registration mark
river view
river view



Research

Sediment Processes and Bottom Environments:
Survey Method and Tools

Obtaining high-resolution, multi-perspective description of the Hudson River bottom required a range of acoustic survey and sampling methods. These included:

Multi-beam bathymetry
Sidescan sonar
Sub-bottom profiling
Sediment sampling


Riverkeeper

The Riverkeeper is one of several vessels we use to acquire data in shallow water.

Multi-beam bathymetry

Multi-beam bathymetry (or swath bathymetry) uses an array of highly focused acoustic beams (up to 120) to measure water depth. The beams bounce off the river bottom and the system measures the time takes for each beam to travel from the boat to the seafloor and back. Because the angle of each beam is precisely known, the return time can be converted to distance with a high degree of accuracy. After correcting for water velocity, ship movements and tides, the individual soundings are combined into digital elevation model of the river bottom.

For the Hudson River Benthic Mapping Project, researchers from SUNY Stony Brook's Marine Science Research Center [http://www.msrc.sunysb.edu] conducted the multi-beam survey on the R/V Pritchard and the R/V Seawulf with a 300 kHz Simrad EM3000 multi-beam system, which provided high-resolution bathymetry (0.1m vertical and less than 1m horizontal accuracy).

Bathymetry diagram River Bottom Bathymetry

Conducting multi-beam bathymetry survey

 

Multibeam bathymetry data showing the river bottom near the Tappan Zee Bridge.

Sidescan Sonar

Sidescan systems send out an acoustic pulse to either side of the receiver and record the amplitude of the returning energy. With each pulse, a small strip (about 100 to 200m to each side) of the seafloor is mapped. As the boat moves, strip after strip is mapped. On a perfectly flat seafloor all energy will be reflected away from the sonar "fish" and no signal will be recorded. In reality, the seafloor is not perfectly smooth. Irregularities such as rocks and ripples cause reflections (backscatter) of the acoustic energy, and the system is able to provide information of the seafloor roughness (a sandy bottom, for example, is much rougher than a muddy bottom).

In addition to roughness, the strength of the returning signal is also determined by the nature of the bottom material. Given the same roughness, a harder surface will return more energy than a softer one. Usually some amount of additional analysis, or ground-truthing, is done to determine the real nature of the bottom.

Bathymetry diagram River Bottom Bathymetry

Sketch of working sidescan system. The yellow tow "fish" (Right: EdgeTech DF-1000) sends a widely spaced signal to each side and records the backscatter reflection. Rough or hard surfaces appear dark gray; smooth or soft appears light gray.



sidescan sonar image

Sidescan sonar mosaic of the Piermont Pier area with aerial photographs of the surrounding land. Dark gray represents areas of high backscatter (rough or hard surface) and light gray represents areas of low backscatter (smooth or soft surface).

Sub-bottom Profiling

Bathymetry diagram profile

In sub-bottom profiling, an acoustic signal is directed into the river bottom. The receiver records the energy that is reflected by different layers beneath the surface.

 

Typical sub-bottom section showing a cross-section of the Hudson River near Nyack. Dark lines mark the boundary between different layers.

A sub-bottom profiling system was used to obtain information of the sediments beneath the surface of the river bottom. Also known as single channel seismic systems, these send out a focused acoustic signal to the river bottom. Some of the energy is reflected by the bottom, but a portion of the energy penetrates into the sediments and rock. Each time the signal encounters a different material, a portion of the energy is reflected, and the system records these reflected signals. Combining these reflections produces an image of the sub-bottom structure of the riverbed.

The resolution and penetration of these systems depend on their frequency. In general, the lower the frequency the lower the resolution, but the better the penetration. The frequency of sub-bottom profiling systems is usually much lower then multi-beam or sidescan systems—typically 3.5 kHz to 20 kHz. The Hudson River Benthic Mapping Survey used an EdgeTech SB-424 Chirp sub-bottom profiler with a nominal resolution of about 0.1m.

Sediment Sampling

To validate the different acoustic data researchers with the Hudson River Benthic Mapping Program collected 410 sediment cores and 600 grab samples that were analyzed for grain size composition and other physical properties. A small number of the samples were also checked for the presence of cesium-137 (137Cs) and beryllium-7 (7Be) to determine their age.

researchers preparing to take core sampleresearchers taking core sample

Researchers preparing to take a sediment core (left) and grab sample (right) on the Hudson River.

 

Reference

Nitsche, F.O., Bell, R.E., Carbotte, S.M., Ryan, W.B.F., Flood, R.D., Ferrini, V., Slagle, A., McHugh, C.M.G., Chillrud, S., Kenna, T., Strayer, D.L. and Cerrato, R.M., 2005. Integrative acoustic mapping reveals Hudson River sediment processes and habitats. EOS Trans. AGU, 86: 225, 229.