Fluid mud plays an important role in navigability in ports and waterways. Characterizing and monitoring the seismic properties of the fluid mud can help understand its geotechnical behavior. Estimation of the wave velocities in fluid mud with high accuracy and repeatability enables investigating the behavior of parameters like the yield stress in a nonintrusive and reliable way. We perform ultrasonic reflection measurements in a laboratory to investigate the wave propagation in a water/fluid-mud layered system. The component of wave propagation in the water layer inevitably brings kinematic dependence on the characteristics of that layer, making the estimation of exact velocities in the fluid mud more challenging. In order to extract the wave velocities only in the fluid-mud layer, we use a reflection geometry imitating field measurement to record the ultrasonic data from sources and receivers in the water layer. We then use seismic interferometry to retrieve ghost reflections from virtual sources and receivers placed directly at the water-mud interface. Using velocity analysis applied to the ghost reflections, we successfully obtain the P-wave and S-wave velocities only inside the fluid-mud layer, and investigate the velocity change during the self-weight consolidation of the fluid mud. Our results indicate that the S-wave velocities of the fluid mud increase with consolidation time, and show that reflection measurements and ghost reflections can be used to monitor the geotechnical behavior of fluid mud.@en

The velocities of the seismic waves propagating in the fluid-mud layer are governed by the rheological properties and density of the fluid mud. Performing seismic transmission measurements inside the fluid mud can give good estimates of the seismic velocities and, thus, of the rheological properties and density. Laboratory ultrasonic transmission measurements of the wave velocities in the fluid-mud layer and their temporal evolution are shown. It is found that the shear-wave velocity and yield stress are positively correlated. Performing a seismic reflection survey for characterization of the fluid-mud layers could be more practical because it allows towing the sources and receivers above the top of fluid-mud layer. Interpretation of the results from a reflection survey, though, is influenced by the water layer above the fluid mud. Applying seismic interferometry to reflection measurements can eliminate the influence of the water layer and retrieve a reflection response from inside the fluid-mud layer. This eliminates the influence of the temperature and salinity of the water layer to obtain information about the seismic properties of the fluid-mud layer. To introduce the approach of retrieving and extracting the reflection response from inside the fluid-mud layer, data from laboratory measurements are used. The obtained compressional- and shear-wave velocities are validated by comparing them with values from current transmission measurements.@en

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)@en