Stochastic Seismic Waveform Inversion Using Generative Adversarial Networks as a Geological Prior

Mathematical Geosciences - We present an application of deep generative models in the context of partial differential equation constrained inverse problems. We combine a generative adversarial...     

Cluster synchronisation: A mechanism for plankton patchiness and a simulation pitfall

We present a study on the emergence of spatial variability, or patchiness, in biophysical simulations of plankton ecosystems. Using a standard approach to modelling such ecosystems, we represent a distribution of plankton as a lattice of non-identical interacting oscillatory populations. Spatial variation is imposed in population parameters, such as maximum growth rate, leading to a spread in the natural (uncoupled) population properties. Using the methods of synchronisation theory, the emergent spatial structure of the coupled system is investigated as a function of the strength of interaction between populations. Surprisingly, a range of coupling strength is found to induce a tenfold increase in the spread in frequency of oscillation of populations in comparison with the uncoupled level of spatial variation. This apparent desynchronisation corresponds to the formation of temporally evolving clusters of local synchronisation: the interplay of grid-cell scale variability and dispersal between populations leads to patchiness at larger scales. However, the occurrence and length-scale of this patchiness is found to be sensitive to typical simulation parameters such as spatial resolution and strength of dispersal, with emergent spatial structure altering abruptly from patchy to homogeneous as these parameters are varied. These results indicate that whilst cluster synchronisation may be a genuine mechanism for the formation of spatial structure in plankton distributions, biophysical modellers should be aware of the possibility of artificial patchiness arising from the basic physical structure of their model.     

Analysis of trophic networks and carbon flows in south-eastern Baltic coastal ecosystems

Carbon flows in five south-eastern Baltic coastal ecosystems (Puck Bay, Curonian Lagoon, Lithuanian coast, Gulf of Riga coast and Pärnu Bay) were compared on the basis of ECOPATH models using 12 common functional groups. The studied systems ranged from the hypertrophic Curonian Lagoon to the mesotrophic Gulf of Riga coast. Interestingly, we found that macrophytes were not consumed by grazers, but rather channelled into the detritus food chain. In all ecosystems fisheries had far reaching impacts on their target species and on the food-web in general. In particular, benthic food-webs were partly affected by indirect fisheries effects. For example, fisheries tend to change the biomass of piscivorous fish, causing a cascading effect on benthivorous fish and macrozoobenthos. These cascades are ecosystem specific and need to be considered when using benthic invertebrates as productivity and eutrophication indicators. Odum’s maturity attributes allowed a ranking of costal ecosystems according to their maturity. Namely, the community development decreased in the following order: Pärnu Bay > Gulf of Riga coast > Lithuanian coast > Puck Bay > Curonian Lagoon.     

Improving sub-salt imaging using 3D basin model derived velocities

The quality of depth imaging is directly related to the accuracy of the underlying velocity model. In most sub-salt settings, lack of angular illumination severely degrades the resolution and accuracy of velocity information derived from the seismic data itself. A standard approach for building a starting velocity model uses more reliable velocity information outboard of salt which is subsequently extrapolated to populate the sub-salt regions. The shortcoming of this method lies in the assumption that the effective stress observed outboard of salt can be extrapolated beneath salt solely as a function of depth below mudline.     

Spatial transferability of the physically based model TRIGRS using parameter ensembles

