As part of the Synergetic Utilisation of CO (Formula presented.) storage Coupled with geothermal EnErgy Deployment project, investigating CO (Formula presented.) reinjection with different seismic methods, both passive and active seismic surveys have been conducted at the geothermal power plant at Hellisheiði, Iceland. During the 2021 survey, two geophone lines recorded noise for a week. We process the passive-source data with seismic interferometry to image the subsurface structure around the CarbFix2 reinjection reservoir. To improve image quality, we perform an illumination analysis to select only noise panels dominated by body-wave energy. The results show that most noise panels are dominated by air-wave energy arriving from the direction of the power plant. We use panels with a near-vertical incidence to create a zero-offset image and a larger selection of body-wave-dominated panels to create virtual common-shot gathers. We process the gathers with a simple reflection seismology processing workflow to obtain stacked images. The zero-offset images show a relatively lower signal-to-noise ratio and only horizontal reflectors. The stacked images show slightly dipping reflectors and possibly lateral amplitude variations around the expected injection region. This could indicate a region of interest for future research into the reinjection reservoir.

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We apply supervirtual interferometry to boost the surface-wave content of two different seismic surveys. The method uses seismic interferometric principles to exploit data redundancy in multi-fold surveys. The effect on the first survey is generally positive, where the signal-to-noise ratio is improved and the relative amplitude of other events, like direct waves or reflections, is decreased. The second survey shows that the effects are not always positive. For some shots, the quality of the dispersion curve decreases and for some a higher mode becomes more dominant. This can be caused when assumptions made for seismic interferometry by corrrelation are not complied with, primarily heterogeneities in the medium and attenuation. As such, the effect of applying supervirtual interferometry could be used as an indication for local heterogeneities.@en

In July 2021, passive seismic data has been recorded on a network of geophones at the CarbFix2 injection site at Hellisheiði, Iceland. This data is processed using seismic interferometry to get an image of the injection reservoir. The data is split up into noise panels. Panels dominated by body-wave energy are selected using an illumination analysis. In panels where the dominant event has a (near) vertical incidence, each trace is autocorrelated to get a zero-offset section. In panels where the dominant event is recognized as a body-wave event, all the traces are crosscorrelated, obtaining virtual common-shot gathers. This is processed with a reflection-seismology workflow to obtain a stacked section. Comparing the two final sections shows that similar reflectors are imaged. The zero-offset section shows a higher frequency content, while the stacked section shows more continuous reflectors. Comparison with a local geological model shows that the results are plausible, but that a better interpretation has to wait for more results of the same survey to be processed.

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