Instrumentation in sediment transport Lecture 31

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Instrumentation in sediment transportLecture 31
Towed Survey Tools (regional reconnaissance) Seism
ic surveying Sidescan imaging multibeam (swath)
bathymetry
Sampling Tools (ground truthing) Coring
devices Grab samplers, water samplers Box corers
In situ instrumentation (process/response
studies) fully digital, high sampling
rates self-logging, complex data sets seabed and
water column sensing
Time and space-resolution
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Surveying methods in regional reconnaissance

• CSS Hudson geophysical survey vessel (offshore)
• single channel seismic surveying
• multi-channel seismic surveying
• analogue/digital systems
• 3-D seismics (DGPS)
• Autonomous vehicles (remote)
• Autosub (SOC)
• Dolphin (BIO)
• swath surveying (0-6000 m)
• 7 days power
• speciality payloads
• chlorophyll
• turbulence
• backscatter
• temperature/salinity

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Swath bathymetry in sediment transport
megaripples
medium sand
sand waves
(1)
(2)
coarse sand

• Bay of Fundy (Scotts Bay)
• (1) shadow relief B/W bathymetry
• tidal sand ridge, with
• flood and ebb oriented sand waves
• flood oriented megaripples
• convergence of sand pathways
• directions of sand transport
• bedrock outcrop in tidal channel
• (2) colour coded backscatter
• grain size
• bed roughness
• bedform patterns
• key sites for process studies
• bounds for extrapolation of results

bedrock channel
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Swath bathymetry - Halifax Harbour

• scouring of main tidal channel
• debris on seabed
• Bedrock Island
• region of dredging
• flood-oriented seabed scour
• natural harbour sediments

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IKU clam-shell grab sampling

• Medium and large IKU grab samplers
• 1 m3 of surface sand
• largely undisturbed, therefore valuable for
  sub-sampling
• push cores
• Reineck box cores (3-D)
• surface sediment samples
• biological sampling
• sedimentary structures in surface mobile layer
• provides link between hydrodynamics, bedforms,
  and geological structures

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Instrumentation for sediment transport

• acoustical
• optical
• electro-magnetic/peizo-electric
• x-ray

• multi-parameter benthic landers
• benthic flumes
• remote sensing tools (turbidity)
• self-contained packages

time-series, usually from fixed location (1)
External sources/sinks (5)
Velocity field (3-D) (2)
Benthic flux (4)
Mass settling rate (3)
Sediment concentration gradients (3-D) (6)
The sediment continuity equation
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RALPH - multi-parameter benthic lander

• Battery packs (12VDC) and electronic driver
  boards
• relocation pinger (12 kHz)
• EMCM 2-D velocity probes (4)
• OBS backscatter sensors (6)
• pressure transducer (waves)
• 4 acoustic altimeters
• 1 sector scanning sonar
• 1 profiling sonar
• 1 digital time-lapse camera
• tilt and roll sensors
• fluxgate compass
• acoustical up-link (1200 baud)

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• Marsh McBirney EMCM current meters
• 2 horizontal components of flow
• up to 5 Hz sampling rate
• 2.5 cm head diameter
• 4 sensors spaced with log(height)
• boundary layer velocity gradient
• bed shear stress estimations
• turbulent frequencies
• problems
• zero drift
• head size minimises sample rate and size of
  turbulent eddies

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IMAGENEX sector scanning sonar

• Up to 2 MHz acoustical output
• fan beam signal
• image of seabed
• backscatter intensity of bed
• 360 degrees (1 degree resolution)
• 20 m range in signal
• 1 cm vertical resolution
• fully digital (0.5 Mbytes/image)
• 1000 image memory storage
• 12 VDC power input
• stand-alone or PC interfaced
• rotating fan beam head

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DA Optical Backscatter sensors

• OBS sampling array for
• vertical distribution of sand
• deep red LED light source
• photo-electric detectors
• 10 cm sampling length
• 10 Hz sampling rate
• problems
• sensitive to sediment size
• highly sensitive to sediment colour
• need in situ calibration

