exploration geophysics : the bulletin of the Australian Society of Exploration Geophysicists}, volume = {46}, journal = {Exploration geophysics : the bulletin of the Australian Society of Exploration Geophysicists}, number = {1}, publisher = {CSIRO}, address = {Clayton}, issn = {0812-3985}, doi = {10.1071/EG14045}, pages = {74 -- 84}, year = {2015}, abstract = {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.}, language = {en} }