Sediment Profiling Imagery (SPI)

Sediment profiling imagery (SPI) uses a camera and prism mounted on a frame to take a picture of the sediment-water interface after weights drive the prism into the sediment. Various features at or below the sediment surface can be identified in the imagery, including infauna, shell hash, sediment layers and subsurface methane pockets. Rhoads and Cande (1971) developed SPI to examine processes occurring at the sediment-water interface. Later, Rhoads and Germano (1986) outlined a paradigm for interpreting SPI in the context of benthic community succession, and Diaz and Schaffner (1988a) used SPI to relate sediment geochemistry, sediment texture, and biological activity.

The apparatus has two basic parts: a camera, encased in a pressure housing, and a prism, with a 15 centimeter x 23 centimeter clear Plexiglas faceplate and mirror to reflect the image of the sediment into the camera lens. The bottom edge of the prism is sharpened to cut through the sediment neatly. The prism is filled with clear freshwater to prevent hydrostatic pressure from distorting the faceplate as the prism is lowered below the sea surface. The lens and light source (strobe light) used to illuminate the sediment are contained inside the water-filled prism. The camera is focused on the prism faceplate and records sediment features pressed against the faceplate. This configuration allows the camera to work in complete darkness with image clarity independent of turbidity. Lead weights fastened to the camera frame push the prism into the sediment, and a hydraulic piston slows the rate of penetration to minimize disturbance to the sediment. A watch with digital time and date displays is attached to the prism and recorded in each picture, enabling photographs to be matched to station locations. In the NY/NJ Harbor study, a switch, triggered by the prism penetrating the sediment, controlled a timer keyed to the camera. In the current design, the camera can be triggered remotely by the camera operator who uses an attached video system to monitor penetration from the survey vessel.

SPI has been adapted to examine impacts from dredging and disposal of dredged material (SAIC 1985, Morton et al. 1985, Diaz et al. 1985, Diaz et al. 1987a, 1987b, Diaz and Schaffner 1988b, Nichols et al. 1990), including mapping of dredged material footprints (e.g., TAI 1987, 1988) and monitoring caps placed on contaminated dredged material (e.g., SAIC 1995). As the technology's potential for quickly assessing environmental conditions was realized, SPI was used by the U.S. Environmental Protection Agency to examine oxygen stress (SAIC 1987); by the National Oceanic and Atmospheric Administration to examine sediment quality and habitat conditions in major U.S. estuaries (San Francisco Bay, Revelas et al. 1987; and Long Island Sound, Day et al. 1988); by Boyer and Shen (1988) to map spatial patterns in sediments and infaunal communities in the Great Lakes; by Valente et al. (1992) and Diaz (1995) to map bottom habitats in Narragansett Bay; and by Cutter and Diaz (1998) to map offshore benthic habitats.

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