Sediment Profile Imagery (SPI) is an underwater technique for photographing the interface between the
seabed
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
and the overlying water. The technique is used to measure or estimate biological, chemical, and physical processes occurring in the first few centimetres of
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
,
pore water
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidated ...
, and the important
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
boundary layer of water.
Time-lapse
Time-lapse photography is a technique in which the frequency at which film frames are captured (the frame rate) is much lower than the frequency used to view the sequence. When played at normal speed, time appears to be moving faster and thus ...
imaging (tSPI) is used to examine biological activity over natural cycles, like tides and daylight or
anthropogenic
Anthropogenic ("human" + "generating") is an adjective that may refer to:
* Anthropogeny, the study of the origins of humanity
Counterintuitively, anthropogenic may also refer to things that have been generated by humans, as follows:
* Human im ...
variables like feeding loads in
aquaculture
Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. lot ...
. SPI systems cost between tens and hundreds of thousands of dollars and weigh between 20 and 400 kilograms. Traditional SPI units can be effectively used to explore
continental shelf
A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
and abyssal depths. Recently develope SPI-Scan or rSPI (rotational SPI) systems can now also be used to inexpensively investigate shallow (<50m) freshwater,
estuarine
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...
, and
marine
Marine is an adjective meaning of or pertaining to the sea or ocean.
Marine or marines may refer to:
Ocean
* Maritime (disambiguation)
* Marine art
* Marine biology
* Marine debris
* Marine habitats
* Marine life
* Marine pollution
Military
* ...
systems.
Advantages
Humans are strongly visually oriented. We like information in the form of pictures and are able to integrate many different kinds of
data
In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted ...
when they are presented in one or more images. It seems natural to seek a way of directly imaging the sediment-water interface in order to investigate animal-sediment interactions in the marine benthos. Rhoads and Cande (1971) took pictures of the sediment-water interface at high resolution (sub-millimetre) over small spatial scales (centimetres) in order to examine benthic patterns through time or over large spatial scales (kilometres) rapidly. Slicing into seabeds and taking pictures instead of physical cores, they analysed images of the vertical sediment profile in a technique that came to be known as SPI. This technique advanced in subsequent decades through a number of mechanical improvements and digital imaging and analysis technology. SPI is now a well-established approach accepted as standard practice in several parts of the world, though its wider adoption has been hampered partly because of equipment cost, deployment, and interpretation difficulties. It has also suffered some paradigm setbacks. The amount of information that a person can extract from imagery, in general, is not easily and repeatedly reduced to quantifiable and interpretable values (but see Pech et al. 2004; Tkachenko 2005). Sulston and Ferry (2002) wrote about this difficulty in relation to the study of the human genome. Electron microscope images of their model organism (''Caenorhabditis elegans'') carried a lot of information but were ignored by many scientists because they were not readily quantified, yet that pictorial information ultimately resulted in a deep, and quantifiable, understanding of underlying principles and mechanisms. In the same way, SPI has been used successfully by focusing on the integration of visual data and a few objectively quantifiable parameters in site reconnaissance and monitoring.
History and application
Conventional diving is limited to shallow waters. Remotely sampling deeper sediments of high water content is often unreliable due to sampler bow waves, compaction upon impact, or variably disrupted surface sediment features (Somerfield and Clarke 1997). In 1971, Rhoads and Cande described an instrument to address the problems of adequately observing and collecting silty sediments. Their remote sampling equipment introduced the field of ''in situ'' vertical sediment profile imagery and what are now commonly called SPI cameras. The device mainly consists of a wedge-shaped box mounted in a frame. The box has an oblique face made of transparent acrylic and a downward-looking camera (Figure 1). Weights force the wedge and its internal mirror into the sediments. The mirror, at 45° to the transparent section, reflects an image of the pierced sediment-water interface to the underwater camera, like a periscope. In order to remain rigid at depth, the wedge is filled with distilled water.
Figure 1. Schematic drawing of the profile camera in partial cross section showing the cradle in the down position intersecting the bottom. A- slack winch-wire; B- oil-filled cylinder; C- piston rod; D- piston containing a small diameter hole; E- battery housing with magnetic reed switch, F- lead weights, G- camera (oriented vertically); H- light; I- Plexiglas guillotine filled with distilled water; J- sediment-water interface; K- 45° angle mirror reflecting the sediment-water interface profile 90° to the camera lens. Taken from Rhoads and Cande (1971).
Their device returned images such as that shown in Figure 2. At first glance SP images may appear unremarkable, but analysis of dozens of images allows the breadth of information they contain to become apparent. In Figure 2 the gross texture and water content of the sediment is immediately apparent. Since resolution allows imaging of individual sand grains, the classic textural parameters (percentage of gravel, sand, and mud) can be assessed and a mean grain size estimated. The sediment-water interface is clear. If the image was taken immediately upon insertion, this observation indicates that the device entered the seabed with little disturbance. Furthermore, the interface is distinct. While seemingly straightforward, some seabeds have, instead, a boundary layer of suspended sediments with a broad density gradient instead of a discrete transition point. This condition has a fundamental importance to many benthic organisms. Biological activity is readily apparent as well. When calibrated using traditional grab samples or cores coupled with a few SP images, resolution allows identification of some infauna including the tubicolous sabellid polychaetes, a bisected nereid, and the mound produced by a sea cucumber seen in Figure 2.
