Ultrasonic parametric investigation of a coral slab: prospects for in situ non-destructive testing of coral microatolls.

Coral microatolls are natural tide gauges whose growth records provide valuable information on long-term sea-level variations that may be linked to global climate change. At present, investigating coral microatolls requires cutting a slab with a hydraulic chain saw to analyze their growth patterns. This approach is logistically demanding, damages both the coral and the surrounding ecosystem, and is inherently destructive. To date, no in situ, non-destructive technique is available for investigating coral microatolls. The present study introduces ultrasonic inspection as a non-destructive approach for visualizing their internal structure without coring, cutting, or otherwise damaging the coral. Developing such an acoustic methodology first requires the identification of relevant acoustic parameters and a thorough understanding of ultrasonic wave propagation within these complex biological structures. Accordingly, this study presents an ultrasonic investigation of a coral microatoll slab and proposes an experimental protocol for measuring mass density, acoustic impedance, and wave attenuation at 1 MHz, while evaluating their anisotropy with respect to the orientation of the coral spans. The results show that these properties vary monotonically from the younger outer growth zones toward the older central core of the microatoll, where more pronounced variations are observed. However, both the mean values and the spatial distributions of these properties remain essentially isotropic with respect to the orientation of the coral spans. Finally, ultrasonic imaging of the slab cross-section demonstrates the potential of this technique for future in situ, non-destructive investigations of coral microatolls.