Geometrical controls on spectral induced polarization in time and frequency domain

Författare

Sara Johansson

Summary, in English

What can we learn by transmitting electrical currents through electrodes in the ground? Geophysical methods can help us to locate the bedrock, valuable mineral ores, remains of ancient buildings and many other secrets hiding below the surface. However, it is not only the universal interest for the unknown that drives the geophysical research forward. Knowledge of, for example, fractured bedrock, contaminated groundwater or unstable ground can prepare us economically during urban expansion, not to mention to predict and prevent catastrophes such as landslides, sinkholes or contaminated drinking water. With the results of this thesis, we are moving one step closer to being able to delineate ground contaminated by the cancerogenic oil-like chemicals that were formerly used in dry cleaners all over Sweden. Furthermore, the results show that we can image the well-known geological boundary that marks the extinction of dinosaurs during the end of the Cretaceous period, as well as how ancient seabeds and their habitats looked like millions of years ago.
What lies below the ground surface is invisible to us, but via investigations of the electrical properties in different materials, we can understand the signals that are returned from the ground when current pulses have been transmitted. The focus of this thesis is to obtain a better knowledge of how geological materials obtain electrical polarization and how the microscale material geometry influence these effects. The induced polarization effect is analogous to a capacitor in an electric circuit and arise by displacement and blockage of groundwater ions in the material pore space. The phenomenon is called Spectral Induced Polarization (SIP) and the measurements result in a spectrum showing how the induced polarization effect varies with different alternating current frequencies. The origin of the spectral dispersion is that different time scales are required for the charge-up and decay processes in different parts of the material. The charge-up time is, for instance, dependent on size distribution and shapes of the grains that builds up the geological material.
Field scale measurements of subsurface conductivity is already a well established geophysical method, which gives two-dimensional images of how the electrical current conduction varies. The images can represent areas of several hundreds of meters length and tens to hundreds of meters depth below the ground surface. Up to three additional images can be produced by simultaneous measurements of SIP-effects, where each image corresponds to different aspects of the induced polarization effects. In this way, there is a great potential for acquiring more interpretable information about the investigated area. However, more research is needed to translate the electrical information to geological or geotechnical information.
Since SIP-effects arise at a microscopic level in geological materials, the electrical measurements have been combined with different microscopic methods in this thesis. Three-dimensional x-ray tomography was used to visualize how oil-like chemicals distribute around sand grains. This geometrical information was analyzed together with laboratory measurements of SIP-effects and showed that different patterns in the distribution of the chemicals also leads to different patterns in the SIP-signals. This knowledge can be used to understand data from large-scale field surveys, which was also done as a part of the thesis work. During field-scale investigations of limestone bedrock, a core was drilled and the fossil composition in the limestone was studied in thin sections. In this way, SIP-effects in a laboratory setting could be related to, for instance, fossil composition at different bedrock levels. An overall conclusion in this thesis is that methods spanning over different length scales, from microscale via laboratory investigations to field scale investigations, are necessary to increase the understanding of how SIPeffects vary in different materials. There is currently a gap in the knowledge of how these different scales relate to each other with respect to SIP-effects. The results in this thesis is a step on the way towards bridging this information gap and improve interpretation of field scale SIP-effects.

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