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1. Novel statistical emulator construction for volcanic ash transport model Ash3d with physically motivated measures

   https://doi.org/10.1098/rspa.2020.0161  

   Yang, Qingyuan; Pitman, E. Bruce; Spiller, Elaine; Bursik, Marcus; Bevilacqua, Andrea (October 2020, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   null (Ed.)

   Statistical emulators are a key tool for rapidly producing probabilistic hazard analysis of geophysical processes. Given output data computed for a relatively small number of parameter inputs, an emulator interpolates the data, providing the expected value of the output at untried inputs and an estimate of error at that point. In this work, we propose to fit Gaussian Process emulators to the output from a volcanic ash transport model, Ash3d. Our goal is to predict the simulated volcanic ash thickness from Ash3d at a location of interest using the emulator. Our approach is motivated by two challenges to fitting emulators—characterizing the input wind field and interactions between that wind field and variable grain sizes. We resolve these challenges by using physical knowledge on tephra dispersal. We propose new physically motivated variables as inputs and use normalized output as the response for fitting the emulator. Subsetting based on the initial conditions is also critical in our emulator construction. Simulation studies characterize the accuracy and efficiency of our emulator construction and also reveal its current limitations. Our work represents the first emulator construction for volcanic ash transport models with considerations of the simulated physical process. 

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2. Reliable emulation of complex functionals by active learning with error control

   https://doi.org/10.1063/5.0121805  

   Fang, Xinyi; Gu, Mengyang; Wu, Jianzhong (December 2022, The Journal of Chemical Physics)

   A statistical emulator can be used as a surrogate of complex physics-based calculations to drastically reduce the computational cost. Its successful implementation hinges on an accurate representation of the nonlinear response surface with a high-dimensional input space. Conventional “space-filling” designs, including random sampling and Latin hypercube sampling, become inefficient as the dimensionality of the input variables increases, and the predictive accuracy of the emulator can degrade substantially for a test input distant from the training input set. To address this fundamental challenge, we develop a reliable emulator for predicting complex functionals by active learning with error control (ALEC). The algorithm is applicable to infinite-dimensional mapping with high-fidelity predictions and a controlled predictive error. The computational efficiency has been demonstrated by emulating the classical density functional theory (cDFT) calculations, a statistical-mechanical method widely used in modeling the equilibrium properties of complex molecular systems. We show that ALEC is much more accurate than conventional emulators based on the Gaussian processes with “space-filling” designs and alternative active learning methods. In addition, it is computationally more efficient than direct cDFT calculations. ALEC can be a reliable building block for emulating expensive functionals owing to its minimal computational cost, controllable predictive error, and fully automatic features. 

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3. Estimation of the cell membrane permeability for gas transport from surface pH measurements

   https://doi.org/10.1088/1361-6420/aceb86  

   Bocchinfuso, A; Calvetti, D; Somersalo, E (August 2023, Inverse Problems)

   Abstract Bayesian particle filters (PFs) are a viable alternative to sampling methods such as Markov chain Monte Carlo methods to estimate model parameters and related uncertainties when the forward model is a dynamical system, and the data are time series that depend on the state vector. PF techniques are particularly attractive when the dimensionality of the state space is large and the numerical solution of the dynamical system over the time interval corresponding to the data is time consuming. Moreover, information contained in the PF solution can be used to infer on the sensitivity of the unknown parameters to different temporal segments of the data. This, in turn, can guide the design of more efficient and effective data collection procedures. In this article the PF method is applied to the problem of estimating cell membrane permeability to gases from pH measurements on or near the cell membrane. The forward model in this case comprises a spatially distributed system of coupled reaction–diffusion differential equations. The high dimensionality of the state space and the need to account for the micro-environment created by the pH electrode measurement device are additional challenges that are addressed by the solution method. 

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4. Regularization and tempering for a moment‐matching localized particle filter

   https://doi.org/10.1002/qj.4328  

   Poterjoy, Jonathan (June 2022, Quarterly Journal of the Royal Meteorological Society)

   Abstract Iterative ensemble filters and smoothers are now commonly used for geophysical models. Some of these methods rely on a factorization of the observation likelihood function to sample from a posterior density through a set of “tempered” transitions to ensemble members. For Gaussian‐based data assimilation methods, tangent linear versions of nonlinear operators can be relinearized between iterations, thus leading to a solution that is less biased than a single‐step approach. This study adopts similar iterative strategies for a localized particle filter (PF) that relies on the estimation of moments to adjust unobserved variables based on importance weights. This approach builds off a “regularization” of the local PF, which forces weights to be more uniform through heuristic means. The regularization then leads to an adaptive tempering, which can also be combined with filter updates from parametric methods, such as ensemble Kalman filters. The role of iterations is analyzed by deriving the localized posterior probability density assumed by current local PF formulations and then examining how single‐step and tempered PFs sample from this density. From experiments performed with a low‐dimensional nonlinear system, the iterative and hybrid strategies show the largest benefits in observation‐sparse regimes, where only a few particles contain high likelihoods and prior errors are non‐Gaussian. This regime mimics specific applications in numerical weather prediction, where small ensemble sizes, unresolved model error, and highly nonlinear dynamics lead to prior uncertainty that is larger than measurement uncertainty. 

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5. Graph Neural Network based elastic deformation emulators for magmatic reservoirs of complex geometries

   https://doi.org/10.30909/vol.08.01.95109  

   Wang, Taiyi A; McBrearty, Ian; Segall, Paul (January 2025, Volcanica)

   Measurements of volcano deformation are increasingly routine, but constraining complex magma reservoir geometries via inversions of surface deformation measurements remains challenging. This is partly due to deformation modeling being limited to one of two approaches: computationally efficient semi-analytical elastic solutions for simple magma reservoir geometries (point sources, spheroids, and cracks) and computationally expensive numerical solutions for complex 3D geometries. Here, we introduce a pair of Graph Neural Network (GNN) based elasto-static emulators capable of making fast and reasonably accurate predictions (error upper bound: 15 %) of surface deformation associated with 3D reservoir geometries: a spheroid emulator and a general shape emulator, the latter parameterized with spherical harmonics. The emulators are trained on, and benchmarked against, boundary element (BEM) simulations, providing up to three orders of magnitude speed up compared to BEM methods. Once trained, the emulators can generalize to new reservoir geometries statistically similar to those in the training data set, thus avoiding the need for re-training, a common limitation for existing neural network emulators. We demonstrate the utility of the emulators via Bayesian Markov Chain Monte Carlo inversions of synthetic surface deformation data, showcasing scenarios in which the emulators can, and can not, resolve complex magma reservoir geometries from surface deformation. Our work demonstrates that GNN based emulators have the potential to significantly reduce the computational cost of inverse analyses related to volcano deformation, thereby bringing new insights into the complex geometries of magmatic systems. 

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