Ultrasonic imaging of analogue scale models

Since the 19^th century pioneering work of Sir James Hall, physical analogue modelling has been proven a valuable method for the study of geological phenomena and has significantly contributed to understanding fundamental mechanisms of crust and lithosphere deformation. Traditionally, in such analogue scale models, structural deformation is monitored and quantified using top-view images or cross-sections, where the latter allow for portraying the final state of internal deformation of the model in great detail. Monitoring the evolution of internal deformation while the experiment is running is however a major challenge, and currently is possible only with X-ray scanning using medical-type CT scanners. These, however, put stringent limitations on size of the model and, thus, the possible geometric configurations related to different modelling setups.

To tackle these limitations, we are developing a novel method to image the evolving interior of analogue scale models using ultrasonic techniques. Similar to reflection seismology used in field studies, the internal structure of the analogue model can be imaged using sound waves. We employ a completely non-contact and non-invasive method, utilizing a laser Doppler vibrometer to detect the arrivals of seismic body waves at the model surface. A laser pulse from a powerful pulsed laser acts as a point source and is used to introduce acoustic waves in the model. By moving the detector and source, acoustic data is recorded for a number of source-recorder combinations, allowing the reconstruction of the internal layering and structure along cross sections, as will be illustrated by the results of several tests with analogue models and other samples. By developing this technique, we provide novel tools to characterize the acoustic behaviour of subsurface structures under well-controlled laboratory conditions with the aim of improving our understanding of waveforms and wave propagation in analogue models and earth materials in general.