The Sea-floor Mapping Group, Woods Hole Science Center, acquires a suite of geophysical and sedimentologic data in order to accurately interpret the geologic framework of the sea floor. Surficial sediment characteristics are mapped utilizing sidescan-sonar systems and collecting sediment samples, bottom-photographs and video. Sidescan-sonar data represent backscatter information from an insonified region of sea floor. In the sidescan-sonar imagery presented here, high backscatter is represented by light tones, low backscatter by dark tones. To accurately interpret the geology of an area, correlations must be made between the surficial sediment characteristics acquired by sidescan-sonar and ground-truth techniques, and the underlying structure defined by seismic-reflection techniques.
The examples to the right display a survey of the northeastern Massachusetts offshore area, collected as part of a cooperative effort between the WHSC and the Massachusetts Office of Coastal Zone Management (CZM), 2003-2004. The acoustic backscatter intensity is shown, also in conjunction with other data types. Bottom-type data and sediment isopach data are overlain on the sidescan imagery to further enhance the interpretation. This interpretation was based on analysis of sidescan-sonar, sedimentologic, and subbottom data. Interferometric-sonar data and bottom photography are also integrated and indicate the degree of detail possible with such an integrated suite of data (see last image in the group). Integrating interferometric swath bathymetry and sidescan-sonar data enable us to generate dynamic maps, in a 3-dimensional perspective. This greatly facilitates data interpretation by showing where backscatter patterns occur relative to bathymetric changes, from which one can begin to infer sediment transport direction and the morphology of bedforms. In some cases, having the combined backscatter/bathymetry data has significantly changed the geologic interpretation of the backscatter patterns from what would be inferred from the backscatter data alone.
During typical at-sea operations, sidescan-sonar data are processed, mosaicked, and ultimately used as a base from which sediment sample and/or bottom photography locations are chosen. This ground-truth component of cruise operations is critical in accurately interpreting the sidescan-sonar data. The figure below displays a line of sidescan-sonar imagery and the associated sediment samples collected within the low- and high-backscatter areas, respectively. In general, the low-backscatter region within the sidescan sonar data represents finer-grained sediment, while the high-backscatter regions represent coarser-grained sediment. However, in the bottom figure to the right, the change in backscatter intensity within the sidescan-sonar data is not attributable to a change in sediment grain-size, but is due to the presence of sand dollars. The orientation of the sand dollars relative to the angle of insonification by the sonar system produced high-backscatter within the image. Without the bottom photographs, the change in backscatter could have been misinterpreted to be due to a textural change in the surficial sediments. This example clearly shows the importance of incorporating ground-truth data in geophysical interpretations.
Barnhardt, W.A., B.D. Andrews, and B.Butman, 2005, High-resolution geologic mapping of the inner continental shelf: Nahant to Gloucester, Massachusetts, U.S. Geological Survey Open File Report 2005-1293, (online at http://woodshole.er.usgs.gov/pubs/of2005-1293/index.html).