Profile extrema for visualizing and quantifying uncertainties on excursion regions. Application to coastal flooding

TL;DR

This paper introduces profile extrema functions for visualizing and quantifying uncertainties in excursion regions of high-dimensional functions, using Gaussian process surrogates, with applications to coastal flooding modeling.

Contribution

It proposes a novel method to visualize high-dimensional excursion sets via profile extrema functions and quantifies uncertainty using Gaussian process models and Borell-TIS bounds.

Findings

Profile extrema functions effectively visualize excursion sets in high dimensions.

Uncertainty quantification is achieved through Gaussian process posterior analysis.

Application to coastal flooding demonstrates practical utility in environmental modeling.

Abstract

We consider the problem of describing excursion sets of a real-valued function $f$, i.e. the set of inputs where $f$ is above a fixed threshold. Such regions are hard to visualize if the input space dimension, $d$, is higher than 2. For a given projection matrix from the input space to a lower dimensional (usually $1,2$) subspace, we introduce profile sup (inf) functions that associate to each point in the projection's image the sup (inf) of the function constrained over the pre-image of this point by the considered projection. Plots of profile extrema functions convey a simple, although intrinsically partial, visualization of the set. We consider expensive to evaluate functions where only a very limited number of evaluations, $n$, is available, e.g. $n<100d$, and we surrogate $f$ with a posterior quantity of a Gaussian process (GP) model. We first compute profile extrema functions for the posterior mean given $n$ evaluations of $f$. We quantify the uncertainty on such estimates by studying the distribution of GP profile extrema with posterior quasi-realizations obtained from an approximating process. We control such approximation with a bound inherited from the Borell-TIS inequality. The technique is applied to analytical functions ($d=2,3$) and to a $5$-dimensional coastal flooding test case for a site located on the Atlantic French coast. Here $f$ is a numerical model returning the area of flooded surface in the coastal region given some offshore conditions. Profile extrema functions allowed us to better understand which offshore conditions impact large flooding events.