Development of mud layer detecting device and its application to the measurement of vertical slices of density profile in Kushiro Port

Abstract

The vertical slices of density profile along the surveying lines provide important information for establishing a countermeasure for the deterioration of coastal marine environments and for the shoaling of waterway or anchorage with a drift of soft mud such as low consistency clay or silt. To detect the density profiles of the mud layer from the surface ship, a mud layer detecting device using wide-band and narrow beam underwater acoustic pulses has been developed for the detection in the sea areas of several tens of meters deep. The device has 3 acoustic transducers that transmit and receive 25, 71 and 250 kHz signals respectively. The beam angle of these transducers is 2 degrees and their band width is equal to the half of the central frequency. The device receives the acoustic signals reflected from the sea bottom, digitizes them, stores them in the hard disk of a computer located on the ship and analyzes them off-line using a wave-form analyzing technique. This device was used to obtain over 10 vertical slices of the density profile in Kushiro Port (Hokkaido, Japan). The density profiles that were obtained by using this device agree well with those of the core-sample. The surface density distribution was low, less than 1.3 g/cm3, at the east side of the port, but over 1.4 g/cm3 at the west side of the port which is open to the waterway. According to results of sounding before and after the Offshore of Tokachi Earthquake in 2003 (Sep. 26, 2003), the depth after the earthquake was about 50 cm deeper than the one before the earthquake. This depth change is considered caused from the movement of the surface sediment by Tsunami from the earthquake. The higher density area over 1.4 g/cm3 at the west side of the port is deemed caused as the result of the movement of the surface sediments. The change of sedimentation can be known easily with this device as described above. It is expected that this device is applicable to the maintenance and control of ports and harbors