Sediment-profile imagery
Sediment-profile imagery (SPI) is a rapid, in-situ
technique, which takes vertical profile pictures of the upper 20cm
of the sediment system.
The SPI camera works like an inverted periscope: the camera
possesses a wedge-shaped prism with a plexi-glass faceplate and an
internal light provided by a flash strobe. The back of the prism
has a mirror mounted at a 45° angle, which reflects the image of
the sediment-water interface at the faceplate up to the camera.
The wedge-shaped prism enters the bottom and is driven into the
sediment by its weight. A "passive" hydraulic piston ensures that
the prism enters the bottom slowly and does not disturb the
sediment-water interface. On impact with the bottom, a trigger
activates a time-delay on the camera shutter release and a
photograph is taken when the prism comes to rest.
A series of images can be obtained by successively dropping the
SPI camera, or the prism can take successive pictures while
deployed in time-lapse (t-SPI) mode.
Images can provide clear insight into the relationship between
benthic communities and the sediment.
It is a useful technique using high-quality images to study in
situ seafloor organisms and processes that cannot be directly
observed using other equipment (for example, grabs).
Image-derived parameters
The quality of SPI images makes it possible to determine several
biological and physical parameters from images, including:
- sediment type
- prism penetration depth (giving an indication of relative
sediment compaction)
- sediment boundary roughness (indicating the degree of physical
disturbance or biotic activity at the sediment water boundary)
- sediment apparent Redox Potential Discontinuity depth
(aRPD)
- infaunal successional status (qualifying the type of animals
living in the bottom).
These image-derived parameters can be combined into indicies
that integrate the information gained from other parameters
into a single index, indicative of health status.
SPI applications
Cefas acquired a SPI system in 2004 and has steadily been
demonstrating its capability and potential application through
monitoring and research programmes.
Due to its rapid and flexible nature in deployment, the SPI
camera can be applied to various environmental questions such
as:
- mapping features and habitats - Sabellaria, eel
grass
- mapping the extent and intensity of human impacts - disposal,
capping, trawling
- impact assessment using key SPI image criteria (biology, redox,
sediment) and conceptual impact models
- integration with other monitoring variables (contaminants,
faunal sampling, acoustics)
- research applications to look at sediment-state changes and the
functional role of faunal biodiversity.