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Abstract
Frequency modulated continuous wave (FMCW) radar in the K-band has been shown to be an effective detector of geomaterial physical properties being highly sensitive to rock characteristics, particularly mineral composition and, for porous rock, variations in liquid water content. This research demonstrates that contrasts in FMCW return signals with time correlate with changes in geomaterial water content. FMCW signal returns were acquired for porous sandstone samples subjected to controlled water injection while also in a neutron beam, taking advantage of the well-known, and well-calibrated, attenuation of neutrons by hydrogen atoms for the water-containing porous sandstone samples. The sequential neutron tomographic images clearly show water moving up the sample with time while the FMCW observations show increases in radar reflection coefficient as a function of water position in the field of view. The observed FMCW detection of flood-front position is corroborated by the synchronous neutron tomographic images. We also observe repeatable variations in the radar reflection coefficient as a function of sample orientation during fluid injection, verifying that FMCW sensing offers real-time insight into the interactions between fluid movement and sample heterogeneity, via non-contact and non-invasive flood-front tracking. This research demonstrates that FMCW has potential to be a more accessible and easily deployable sensing modality than neutron tomography, enabling dynamic geomaterial testing to be conducted outwith the confines of the highly controlled laboratory environment required for neutron investigation.
Highlights
One current matter of considerable societal importance is the need for a complex multi-objective optimization of environmental objectives, energy security and energy equity [1]–[3]
The further purpose is to identify whether K-band Frequency modulated continuous wave (FMCW) may detect water front variations caused by the damage-induced increase or decrease of porosity in sub-millimetre width fractures and deformation bands
The FMCW was set to transmit and receive a single chirp of 300 milliseconds duration, and of bandwidth 1500 MHz (24 – 25.5 GHz), once every 5 seconds. These settings provide a spatial resolution of approximately 1 cm in a vacuum or in air [51], resulting in a region of the Fourier transformed return signal representing an internal view of the sandstone samples
Summary
One current matter of considerable societal importance is the need for a complex multi-objective optimization of environmental objectives, energy security and energy equity [1]–[3] This approach mirrors the wider need to optimize all subsurface energy source uses, recognizing benefits, costs and consequences. Nuclear energy creates wastes as part of the extraction of uranium, and the spent fuel plus the reactor facilities wastes must be isolated. These examples and others have direct links to the subsurface which can be both the source and the repository over both shorter (months to decades) and longer (millennia) timeframes. Research streams focused on fluid flow processes that occur naturally, and those caused
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