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Behaviour of aggregates using digital laser holography.
David M. Paterson, Giselher Gust (Technical University of Hamburg/Harburg)
Mike Player and John Watson (University of Aberdeen)
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Project Motivation
All natural sediment deposits, whether deep sea, coastal or riverine support a biota of highly adapted organisms. Even sediments deep within the ocean bed support a "deep biota" of bacteria living and reproducing under extreme and inhospitable conditions. The study of depositional systems as a habitat and resource for benthic organisms has a long history of research but it is only in the last decade that interdisciplinary programmes (eg LISP, LOIS) have attempted to establish and predict the influence of organisms in mediating the response of sediments to hydrodynamic forcing (Paterson and Black, 1999a and b). Most of this work has focused on arguments concerning the sedimentological determination of the response of the sediment bed to fluid stress versus the potential of biological processes to influence stability (e.g. biomass, organic content, polymeric content).
However, these arguments are bound to continue until the erosion process can be investigated at an appropriate temporal and spatial scale under defined conditions. The problem of studying the biological influence on sediment erosion requires the combination of three area of expertise. Firstly, the hydrodynamics of the system must be highly defined, controllable and replicable. Secondly, the process of erosion, once initiated under characterised flow, must be recorded on a suitable temporal and spatial scale. Thirdly, biological variables and process must be introduced into the test system in a controlled manner. The PI's are now at a stage when the required technology and experience is in place to examine the process of sediment erosion on a scale and resolution that has not yet been equalled. This work will provide entirely new information describing the process of sediment erosion for both biotic and abiotic sediments and will lead to a new understanding of the nature of biogenic mediation of sediment erosion.
The new steps that make digital laser holography a practical possibility are the introduction of electronic recording and digital reconstruction. In conventional holography, the hologram is recorded on photographic emulsion; a high-energy pulsed laser is essential to “freeze” motion and provide adequate illumination (Sun et al. 2002). In DLH, the photographic plate is replaced by an electronic (CCD or CMOS) imaging sensor, and physical reconstruction is replaced by numerical reconstruction using algorithms that simulate the physical processes of light propagation and diffraction. Thus, not only can video holographic sequences be recorded, but complete reconstructions can be achieved in near real-time, without the need for wet processing or mechanical scanning. In addition, the high sensitivity of the electronic imaging sensor allows smaller and less costly lasers to be used for recording.
DLH is currently restricted by available sensor resolution to “in-line” or to very narrow angle “off-axis” recording geometries. However, in-line geometry is already preferred in this application for its superior resolution (Sun et al. 2002). DLH does suffer two disadvantages in comparison with photographically-recorded in-line holography: the sample volume is restricted by the size of the imaging sensor, and resolution is restricted by the sensor pixel pitch. Both of these issues are being resolved as large-area multi-megapixel sensors become increasingly available, and the resolution restriction can also be traded off against sample volume by choice of recording optics and reconstruction algorithm (Dong et al. 2004). Our existing prototype system for digital laser holography (DLH) utilises in-line geometry with low-power continuous (CW) laser illumination, provided either by a laser diode or a low-power HeNe laser (below).
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Play video clips of laser holography
Brief explanation: The video clips are 2-dimensional representations of holographic images. Each frame of the video contains the information to reconstruct the three dimensional image capture at the time. This is therefore a three dimensional video, but we can only provide a 2 d image on the web without a laser system to replay the scene. However, we can reconstruct this image electronically and interpret it using data analysis systems. |
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Project Aims and Objectives
We have proposed a new programme of work. The objectives of the new programme are:
1. To establish and utilise digital pulsed laser holography (DLH) as an important tool to study the process of incipient erosion and aggregate suspension and resilience.
2. To determine the influence of biofilm type on the nature of cohesive aggregate material eroded from the bed and their resilience under varying conditions of turbulent flow.
3. To determine the erosion criteria and resilience of faecal pellets eroded from the bed under varying conditions of turbulent flow.
4. To determine detailed time history of individual aggregates (both faecal and biofilm produced) eroded from the bed under varying conditions of turbulent flow.
Scientific and socio-economic benefits.
