August 2021: Thibaut Astic

Implementing geological rules within geophysical inversion: A PGI perspective

Thibaut Astic



Inferring geologically meaningful information from a geophysical inversion is a challenging task. Moreover, prior knowledge about the petrophysical contrasts or the relationships between various geological units can also prove difficult to translate into quantitative input for the inversion. In previous works, we developed a Petrophysically and Geologically guided Inversion (PGI) framework that enables desired petrophysical characteristics to be reproduced. This information is encoded into the objective function's smallness through a Gaussian Mixture Model (GMM). The resulting discrete geological representation of the subsurface thus fits both the geophysical and petrophysical information. The way we included geological information was limited to favouring the occurrence of chosen rock units in user-defined areas on a cell-by-cell approach. Transferring geological information from one area to another, such as an expected stratigraphy, was not easily done. Moreover, structural information (dip orientation, etc.), which by definition depends on multiple cells at once, was left to the objective function's smoothness, which acts on the physical property models rather than on the geological representation itself. We improve upon the existing PGI framework to make the inversion result geologically realistic by including geological rules as part of the process that builds the geological representation throughout the inversion's iterations. For this purpose, we incorporate image segmentation tools using Markov Random Field (MRF) as part of the PGI framework. The final recovered model fits geophysical and petrophysical information while reproducing geological characteristics, thus providing a more faithful and informed representation of the underground.


Thibaut Astic received a Ph.D. (2020) from the Geophysical Inversion Facility (GIF) at the University of British Columbia (Vancouver, BC, Canada), where he is currently working as a postdoctoral researcher. His research focuses on joint inversion coupled by petrophysical and geological information and the development of open-source tools for the geosciences community, mostly through the Python package SimPEG. Before his Ph.D., Thibaut worked in geological mapping and geophysical data acquisition and processing at the Quebec Department of Natural Resources and various geophysics companies.