Monitoring shear-stress changes using seismic measurements from controlled sources and ambient noise and optical fibres

Abstract

Monitoring the nautical depth is vital for the safe passage of water transport. Port authorities worldwide have different navigable depth criteria and use various methods to ensure the safe navigability and manoeuvrability of ships in ports and waterways. These measurements often require a surveying vessel and are limited in repeatability and accuracy. Often, it is challenging to survey at heavily occupied quay walls; this may hinder economical activities. Additionally, because the current monitoring techniques depend on a surveying vessel's availability, monitoring significant changes in the nautical depth after, for instance, storms, is challenging, especially over large areas. Reliable continuous depth measurements could therefore help to optimise ships’ docking operations in heavily occupied areas. We show how the nautical depth can be measured and demonstrate the potential for estimating shear stresses using distributed acoustic sensing. Our laboratory study and our field test show that the acoustic energy differs for non- Newtonian fluids with different shear strength. For our laboratory experiment, we use natural and synthetic sediment suspensions for measuring the difference in acoustic attenuation with an optical fibre wrapped around a polyvinyl chloride (PVC) pipe. Our first acoustic measurement conducted one hour after mixing has a shear strength of 17 Pa and shows very high attenuation. The second laboratory test recorded 24 hours after mixing, with the shear strength of 48 Pa, reveals a tremendous signal-attenuation decrease and thus amplitude increase. In our field experiment, we observe a similar increase in amplitude with increased shear strength when recording propeller noise from passing vessels for frequencies < 60 Hz. We also observe a reverse trend for frequencies > 100 Hz. This difference in amplitude with depth might be related to a difference in fibre coupling and a difference in attenuation of acoustic waves. Additionally, our field experiment shows the potential to use Distributed Acoustic Sensing for continuous depth measurements.