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RALPH - other sensors

• Downlooking hi-8 camera
• 2400 images on film (1 x 1.5 m)
• downlooking digital video camera
• 1000 frames (460 x 680 pixels)
• profiling Imagenex sonar
• 5 m range
• 0.2 cm vertical resolution
• backscatter in water column
• backscatter from seabed
• 1 minute sampling rate

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• digital video camera (DULSE)
• Mesotek acoustic altimeters
• pencil beam (2 MHz)
• 10 Hz sampling rate
• 1 cm bin size
• backscatter/sand in suspension
• depth changes to ? 0.5 cm

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ADCP (Acoustic Doppler Current profilers)

• RD instruments (ADCP)
• 0.25 m bin sizes
• low frequency sampling (0.5 Hz)
• from 2-100 m depth ranges
• 3-D flow structure
• backscatter/turbidity
• traversing (ship speed removal)
• Sontek (ADP)
• 2 MHz
• 1 cm bin sizes
• 10 Hz sampling rate
• 3-D flow (boundary layer)
• backscatter

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Sea Carousel - a benthic annular flume

• Power pack and electronics storage
• benthic flume (15 cm x 30 cm x 1 m)
• 3 OBS sensors (3 heights) - 1 Hz
• tangential velocity - 1 Hz
• vertical velocity - 1 Hz
• 3-D turbulence - 25 Hz
• communication cables, power, pumped water
  samples, lifting
• interfaced to PC data controller/logger
• motor controller, water pressure
• low resolution video, pitch/roll
• Sontek ADV data
• digital stepping drive motor
• accurate speed and acceleration

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• high resolution video camera
• particle tracking
• turbulence
• erosion type
• shaft end-coder (lid rotation)
• mechanical motion of lid
• ballast (lead)/penetration skirt
• port for pumped water samples

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• Deep-water deployment strategy
• communications (100 m)/RS232
• water sampling tube
• reaction weight against cable drag
• springs for control while deploying

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CT scanning in sediment dynamics

• Computer tomography
• multiple path scanning
• X-ray attenuance
• Photoelectric effect
• atomic density related
• Compton effect
• atomic weight related
• Hounsfield Unit (HU)
• 3-D real time imaging
• DICOM3.0 format
• medical imaging standard
• multi-system compatible
• portable for image analysis

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CT scanner specifications

• 0.5 s scan rates
• 0.5 mm slice thickness
• mobile gantry
• real-time 3-D imaging
• overlapping/contiguous
• topogram length - 1024 mm
• HU scale -1024 to 3071
• Extended HU scale x 10
• image res 1024 x 1024
• image rate 10/s
• scan field 500 mm

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CT-derived bulk density (Orsi, 1994)

• CT number (1 HU/1000)
• CT 0 (air)
• CT 1 (water)
• CT 2 (sediment)
• CT 3 (rock)
• linear response to marine sediments (clastics or
  carbonates)
• independent of grain size
• dependent on salinity of porewater

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Inshore surveying - small boat

• Differential GPS ( 1 m accuracy)
• high-resolution sub-bottom profilers
• Seistec, Huntec, Imagenex
• 1 m water depths
• digital control
• PC-based
• on-screen profiles
• software processing packages
• high-frequency sidescan sonars
• 500 - 2000 kHz
• Imagenex, Klein, EGG
• digital control
• PC-based
• 12 VDC (battery)

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Deep water geophysical surveying for sediment
transport studies

• Multiple systems/multiple frequencies
• sidescan (seabed imagery/bedforms)
• surface patterns only
• 0.1 m spatial resolution
• may not be relevant to processes active
• Huntec (sub-bottom profiler 0-10 m)
• good in mud
• not good in sand
• high resolution (0.1 m)
• does not discriminate cross-bedding
• guide for sampling
• airgun seismics (5-100 m)
• major lithological changes
• 1 m resolution
• formational outcrops at seabed
• guide for sampling

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Strategic seabed sampling

• recovery of seabed material
• calibration/classification of survey tools
• design of process studies
• fundamental surface samplers
• grab samplers (sand/mud)
• Eckman
• van Veen
• IKU
• Videograb
• dredges (rock/gravel)
• ROV/submersible
• box corers
• fundamental sub-surface samplers
• drills (rock)
• rotary drills (unconsolidated sediments)
• piston corers (clay)
• gravity corers (mud)
• Vibrocorers (sand)

