Purpose Current surveying techniques used by port authorities to estimate the nautical depth are limited in depth resolution and temporal resolution. Because of this, certain heavily occupied quay walls cannot be optimised in terms of utilisation. Therefore, a permanent continuous measuring system with a higher depth resolution is needed to optimise the occupation at these quay walls. We show how this could be achieved with distributed acoustic sensing (DAS) using fibre-optical cables. Materials We analyse recordings from a dual-frequency echo-sounder source along a standard communication optical fibre coiled vertically around a PVC pipe to represent vertical seismic profiling. This PVC pipe is placed inside a transparent plastic cylindrical tank which is partly filled with water and mud. This allows us to track the water-mud interface visually. We use a Silixa iDAS v2 and a Febus A1 DAS interrogator to convert the optical fibre into a seismic sensor. We use a wave generator to select the source frequency and an amplifier to amplify the output of the wave generator to a SIMRAD 38/200 COMBI C dual-frequency echo-sounder. Results We identify standing waves and use them to make accurate depth estimates of the water-mud interface inside the column we measure. Due to the high apparent velocity, the standing waves are easy to identify in the time domain. Due to the constructive interference, standing waves also show the water-mud interface in a power spectral density plot. We demonstrate that these standing waves could be used with an on-demand permanent continuous measuring system using ambient noise sources. Conclusion Our laboratory experiment showed that DAS could be used to estimate the water-mud interface. In addition, we showed the potential for on-demand monitoring in ports and waterways using DAS. Furthermore, due to the low cost of optical fibres, and the possibility of utilising ambient noise sources, DAS could be used for continuous depth monitoring purposes in ports and waterways.

Ultrasound measurements are routinely used to evaluate the safe depth for ships navigationnautical depth -at waterways and ports using single-beam dual-frequency echo-sounders. The nautical depth is routinely defined by suspension density in the range of 1100-1300 kg/m3 in the mud layer. While ultrasound measurements have a weak sensitivity to density variations, calibration is always needed to convert ultrasound measurements into reliable indicators for nautical depth levels in the mud layers using densely distributed density rheological in-situ measurements.We present a laboratory ultrasonic transmission experiment to monitor the fluid mud's settling and consolidation processes using a sample from the Port of Rotterdam. We use P-and S-wave ultrasonic transducers in the frequency range between 200 to 1000 kHz. Our results show that the P-wave velocities slightly increase during the consolidation and settling process while the P-wave amplitudes decrease. On the other hand, we observe a high S-wave velocity that increases together with amplitudes over time. The P-and S-wave amplitude and S-wave velocity variation over time correlate well with the mud average density variation. The presented results can be very useful for fluid-mud monitoring at a lab scale, besides possible utilization for large-scale monitoring field campaigns.

In ports and waterways, the bathymetry is regularly surveyed for updating navigation charts ensuring safe transport. In port areas with fluid-mud layers, most traditional surveying techniques are accurate but are intrusive and provide one-dimensional measurements limiting their application. Current non-intrusive surveying techniques are less accurate in detecting and monitoring muddy consolidated or sandy bed below fluid-mud layers. Furthermore, their application is restricted by surveying-vessels availability limiting temporary storm- or dredging-related bathymetrical changes capture. In this chapter, we first review existing non-intrusive techniques, with emphasis on sound techniques. Then, we give a short review of several seismic-exploration techniques applicable to non-intrusive fluid-mud characterization and monitoring with high spatial and temporal resolution. Based on the latter, we present recent advances in non-intrusive fluid-mud monitoring using ultrasonic transmission and reflection measurements. We show laboratory results for monitoring velocity changes of longitudinal and transverse waves propagating through fluid mud while it is consolidating. We correlate the velocity changes with shear-strength changes while the fluid mud is consolidating and show a positive correlation with the yield stress. We show ultrasonic laboratory results using reflection and transmission techniques for estimating the fluid-mud longitudinal- and transverse-wave velocities. For water/mud interface detection, we also use distributed acoustic sensing (DAS) and distributed temperature sensing (DTS).

This work shows the potential of using DAS for continuous water-depth monitoring by using the difference in acoustic energy in water and mud. The advantage over conventional methods is that our method can be used continuously and remotely, given that there is traffic nearby. Due to the low cost of fibres and the far-reaching dynamic range of interrogators, DAS could be a very attractive alternative for waterdepth monitoring using propeller noise in shallow marine environments, ports and waterways.

We show results of of using distributed acoustic sensing (DAS) for continuous relative water-column changes monitoring by relating the oscillating frequencies to measurements of a nearby tidal-station. The oscillations have a great qualitative agreement with the tidal-station, having a period of 12 hours and 25 minutes. No calibration is required to measure the tides and the relative difference in water height, though calibration would allow measuring the absolute water height at any location. Because we used two poles with different exposure lengths to air, at different depths and only 38 m apart, we can interpret he spectral oscillations are a result of constructive interference in our poles, likely generated by the wind. DAS could be a very attractive alternative for tidal monitoring in shallow marine environments, ports and waterways. DAS could potentially resolve spatial resolution problems with tidal monitoring, which is currently cost-prohibited, at a relatively low expense by wrapping a fibre around a pre-existing structure such as a docking pole. Furthermore, DAS can be used remotely and continuously, allowing for better model calibrations or local tidal fluctuation monitoring. This monitoring system could help determine if ships have enough water clearance to dock and, in turn, increase the occupation rate.