1.5 GRAIN SIZE ANALYSIS METHODS

Question 1

What questions must be addressed during formulation of a strategy for analysing grain size of sediment samples?

Answer 1

• what pre-treatments are required? (do we want to get rid of organic matter or carbonate?)
• what is the grain size and size range of samples?
• how much volume of sample material is available for analysis?
• what are the relative merits of techniques that measure simple physical size or particle volume vs settling diameter?

Question 2

What physical basis and size range is appropriate for callipers?

Answer 2

• Physical measurement (DIRECT)

- Pebble +

Question 3

What physical basis and size range is appropriate for sieving (sieve diameter)?

Answer 3

• Physical measurement (INDIRECT)
• Sand & granule (silt > 10 microns)

need to split sample to correct size if necessary

Question 4

What physical basis and size range is appropriate for pipette analysis/ settling tower/balance/ sedigraph?

Answer 4

• Settling
• Silt/(clay)/ sand/ silt/(clay)

for all techniques based on settling, concentration is CRITICAL - should be <1%

Question 5

What physical basis and size range is appropriate for electrical sensing zone (coulter counter/particle data)?

Answer 5

• Particle volume (resistivity)
• silt ((clay)) / (sand)

coulter counter advantages: produces size & number data, can analyse small sample volumes

Question 6

What physical basis and size range is appropriate for laser size analyser?

Answer 6

• Diffraction of light
• Silt (clay) / (sand)
• large particles scatter light at narrow angles with high intensity
• small particles scatter at wider angles but with low intensity

advantage of laser diffraction: some can be used in situ e.g. deployed from small boats; can measure natural size populations; minimal damage to paticles/aggregates

Question 7

What is used for separation of different size fractions?

Answer 7

CENTRIFUGATION (CALIBRATED)