The development of better, more reliable and more efficient susceptibility assessments for shallow landslides is becoming increasingly important. Physically based models are well-suited for this, due to their high predictive capability. However, their demands for large, high-resolution and detailed input datasets make them very time-consuming and costly methods. This study investigates if a spatially transferable model calibration can be created with the use of parameter ensembles and with this alleviate the time-consuming calibration process of these methods. To investigate this, the study compares the calibration of the model TRIGRS in two different study areas. The first study area was taken from a previous study where the dynamic physically based model TRIGRS was calibrated for the Laternser valley in Vorarlberg, Austria. The calibrated parameter ensemble and its performance from this previous study are compared with a calibrated parameter ensemble of the model TRIGRS for the Passeier valley in South Tyrol, Italy. The comparison showed very similar model performance and large similarities in the calibrated geotechnical parameter values of the best model runs in both study areas. There is a subset of calibrated geotechnical parameter values that can be used successfully in both study areas and potentially other study areas with similar lithological characteristics. For the hydraulic parameters, the study did not find a transferable parameter subset. These parameters seem to be more sensitive to different soil types. Additionally, the results of the study also showed the importance of the inclusion of detailed information on the timing of landslide initiation in the calibration of the model.     

Neogene kinematics of the Giudicarie fault (Central-Eastern Alps, Italy): new apatite fission-track data

The Neogene kinematics of the Giudicarie fault (part of Periadriatic lineament, NE Italy) have been re-examined using apatite fission-track analysis. Twenty samples were collected along two geological sections; the first one crossing the Tertiary Corno Alto pluton (Adamello batholith) and the Variscan basement (Southalpine domain) adjacent to the South Giudicarie fault, the second one close to the North Giudicarie fault, in the Variscan basement of the Tonale nappe (Austroalpine system). Samples from the southern section show short tracks and ages between 14.7±1.2 Myr and 22.5±2.2 Myr along 1570 m of the profile; samples from the northern profile present long tracks and ages between 11.3±1.3 Myr and 14.7±3.4 Myr along 1225 m of the vertical profile. In the former, the presence of short tracks might indicate either a long permanence of the rocks in the apatite partial annealing zone, or a more complex thermal history; in the latter case we are dealing with rocks which experienced more rapid cooling.
  The two differing segments of the Giudicarie fault can be explained either as two completely independent tectonic features or, more likely, by hypothesizing a single fault active in its southern and northern parts at different times. Fission track data support a first exhumation of this single fault c. 15 Ma along the North Giudicarie, with a final exhumation towards the south, in the Adamello area, at c. 8–10 Ma (Mid Tortonian). This age fits with the so-called 'Giudicarie' phase, during which σ₁ in the stress field was orientated N280–290°.     

Symplectic integrators for long-term integrations in celestial mechanics

We discuss the use of symplectic integration algorithms in long-term integrations in the field of celestial mechanics. The methods' advantages and disadvantages (with respect to more common integration methods) are discussed. The numerical performance of the algorithms is evaluated using the 2-body and circular restricted 3-body problems. Symplectic integration methods have the advantages of linear phase error growth in the 2-body problem (unlike most other methods), good conservation of the integrals of the motion, good performance for moderately eccentric orbits, and ease of use. Its disadvantages include a relatively large number of force evaluations and an inability to continuously vary the step size.     

A consistent formulation of a dilatant interface element

The paper considers a plane joint or interface element suitable for implementation into a standard non-linear finite element code. The element is intended to model discontinuities with rough contact surfaces, such as rock joints, where dilatant behaviour is present. Of particular concern is the formulation of a constitutive model which fully caters for all possible histories of opening, closing and sliding (accompained by dilation or contraction) in any direction. The non-linear incremental constitutive equations are formulated in a manner appropriate for a back-ward difference discretization in time along the path of loading. The advantage of such an approach is that no essential distinction need be drawn between opening, closing and sliding. Further, a convenient formulation of the constitutive equations is facilitated by representing the different contact conditions in relative displacement space. The state diagram in relative displacement space, however, changes from one time step to the next, and evolution equations for the updating must be formulated. These concepts are illustrated for two rock-joint models: a sawtooth asperity model and a limited dilation model. The models are based on a penalty formulation to enforce the contact constraints, and explicit equations for the tangent stiffness matrix and for the corrector step of the standard Newton–Raphson iterative algorithm are derived. These equations have been implemented as an user element into the finite element code ABAQUS⁷. Three examples are presented to illustrate the predictions of the formulation.