Maritime transport facilitates close to 80 % of global trade, standing as an unparalleled cornerstone in international commerce. Projections herald a further increase in maritime activity, due to the low carbon footprint and cost-effectiveness, elevating the need for ensuring navigational safety within ports and waterways. At the heart of safe maritime navigation lies the pivotal concept of maintaining adequate nautical-depth, a threshold where a ship’s keel encounters navigational constraints. However, this necessity inherently demands incessant monitoring and recurring dredging operations, resulting in high costs amounting to millions of euros and, contributing significantly to carbon emissions. These factors underscore an urgent call for optimization of monitoring the nautical-depth to reduce monitoring and dredging costs, and increase marine navigational safety.
Presently, common methods for nautical-depth monitoring rely heavily on acoustic echo sounders, rooted in dated methodologies with limited innovation over nearly a century. Acoustic echo sounders measure the two-way travel time of sound pulses, assuming a known propagation velocity of the acoustic energy. However, accurately approximating the pressure-wave velocity in shallow marine environments poses challenges due to variations in temperature and salinity among different water layers, leading to depth measurement inaccuracies. Furthermore, this method is limited by vessel availability and requires access to quay walls, often occupied by loading or unloading ships.... ... Read more

Safe navigation in ports and waterways subjected to siltation requires nautical depth monitoring. For this purpose, surveying vessels equipped with a zero-offset echo sounder and intrusive point measurements are frequently used. Because these measurements depend on the availability of a surveying vessel and require access to quay walls, such as at the container terminals in seaports, the temporal resolution is limited. Especially at these locations, a high temporal resolution monitoring system could allow for a higher occupancy rate. We propose to use Distributed Acoustic Sensing to monitor the nautical depth using fiber-optical cables. We install five horizontal fibers at different heights between two points and continuously record along the complete installation. Analysing the continuous recordings, we show that horizontal fibers can be used to monitor the water-mud interface depth with a vertical resolution around six mm. Multiple passive sources, like vessel movements and water currents, are used to estimate the water-mud interface. ... Read more

We conducted laboratory experiments using large-scale samples (height: 0.47, diameter: 0.39 m) of basalt and marble coiled with telecommunication fibre. The fibre optical cable was converted to an array of densely spaced receivers (0.01 m) using distributed acoustic sensing (DAS) technology, and the source was placed on top of the samples. We demonstrate with an active acoustic setup how we can capture both the natural and artificial fracture responses. Therefore, this work investigates the applicability of the DAS method for seismic imaging on the lab scale for further technological advancement of vertical seismic profiling using DAS. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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) ... Read more

Ultrasound measurements are routinely used to evaluate the safe depth for ships navigation-nautical 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. ... Read more