Figure 2. Sediment profile photograph of a mud bottom 35 m deep in Cape Cod Bay, Massachusetts. The place of the photograph passes through a fecal mound produced by Molpadia oolitica (holothurian). The apex of the cone is populated by the sabellid polychaete Euchone incolor (A). An errant polychaete has been cut by the guillotine (B). Void spaces at depth are produced by the feeding activities of M. oolitica (C). Light-coloured oxidized (sulfide-poor) sediment extends about 3 cm below the sediment surface. Taken from Rhoads and Cande (1971).
Another significant feature of Figure 2 is the distinct colour change between surface sediments and those deeper. This gradient of colour change, though continuous, is known as the apparent redox potential discontinuity depth (ARPD) when reduced to an average transition point. When properly considered in conjunction with local geology and bioturbation levels, the depth and character of the ARPD can provide profound insights into the interactions between sediment geochemistry and biologic activity. Graf's review (1992) supports the early observations of Jorgensen & Fenchel (1970) that sediments can be divided into
oxic
{{Short pages monitor
The first prototype was constructed as a proof-of-concept exercise. The glass cylinder was unlikely to survive repeated use in the field. The device was subjected to a simulated SPI application: spoil mound cap monitoring. A 450 L drum was filled with fine sand from a local beach. Glutinous silt and clay-sized material was then laid down in discrete layers with the sand. A coarse-sand ‘cap’ was then laid on top and the whole drum filled with seawater. Penetration was satisfactory (13 cm of image, another 15 cm for the penetrator head), but resolution was poor as expected.
Second prototype
Experience building and testing the first prototype identified a number of key issues. The scanner technology chosen provided great depth of field (useful for identifying surface features), but required a large volume for the mirror assembly (which had to be strengthened to withstand vibrations). Furthermore, the armour, support flanges, and water pipes limited further sediment penetration and caused sediment disturbance. It was desirable to move the entire water gallery into the centre of the scanner module so that penetrator heads could be rapidly changed in the field. It was likely that different shapes would be more effective in different sediment textures and fabrics.
These decisions led to an alternate scanner technology that had been developed and marketed mostly in the early 2000s. It is known by various names such as contact imaging, direct imaging, or LED indirect exposure (US Patent 5499112). In this technology, a string of LEDs strobe the primary colours onto an imaging plane. Illumination is crucial so the imaging plane must be close. Reflected light from the imaging plane is directed into an array of light guides which lead to CCD elements. The physical arrangement between the light guides and the imaging plane is what limits the depth of field using this technology. Tests using consumer scanners indicated that the imaging plane could be 1–3 mm away from the scan head for full resolution images, but dropped off quickly beyond that. Scene features 5 mm or more away from the scan head were almost unidentifiable. Since the primary value of SP imagery is two-dimensional, this limitation was a small trade off for the great savings in space. The solid-state technology is robust to vibration and no mirrors are necessary. Unfortunately, UV illumination was difficult to provide without a custom-designed scan head and was therefore not included in the second prototype.
One major advantage of SPI is that it reliably provides sediment information regardless of water clarity. However, many SPI applications such as habitat mapping and side-scan sonar ground-truthing, would benefit from imagery of the seabed's surface when visibility permits. Since the tether provided a source of power and computer connectivity with the surface vessel, adding a digital camera to image the seabed surface immediately adjacent to the sediment profile was another conceptually simple addition. A laser array surrounding the camera provided a means to correct the geometry of the seabed surface image (since it is captured at a variable angle) and its scale. Such imagery provides a larger reference frame in which to interpret the adjacent sediment profile and permits a more informed estimation of the habitat connectivity of multiple profiles. A longitudinal section of the second prototype with the seabed surface camera is presented in Figure 11. The typical deployment configuration is shown in Figure 12.
A longitudinal section through the second prototype sediment imager.
Figure 11. A longitudinal section through the second prototype SPI-Scan imager produced b Benthic Science Limited A) electronics space, B) motor/gearing assembly connected to vertical drive shaft, C) one of five lasers, D) seabed surface CCD, E) camera pod, F) scan head, G) field-changeable penetrator with water galleries and jets, H) field-changeable cutting blade, I) scan head holder, J) central pressurised water gallery, K) transparent polycarbonate cylinder, L) water pump.
Diagram of second prototype (one leg of frame removed for clarity) as envisioned in situ with scale/geometry lasers active emanating from surface camera pod.
