Sub Bottom Profiler Project   

PSBP PVC pipe enclosure

Introduction

A parametric sub-bottom profiler is a tool used by professionals to measure bathymetry, depth and the types of sub-layers on the bottom of a lake or ocean. The key feature of this piece of equipment is its ability to measure the thickness of different types of sediment at the bottom. This is especially important in  research that needs to compare the bottom of a lake now with how it was years ago.

Since the PSBP work best in water depths greater than 1m, a new improvement was designed so that it could be used in water depths less than 0.5m. Problems arose with data collection so changes needed to be made so that future research could be performed with better results. The purpose of this project was to improve the quality of the signal received and improve the overall performance.

“We are UW-Badger!”

Nick Janous

 

Coastal Engineering CEE 514

Fall 2009

University of Wisconsin-Madison

Civil/Environmental Engineering

Professor: Chin Wu

 

 

Seaking Parametric Sub Bottom Profiler

Equipment

The PSBP Professor Wu and the graduate students use is called “Tritech SeaKing Parametric Sub Bottom Profiler” and is used on many projects here. It is very expensive at $15,000 so it is important this piece of equipment is being used correctly. The PSBP uses a technique that is similar to simple echosounders. A sound source emits a signal vertically downwards into the water and a receiver monitors the return signal that has been reflected off the seafloor. Some of the acoustic signal will penetrate the seabed and be reflected when it encounters a boundary between two layers that have different acoustical properties (acoustic impedance). The system uses this reflected energy to provide information on sediment layers beneath the sediment-water interface.

Acoustic impedance is related to the density of the material and the rate at which sound travels through the material. When there is a change in acoustic impedance, such as the water-sediment interface, part of the transmitted sound is reflected. However, some of the sound energy penetrates through the boundary and into the sediments. This energy is reflected when it encounters boundaries between deeper sediment layers having different acoustic impedance. The system uses the energy reflected by these layers to create a profile of the sub-bottom sediments.

Because the PSBP uses sound waves, it is important to investigate the different wave lengths the instrument uses and see what restrictions it will have. The high frequency will provide seabed returns up to 200m altitude and plot a rolling display. The low frequency will penetrate the seabed up to 10m in soft silt or sand and is most effective at altitudes up to 30m with optimum performance and resolution at around 5m. The unit must be at least 1m from the seabed to allow the parametric process to form the low frequency beam. The beam has a 4 degree range

 

Also used in the apparatus was a 12” diameter PVC pipe. An aluminum mount on the inside of the pipe fastened the profiler to the wall securely and at a constant height.

 

Materials for the use of sound damping were 1/2” black pipe insulation, 1/8” pink insulation and small bubble wrap.

 

 

 

The Problem

Professor Wu and graduate students are interested in the sub layers of lakes but find that when the PSBP is used in water depths less than 1m, the data collected is not accurate. In order to collect usable data in the shallow water, a device was built to trap water, lift it above the lake water level and measure the sub layers with the PSBP inside. This created a “false” water depth greater than 1m, enabling the PSBP to function properly in shallow water.

 

Students found that when the piece of equipment was surrounded by a PVC pipe, the data had lots of noise. This was believed to be from the waves reflecting off the sides of the pipe, creating false signals. This is demonstrated in diagram “A” below. A solution was needed to prevent the sound waves from reflecting off the sides of the pipe and distorting the data. Diagram “B” shows the correct quality of data.

 

 

Seaking Parametric Sub Bottom Profiler      within PVC pipe addition