GEOPHYSICAL MAPPING OF A ROCKY COASTAL EMBAYMENT: PRELIMINARY RESULTS FROM USGS SEA FLOOR MAPPING EFFORT IN NORTHERN MASSACHUSETTS

Historically, seafloor mapping in shallow water regions has posed technological challenges, primarily due to the limitations of surficial mapping systems and the dynamic nature of the environment. However, technological advances over the past decade now enable high-resolution, systematic mapping within shallow water regions. In September-October 2003, swath-bathymetry, sidescan-sonar, and seismic data were collected within a 113 km2 survey area off the northern Massachusetts coast as part of a cooperative agreement between the U.S. Geological Survey and the Massachusetts Office of Coastal Zone Management. This mapping effort was designed to define seafloor morphology, surficial sediment distribution and underlying geologic structure of this complex glaciated coastal region. Survey line spacing of 100-150 m in a depth range of 7 m to roughly 45 m allowed for 100% overlap of sidescan-sonar and swath bathymetric data. Real-time kinematic GPS was used for horizontal and vertical navigation to ensure maximum positional accuracy. Preliminary analysis of sub-bottom profiles reveal a bedrock-framed valley fill in Salem Sound, which is likely a mid-late Holocene extension of the Danvers River. The backscatter and bathymetric images show heterogeneous seafloor morphology ranging from uniform low-backscatter regions to areas dominated by multiple bedrock exposures. Several bedform fields lie adjacent to offshore islands and ledges and exhibit wavelengths of 10-30 m and are several meters in height. These geophysical data will also be used to identify and map potential paleo-shoreline features, as well as to gain a better understanding of the active geologic processes within the study area.