Combining Spectral Induced Polarization and X-ray micro-Computed Tomography imaging to reveal pore-scale dynamic processes occurring in volcanic hydrothermal systems

Many volcanoes host a hydrothermal system,  responsible for a large fraction of volcanic eruption. These eruptions do not expel magma but involve the forceful ejection of pre-existing rocks, volcanic gases, and steam, posing a significant threat to human safety. Recent catastrophic incidents underscore the difficulty in foreseeing sudden hydrothermal explosions, exposing our limitations in prediction. The challenge lies in the absence of distinct precursory signals, making it difficult to anticipate these events. These eruptions may be triggered by the introduction of mass and energy originating from magma, or alternatively, by the development of mineralogical seals above vents, devoid of any direct magmatic influence. Understanding and predicting these hydrothermal phenomena remain critical for mitigating their potential human and environmental impacts.

In the ERUPT research project, we study the geoelectrical response of volcano hydrothermal systems (VHS).  Here, we focus on the laboratory scale, where we amalgamate electrical properties, namely SIP (Spectral Induced Polarization) measurements, with X-ray pore-scale (4D µCT) imaging to unravel the intricate electrical signatures of volcanic systems on rock samples collected from Gunnuhver region (Iceland). SIP  is a geophysical method that measures the complex electrical impedance of a material as a function of a wide range of frequencies (Zimmermann et al., 2008). It is particularly useful for characterizing the electrical properties of porous media, and have been widely used to study rock samples from VHS (e.g., Lévy et al., 2019). SIP responses are sensitive to factors like surface area, pore size distribution, fluid content, as well as movement of fluids within the rock. On the other hand, X-ray µCT is an imaging technique that uses X-rays to create detailed, 3D images of the internal structure of a sample, such as internal morphology, porosity, other structural features of rocks at a micrometer scale, and quantify fluid pathways and flow dynamics within the rock. The synergy of combining these two methods can provide a more comprehensive understanding of the geoelectrical properties and internal structure of a rock sample as follow: by analyzing SIP responses at different frequencies and correlating them with the µCT images, we gain insights into how variations in geoelectrical properties relate to the movement of fluids within the rock matrix, as well as the influence of alteration or precipitation of minerals. As a first step, we developed a unique experimental set-up that enables to combine both methods (SIP and µCT) simultaneously. The noval prototype was thoroughly designed following specific technical features (e.g., dimensioning, materials) to ensure an optimal SIP signal acquisition under well controlled conditions of temperature and pressure, together with a high resolution 4D µCT imaging. This integrated approach is valuable for studies in geophysics, hydrogeology, and reservoir characterization, among other various relevant domains.

 

References

Lévy, L. et al. (2019) ‘Electrical resistivity tomography and time-domain induced polarization field investigations of geothermal areas at Krafla, Iceland: Comparison to borehole and laboratory frequency-domain electrical observations’, Geophysical Journal International, 218(3), pp. 1469–1489. https://doi.org/10.1093/gji/ggz240.

Zimmermann, E. et al. (2008) ‘A high-accuracy impedance spectrometer for measuring sediments with low polarizability’, Measurement Science and Technology, 19(10). https://doi.org/10.1088/0957-0233/19/10/105603.