Non-destructive measurements of contaminated soil core samples are desirable prior to destructive measurements because they allow obtaining gross information from the core samples without touching harmful chemical species. Medical X-ray computed tomography (CT) and time-domain low-field nuclear magnetic resonance (NMR) relaxometry were applied to non-destructive measurements of sandy soil core samples from a real site contaminated with heavy oil. The medical CT visualized the spatial distribution of the bulk density averaged over the voxel of 0.31 × 0.31 × 2 mm3. The obtained CT images clearly showed an increase in the bulk density with increasing depth. Coupled analysis with in situ time-domain reflectometry logging suggests that this increase is derived from an increase in the water volume fraction of soils with depth (i.e., unsaturated to saturated transition). This was confirmed by supplementary analysis using high-resolution micro-focus X-ray CT at a resolution of ∼10 μm, which directly imaged the increase in pore water with depth. NMR transverse relaxation waveforms of protons were acquired non-destructively at 2.7 MHz by the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence. The nature of viscous petroleum molecules having short transverse relaxation times (T2) compared to water molecules enabled us to distinguish the water-saturated portion from the oil-contaminated portion in the core sample using an M0–T2 plot, where M0 is the initial amplitude of the CPMG signal. The present study demonstrates that non-destructive core measurements by medical X-ray CT and low-field NMR provide information on the groundwater saturation level and oil-contaminated intervals, which is useful for constructing an adequate plan for subsequent destructive laboratory measurements of cores.

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Underground pipeline infrastructures, such as water supply and industrial water pipes, were rapidly constructed in Japan during the 1970-1980s economic boom and have been ageing quickly. In general, corrosion of buried metal water pipes depends on the physicochemical properties of the soil around them. Conventionally, when conducting such investigations, the soil is excavated and sampled to analyse these properties in laboratories. As this damages the paved road surface, an alternative method is required. Resistivity is a significant physical property measured when investigating the corrosion risk of underground pipelines. Therefore, if geophysical exploration can facilitate the investigation of soil resistivity from the ground surface, it will play a key role in the renewal planning of water pipes. Traditionally, geophysical methods such as electrical and electromagnetic exploration have been used for measuring subsurface resistivity. Electrical exploration is a robust and noise-tolerant method; however, it requires electrode installation. Most roads over the pipelines are paved, making conventional electrical exploration using metal electrodes a challenging task. Therefore, the Geological Survey of Japan, the National Institute of Advanced Industrial Science and Technology, has developed a very-low-frequency band alternating current electrical surveying technique that uses water-saturated polyvinyl alcohol sponge electrodes. In this technique, electrodes are placed on an asphalt or concrete paved surface and the electrode measures the soil resistivity profile and detects the corrosive soil distribution without damaging the paved surface. Moreover, we improved this equipment to enable vertical electrical survey of numerous points and acquired data for 740 m of the survey line in 1 day at Maborikaigan coast in Yokosuka City. We found that the resistivity values varied with the depths and locations of the buried water pipes. These variations indicated that surveys and evaluations on long survey lines along water pipe burial routes are essential for risk management in water pipelines.

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