Detection of fluid mud layers using tuning fork, dual-frequency echo sounder, and chirp sub-bottom measurements

Abstract

Many ports and navigation channels around the world suffer from silting and the presence of fluid mud layers, which can cause reductions in nautical depth. In this context, the existence of fluid mud layers makes the definition of the bottom ambiguous because the location of interfaces between water, fluid mud, and consolidated mud is dynamic. The present paper presents an integrated approach to analyzing the fluid mud layers in different port regions of South America and under controlled conditions in a laboratory column. In situ measurements were obtained with acoustic equipment, a chirp sub-bottom profiler and density profiles. The laboratory experiment was operated in a sedimentation column 4.3 m high, and dual-frequency echo sounder and density measurements were used to monitor sedimentation and resuspension events. Concerning the detection of the mud layers through dual-frequency echo sounder measurements, the high frequency return (HF) is associated with the water mud interface (lutocline), and the low frequency return (LF1) is more difficult to interpret. The high frequency recorded the position of the upper layers formed, either by resuspension or by the presence of diluted suspension. The depth measured by the low frequency can be either related to higher densities or associated with a second density gradient observed in the profiles. It is recommended to interpret and visualize the echograms when there is interference in the signal due to suspensions. The combination of techniques to detect and measure the fluid mud layers is a promising approach to empower and develop tools using fluid mud for navigation, with a potential increase in draft, as well as to define critical densities for port and channel areas, for safe navigation.