We are very pleased to announce Nicolas' PhD defense on March 05, 2020 in ENSG, Amphi G at 2:00 pm
Stochastic seismic interpretation of salt bodies:
detection, sampling and impact on imaging
Abstract
Building a numerical 3D model of the subsurface requires to integrate sparse and ambiguous data. This task is especially difficult in the presence of salt bodies, due to the peculiar salt physical properties. On the one hand, salt behaves as a viscous fluid over geological time scales. As a consequence, salt develops highly convoluted shapes when undergoing deformation. Current geomodeling methods struggle to handle such geometries as they are usually designed for generating minimal surfaces. In addition, the presence of disconnected salt bodies introduces topological singularities that require to be handled separately. On the other hand, salt bodies introduce large velocity contrasts in the subsurface, that require the use of advanced seismic processing methods and the definition of detailed models of the subsurface parameter fields. The inaccuracy of these models induces uncertainties that propagate throughout the seismic imaging workflow and impact seismic interpretation. In this thesis, I am interested in the assessment of structural uncertainties related to the interpretation of ambiguous seismic images of salt tectonics environments.
The main contribution of this thesis is a numerical method for stochastically modeling variable shapes of salt bodies and their connectivity. The modeling is based on an a priori definition of the uncertainties, represented as a buffer zone encompassing the salt boundary. The boundary is defined as the combination of a reference scalar field, computed from the buffer zone, and a spatially correlated random field that is used as a perturbation. This implicit formulation allows for the simulation of both varying salt geometries and topologies while ensuring the validity of the simulated boundaries. When the result of the simulation is a diapir whose bulb is detached from its pedestal, a weld is simulated to connect them. The position of the weld is determined from the scalar field representing the salt boundary, to ensure its consistency with the simulated salt bodies. The method is automatic and proposes to integrate punctual information (e.g., well data or manual seismic picks) and, to some extent, prior geological knowledge.
The second contribution of this thesis is an application of this method to the characterization of structural uncertainties underlying seismic imaging on a 2D synthetic data set. Starting from a rough uncertainty envelope, I simulate a set of possible interpretations of the salt boundaries. I use these interpretations to define a set of equiprobable migration velocity models, that are used in turn to generate as many seismic images. The statistical analysis of this image set, both directly and from derived seismic attributes, permits to highlight the image parts which are most sensitive to migration velocity variations, and provides insights on the nature of the imaged salt bodies.
These contributions open new perspectives for uncertainty quantification in an automatic velocity model updating framework in seismic imaging.