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
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
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.