Data collected in the last decade have provided a greater understanding of the bottom of Lake Champlain. Two highly detailed data sets have been developed using sonar systems by faculty in the geology department at Middlebury College. These sidescan sonar and CHIRP data have provided an unprecedented view of not only the lake bottom and natural and man-made underwater features but also of sub-bottom stratigraphy.
A sidescan sonar unit transmits a fan shaped sound beam to either side of the sonar fish instead of directing it downwards as is the case for conventional echo sounders. As a result, side-scan data provides information about the upper few centimeters of the bottom sediment. The strength of the returned sound beam is affected by topography of the sediment surface as well as by differing lithologies and bottom surface roughness. The rougher the bottom is, the more energy is returned. As a result, a qualitative measurement of percent sand and mud can be determined by the strength of the return sonar signal. Side-scan sonar records are not plan views of the sediment surface, but with application of simple algorithms, the records can be interpreted to determine bottom morphology.
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Sound from the Full Spectrum CHIRP (Compressed High Intensity Radar Pulse) sonar system is directed downward to the lake bottom and penetrates depths up to 80 m. The sound is returned from the sediment water interface as well as from layers in the sub-bottom. In essence it allows a view of the subsurface in a vertical plane. In Lake Champlain three characteristic sediment units are easily identified: Lake Vermont sediments, Champlain Sea sediments, and recent Lake Champlain sediments. CHIRP sonar images allow for the interpretation of the lake’s history by showing the sediment layers as they were deposited.
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For more information on these data sets, contact Pat Manley, Professor of Geology, Middlebury College, (802) 443-5430