Guidance for better mapping the horizontal variability of the subsoil so that interpolations are useful for geological schematization.
Motivation and practical challenge
Geophysical methods are increasingly popular for engineering applications such as dikes and roads because they map the subsoil in a horizontally continuous manner. However, the operational effort required by many geophysical methods does not always pay off. Expectations are, in some cases, beyond the physical limits of the methods. Moreover, the scale of the subsoil variability is often shorter than the distance between the point data that is measured with traditional methods. The practical challenge is, therefore, to find the scale of heterogeneity that geophysical methods can resolve which at the same time gives valuable information for geotechnical calculations.
I formulate methods to answer the question: how to use geophysical data for better mapping the subsoil variability? Specifically, I work on uncertainties related to geological schematization, such as geometrical variability.
Combination of geophysical data and point data measurements to improve the schematization of the subsoil below the dikes (source: left-figure adapted from van Beek (2015), right-figure based on scheme prepared by Richard Marijnissen and bottom figure provided by Juan Chavez Olalla).
I look at the type of subsoil variability that plays a role in failure mechanisms of dikes (top-left figure). For example, in clay-over-sand dikes, the thickness of the clay layer in the landward side provides resistance against piping (top-right figure). Dikes are longitudinal structures, so it is challenging to map variability with point data, such as cone penetration tests (CPTs). One component of my research is to study the geometrical variability of layers with geophysical methods (bottom figure). I pay special attention to electromagnetic methods that can cover, quickly, large distances. The innovative aspect is the statistical combination of geophysical data and point data. Previous approaches use only point data, so they do not consider explicitly the geometrical variability between data points. Another component of this research is related to more elaborate geophysical methods, such as seismic exploration, which require large operational efforts. I study the cases where these methods could bring useful information for geological schematization.
Relevant for whom and where?
The output of this research is relevant for advisors who assess dike safety where horizontal variability of geological layers is uncertain.
A test site in Montfoort is studied in collaboration with the group of Physical geography from Utrecht University.
Progress and practical application
Preliminary surveys in test sites show that geological architecture is captured in geophysical data. The level of detail with which it is captured, is smaller than that of cone penetration tests. However, the horizontal coverage is larger. By studying the patterns in geophysical data, it is possible to describe more extensively geological architecture. An approach to combine geological knowledge, point data, and geophysics is formulated in this research. Part of this approach is aimed at retrieving geometrical variability from tomograms. Another part of this approach is aimed at using physical properties derived from geophysics to correlate geotechnical properties. For example, electrical resistivity is highly correlated to the cone resistance of a cone penetration test.
Status for day-to-day practice
Uncertainties in geological schematization are taken into account in calculations via safety factors. The goal of this project is to propose a method to use geophysics so that these uncertainties are reduced.
The next step of this research is to use estimates of physical properties as proxy for average composition of layers. A statistical framework to estimate layer composition from geophysical data is being developed.
Last modified: 21/06/2020
Juan Chavez Olalla
Delft University of Technology
Main research outputs are under preparation, currently the following abstract is available:
- Chavez-Olalla, J. (2020) Layer interpolation with tomographic aid. Athens, Third European Regional Conference of IAEG, 20-24 September 2020.