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Vibrocoring sandy sediments

• Vibrating head
• core barrel (3 m)/sliding gantry
• core cutter/core catcher
• supporting base
• pivoting corer for removal of barrel and sample

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Box coring soft/muddy sediments

• Box corer cocked for deployment
• retraction mechanism
• sliding mechanism/bucket penetration
• rotating cutter blade
• sample bucket
• support frame
• ideal for biological assays, geochemical
  analyses, sedimentary fabric studies
• works in mud only

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Box corer after recovery

• Full retraction of bucket
• access windows
• lugs to remove bucket sample
• cutter fully in place
• heavy/difficult to deploy and recover
• difficult to remove samples without disturbance
• not oriented wrt north

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Long-coring device (piston corer)

• Pilot gravity corer (with flights and open core
  barrel for ventilation)
• core weights (lead donuts)
• threaded collar for barrel attachment
• core barrel with plastic liner
• piston corer head in horizontal position
• deployment frame

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Piston corer upon recovery

• 2000 lb coring weight
• open ocean sampling
• soft muddy sediments
• recovery up to 30 m
• possible disturbance high
• can provide a long/continuous record
• depth of penetration of barrel
• threaded collar for barrel segments

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Core segment upon recovery

• Plastic core liner with sediment sample
• cut into 2 m segments
• annotation/way-up important
• storage vertical
• 4C temperature control
• contamination by plastic liner
• structures difficult to see
• limited sample volume from each level
• not oriented wrt north
• compaction usually high (unknown)

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Core descriptions/analyses

• Core cutting (longitudinal)
• core description
• structure
• composition
• colour
• inclusions
• core photography
• core sub-sampling
• granulometry
• micropaleontology
• C14 age dating
• physical properties
• geotechnical properties
• provides regional stratigraphic control
• modern versus relict (long-term)

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Video grab for selective sampling

• Video grab for selective sampling of seabed
• real-time video camera
• hydraulic retraction machanism
• sampling clam shell bucket

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BOSS (Benthic Organic Seston Sampler)

• single line deployment, trigger weight released
  once on seabed
• vane to rotate sampling array into flow
• sampling array of 100 ml syringes
• (damped and timed asperation of sample by plunger
  retraction)
• array frame

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BOSS sampling array

• Water samples of benthic boundary layer
• samples every 2 cm from about 5 cm above bed,
  used for
• defining profiles of suspended solids
• defining profiles of organic matter
• sampling benthic fluff layers
• synoptic undisturbed sampling for floc analyses

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• (5i) Deposition (D) from cores (Pb210 or Cs137)
  or sediment traps or altimeter measures
• (5ii) Erosion (E) ?????? (closed systems) or
  acoustic profiling or long-term swath bathymetry
  (accuracy, resolution)
• (6) Sediment traps, geophysical surveys of
  bedform migration

(1) only possible in closed system !!!
(mesocosm) (2) bottom sampling (3) acoustic
velocity, resistivity, CT scanning (4) acoustic
altimeter, 2-axis sector scanning
sonar repetitive swath bathymetry
Time-series of bed level change (4)
Benthic flux (1)
Sediment density (2)
Porosity (3)
Divergence in bedload transport rate (6)
Deposition(D) Erosion(E) (5)
The Exner equation
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Instrumentation on sediment transport
time-series, usually from fixed location (1)
Velocity field (3-D) (2)
External sources/sinks (5)
Benthic flux (4)
Mass settling rate (3)
Sediment concentration gradients (3-D) (6)
(1) Fixed moorings/benthic landers (custom
made) (2) Traversing ADCP (RDI) or profiling ADV
(Sontek) (3) In situ settling columns, INSSEV,
benthic flumes (4) Settling traps (D) and benthic
flumes (E) (5) Sequential sector scanning sonars,
profiling sonars (also geophysical tools) (6)
Backscatter from ADCP or remote sensing or
profiling sensors
Orchestrated suite of tools