HOW is Benthic imaging done?

The Benthic Mapping Project utilizes a battery of techniques to chart the landscape under the river (known as the benthos). Each method provides information about different features of this underwater habitat. Combining the data enables us to create a topographical picture of the river floor, know the composition and thickness of the sediment layer beneath, and also tells us about its inhabitants and their health.

Beaming sound waves underwater and measuring the way they bounce back off the river floor is the first step in benthic mapping (called acoustic profiling). The time it takes for an echo to return indicates the depth. This echo needs to be measured at a huge number places in the river to collect enough information to construct a benthic map.

The Benthic Mapping Project scientists go out in boats towing buoys which send out the repeated pulses of sonic energy (sound waves). We call these buoys "towfish," and call the data collecting process acoustic sonar tomography. When the buoy sends the sound waves out sideways this is called sidescan sonar, which indicates lateral distance from objects. The comparison of both sets of data enables reconstruction of an image of all the underwater hills, valleys, caves and tunnels in the Hudson.

Sound waves have other characteristics which we can use to learn about the benthos. The energy attenuation (loudness of the echo) provides a measure of hardness and particle size. From this we can project the composition of underwater structures, whether they're rock, coral, sand or mud. Two different frequencies are used as probes and the differece in response to them fine tunes our knowledge about buried structures.

Optical reflectivity (studying the way light reflects light off underwater surfaces) also reveals information about their composition.

 

We retrieve samples of sediment for study (core sampling), and take pictures of these compounds using a sediment profile camera, designed to get into inaccessible areas. Analyzing the makeup of sediment samples is known as Sediment Profile Imagery (SPI). SPI tells us a great deal about the potential for life in the benthos.

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