Shallow Water Data Acquisition with a Sub Bottom Profiler
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Figure 1: Tritech Seaking Sub Bottom Profiler
An acoustic sub bottom profiler (SBP) is a device used to collect sediment data from bottoms of lakes, rivers, seas, and other bodies of water. The SBP pictured to the left is a Tritech Seaking model (Copyright 2010, Tritech International). It transmits acoustic pulses toward the sediment layer, and the pulse reflects off the layer, returning to the SBP. The SBP measures the strength of the reflected signal, and sediment properties like saturated bulk density and grains size can be calculated based on the energy lost by the signal. These properties are used in sediment erosion and deposition calculations. The SBP also calculates the depth at which the signal was reflected based on the known velocity of the signal and the time it takes for the signal to return to the SBP. Many properties of waves, like propagation speed, bottom orbital velocities, and wave height, depend on water depth, so it is important to obtain depth measurement for all areas of a body of water.
Two frequencies are generated by the SBP; a 20kHz and 200 kHz pulse. The higher frequency pulse carries less energy and does not penetrate very far into the sediment layers. Therefore, it is primarily used for gathering depth data and sediment properties of the first inch or two. The lower frequency pulse carries more energy and is used to obtain information for sediment layers below the floor surface. It can penetrate up to 30 meters below the surface layer.
Two frequencies are generated by the SBP; a 20kHz and 200 kHz pulse. The higher frequency pulse carries less energy and does not penetrate very far into the sediment layers. Therefore, it is primarily used for gathering depth data and sediment properties of the first inch or two. The lower frequency pulse carries more energy and is used to obtain information for sediment layers below the floor surface. It can penetrate up to 30 meters below the surface layer.
Problems with Shallow Depths
The SBP has problems obtaining measurements in shallow depths because of the noise generated by vibration of the transmitter head. The noise from the lower frequency signals obscure readings for the first meter of depth below the SBP, and noise from the higher frequency signals obscures data for the first 1/2 meter. In order to obtain sediment data and facilitate a quick method of obtaining bathymetry data in shallow waters, a PVC housing for the SBP was constructed, which is described in the Approach section.
