Ultrasonic microscopic imaging and evaluation of mineral and soil block surfaces

Ultrasound-based microscale studies for soil property evaluation remain limited. This study employed ultrasonic microscopy to obtain acoustic images of soil blocks and minerals. A granite plate and its constituent minerals (quartz, feldspar, and mica), as well as resin-impregnated blocks of Andosol and sandy soil, were examined. Pulse-echo signals were acquired using a microscope scanning system (interval: 0.01 mm; frequency: 80 MHz). Acoustic impedance images of the sample surfaces were generated using the cross-correlation method. Optical images were subsequently captured. Quartz and mica exhibited acoustic impedances of 14–16 and 10–14 MRayl (= ×106 Ns/m³), respectively. Feldspar showed two impedance ranges (17–19 and 20–26 MRayl), reflecting its variable crystal structures. Mineral overlap in granite was difficult to observe in optical images but was clear in the acoustic impedance images. The soil blocks showed impedances of 3–4 MRayl (Andosol) and 10–15 MRayl (sandy soil). In the acoustic image of Andosol, numerous signal-free gaps of approximately 100 μm were observed, consistent with its porous structure. At a depth of 4 cm in sandy soil, the average impedance increased by 24%, revealing characteristics not visible in optical images. These results demonstrate the utility of ultrasonic microscopy.