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The Boston Harbor Story:
Applying Earth Science to issues of immediate public concern
Sidescan Sonar and Seismic Reflection Diagram

MAPPING THE SEAFLOOR

In order to determine where contaminated sediments may accumulate, maps of seafloor topography and geology are generated using two specialized techniques: sidescan sonar and seismic reflection. Sidescan sonar uses the reflectivity of sound to distinguish different sediment types on the seafloor. Sediment-free boulders and bedrock are strong sound reflectors and are shown as light areas on the sidesacan record. Intermediate grays are areas of sand and gravel; dark zones are deposits of fine-grained material, typically found in the sheltered topographic lows between hills. Seismic reflection helps to understand the types and orientation of sediments and rocks below the sea floor. Sampling sites for ongoing monitoring of contaminants in the sediments of Boston Harbor are located in fine grained areas.

Below: Bottom photographs of different sediment types located near the Massachusetts Bay sewage outfall. Click on images to enlarge.

Photo of muddy seafloor Photo of mud and gravel seafloor Photo of rocky seafloor
A muddy location in a depression with numerous burrows caused by benthic organisms. This bottom type causes weak, reflections of the sidescan signal. Photograph displaying poorly sorted mixture of mud and gravel which causes intermediate strength reflections of the sidescan signal. Boulders on top of a drumlin covered with biological growth including a red algae. This bottom type creates strong reflections of the sidescan signal.

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Updated: 11 September 2000