APPLICATIONS OF CHIRP SONAR ACOUSTIC REFLECTION COEFFICIENTS FOR BOTTOM SEDIMENT CHARACTERIZATION: RESULTS FROM THE UPPER DELAWARE ESTUARY

Results from previous studies indicate that a direct correlation exists between the acoustical reflection properties of the sea floor and its sedimentary characteristics. Here, based upon that concept, a highly detailed and comprehensive correlation between chirp sonar reflection coefficients and the physical characteristics of the sediment-water interface is established. The physical sedimentary characteristics (e.g., percentages of sand, silt, clay) of over 300 grab samples from the floor of the upper Delaware Estuary were correlated to chirp sonar reflection coefficients. This allowed for the graphical examination of the reflection coefficient as a function of bottom sediment type. When plotted, the data points for each sediment classification type cluster around a specific reflection coefficient range, which is representative of the grain size of that particular sediment. The results yield a geoacoustical database that can be used to indicate the type of sedimentary environment each specific sonar reflection coefficient range represents.

This study is part of a larger sedimentological and geophysical survey of the upper Delaware Estuary that was conducted during 2001-2002 by the University of Delaware in cooperation with the Delaware River Basin Commission (DRBC). The overall purpose of the survey was to acquire geologic data pertinent to hydrodynamic and sedimentation models implemented by DRBC to develop a Total Maximum Daily Load for PCBs in the estuary. The industrialized sector of the river-estuary between Burlington, New Jersey and New Castle, Delaware was systematically surveyed using sidescan and chirp sonars. A total of 217 kilometers of sonar tracklines were collected, covering 100% of the estuarine floor below the 5-meter isobath. Sidescan backscatter patterns and grain-size data, based on the grab samples, were used to develop interpretations of bottom sedimentary environments with regard to dominant sediment type and mode of transport. The four major environments identified were reworked bottom, fine-grained deposition, coarse-grained bedload, and non-deposition or erosion. The chirp reflection coefficient correlation with sediment type described in this study further constrains these estuarine sedimentary environments.