Figure 12. Diagram of second prototype (one leg of frame removed for clarity) as envisioned ''in situ'' with scale/geometry lasers active emanating from surface camera pod.
Field trial results
Several decisions during the design phase affected the ultimate utility of this device. The REMOTS system is well suited to providing point SP imagery in deep water from large vessels. SPI-Scan prototypes were specifically intended for shallow water work from small vessels. Although the design can be modified to work deeper, a 50 m tether was used to allow effective operations in 30 m of water. Field tests were first conducted in 29 m water depths from the R/V Munida of the University of Otago Department of Marine Science.
Image:SPIScanTrials.jpg, The second SPI-Scan prototype in field trials. Seen here deploying from the 6 m R/V Nauplius (upper left), on the seabed though locked in the up position (upper right and lower left – lasers not visible here), and starting to dig into the sand (lower right).
Figure 13. The second prototype in field trials. Seen here deploying from the 6 m ''R/V Nauplius'' (upper left), on the seabed though locked in the up position (upper right and lower left – lasers not visible here), and starting to dig into the sand (lower right).
The next set of sea trials were conducted near an aquaculture facility from a 5 m research vessel. Seventy-eight images from about 20 deployments were collected. Figure 14 presents two representative images. The digital images carry much more detail than reproduced here as Figure 15 demonstrates.
Here are two portions of sediment profiles taken 1 km from an aquaculture facility along the tidal current (left) and across (right). The right hand scale divisions are 1 mm apart.
Figure 14. Here are two portions of sediment profiles taken 1 km from an aquaculture facility along the tidal current (left) and across (right). The right hand scale divisions are 1 mm apart.
Image:SPIScanSample2.jpg, Portions of images in figure 14 are shown in panels 6, 7, and 8. Sediment texture is detailed in panel 6, a polychaete worm is evident in panel 7, and panel 8 shows ''Echinocardium'' (heart urchin) shell fragments in silt matrix. Panel 9 shows a diver giving the ‘thumbs up’ sign to the scanner to illustrate the limited depth of field of the second prototype. Poor water visibility is also in evidence by the heavy background lighting. All scale divisions are in millimetres.
Figure 15. Portions of images in figure 14 are shown in panels 6, 7, and 8. Sediment texture is detailed in panel 6, a polychaete worm is evident in panel 7, and panel 8 shows ''Echinocardium'' (heart urchin) shell fragments in silt matrix. Panel 9 shows a diver giving the ‘thumbs up’ sign to the scanner to illustrate the limited depth of field of the second prototype. Poor water visibility is also in evidence by the heavy background lighting. All scale divisions are in millimetres.
The surface computer stamped the date and time of collection directly onto the SP image. Custom software integrated an NMEA data stream from a GPS connected to the computer's serial port to also stamp the geographic position of the surface vessel (or of the device if corrected by NMEA output from an acoustic positioning beacon array). The software further uses a modification of the GEOTiff graphic standard to embed geographic position and datum information into the image tags. This permits automatic placement of SPI and seabed surface images into spatially appropriate positions when opening within a GIS package. This functionality allows real time assessment of benthic data in the field to inform further sampling decisions.
Future directions
Field trials have proven that the device produces usable images (image analysis is a separate topic covered in the broader literature). The technology is substantially more cost-effective than other existing SPI devices and able to be deployed from small vessels (ca. 5 m) by two persons operating a light frame or davit. Development of the device continues with better penetration geometries and technologies, more hydrodynamic housings, and extra sensor options. Koenig et al. (2001) reviewed some exciting developments in optical sensors (also known as optodes or reactive foils) capable of resolving sub-centimetre oxygen distribution (using the non-consumptive ruthenium fluorescence method) and pH. Very small redox (Eh) probes have also been available for quite some time. Vopel et al. (2003) demonstrated the utility of combining such instruments in studying animal-sediment interactions. These instruments can be integrated into the sediment imager relatively easily and would allow absolute quantification of sediment geochemical profiles at a small number of sites to inform the analysis of the surrounding SP images. Adding UV illumination is only a manufacturing issue. UV capabilities could extend the role of SPI in direct pollution monitoring of harbours or assessing the effects of petrochemical spills. SP image resolution is high enough to permit sediment tracer studies without expensive dyeing if the tracer mineral presents unique colour or fluorescence characteristics.
Keegan et al. (2001) pointed out that chemical and physical environmental measurements alone are easily quantified and readily reproducible, but are overall poor monitors of environmental health. Biological and ecological theory is well enough advanced to be a full partner in environmental legislation, monitoring, and enforcement (Karr 1991) and can provide the appropriate local context for interpretation of physico-chemical results. In a typical assessment of mariculture impacts on the benthos Weston (1990) found that sediment chemistry (CHN, water-soluble sulfides, and redox measures) measures of organic enrichment effects extended only 45 m from the farm, but benthic community effects were apparent to 150 m. SPI can elucidate many of these important biological parameters. Benthic Science Limited continues development of SPI-Scan technology.
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