The prediction of sediment transport pathways and patterns has major implications for the sustainable management of marine and coastal systems. Sediment erosion and transport influences:
- Maintenance of navigation channels
- Distribution of sediment-borne pollutants
- Coastal erosion
For the above reasons efforts to understand the natural behaviour of particles and aggregates under stress is critical. This information allows the models of sediment erosion and transport to be refined and in some cases revised. Recently a new integration of modelling effort has led to the development of a bio-sedimentary modelling approach which includes the influence of biological processes (bio-stabilisation, bio-turbation; e.g. Orvain et al, 2003).
The use of laser holography for the study of sediment dynamics has been pioneered by the team from St Andrews and Aberdeen and we believe we are the first research group in the world to be working on laser holography as an approach to study the micro-processes of sediment erosion and transport.
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DLH system. The object volume can incorporate either a CSM (bottom, left) or a Gust “microcosm” (bottom, right). The systems can be combined where the incipient erosion threshold of the bed is greater than the maximum bed shear stress produced by the microscom.
The hologram is recorded on a standard analogue camera with 768´576 pixel CCD sensor (CCIR monochrome format), and digitised by an 8-bit frame grabber. Although this arrangement has operated sufficiently well to prove the concept of DLH for recording erosion events, its components are not state-of-the-art, reducing its overall performance. In particular, the use of CW laser illumination limits the ability to “freeze” rapid movements, and in fact the system relies on the camera’s electronic shuttering (100 ms compared to 50 ns for a typical pulsed laser) to provide any motion tolerance. This is turn limits the imaging sensor in other respects, since larger sensors do not in general support fast electronic shuttering. A more satisfactory design would use a pulsed laser in conjunction with a large high-resolution sensor.
Low-temperature scanning electron micrographs of faecal pellets. A: Hydrobia ulvae. B. Detail of H. ulvae pellets. C. Corophium volutator pellets. D. Detail of C. volutator pellets. (Source: SERG) |
However, a ruby laser as used in our previous photographically-recorded holography is unsuitable, as ruby lasers are intrinsically limited to very low pulse repetition rate: instead a diode-pumped Q-switched frequency-doubled Nd:YAG laser is proposed. This will enable stroboscopic illumination at up to video frame rates. Additionally, we will use a multi-megapixel sensor with at least 10 bits per pixel digitisation.
Refs:
- Sun, H., Perkins, R.G., Watson, J., Player, M.A. and Paterson, D.M. (2004). Observations of coastal sediment erosion using in-line holography. J.Optics A. Pure Appl. Opt. 6: 703-710.
- Kovalchuk A, Grandjean V, Hurst A, Player MA & Watson J (2004) Characterisation of the physical properties of porous media using monochrome imaging SPIE Proceedings 5477 (Correlation Optics 2003) 138-150
- Dong H, Khong C, Player MA, Solan M & Watson J (2004) Algorithms and applications for electronically-recorded holography SPIE Proceedings 5477 (Correlation Optics 2003) 354-365
- Perkins, R.G., Sun, H., Watson, J., Player, M.A., Gust, G. and Paterson, D.M. 2004. In-line laser holography and video analysis of eroded floc from artificial and estuarine sediments. Env. Sci. Technol. 38: 4640-4648.
- Paterson, D.M. & Black, K.S. (1999a). Siliclastic Intertidal microbial sediments. In: Microbial Sediments (Riding. R.E & Awramik, S.M. (Eds). Spriger Verlag, Berlin. pp 217-241.
- Paterson, D.M. & Black. K. S. (1999b). Water flow, sediment dynamics, and benthic biology. In: Advances in Ecological Research (Raffaelii, D and Nedwell, D. eds). OUP. Oxford 155- 193.
- Black, K.S., Craig, G., Paterson, D.M., Watson, J. Visualisation of sediment transport processes using underwater laser holographic inspection. Environmental Science and Technology, in press).
Note: Dr Rupert Perkins completed the work on NER/A/S/2000/00513 and we would like to acknowledge his excellent work and wish him success in his new position at Cardiff University (contact: PerkinsR@Cardiff.ac.uk)
A Multidisciplinary
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