Particle Insight
PI App notes

PARTICLE SHAPE APPLICATION EXAMPLE: Ocean Sediments


The challenge


Ocean floor sediments are studied by marine scientists. They are composed of fine particles of sand, clay, volcanic ash, minerals and organics. The small grain size makes measurement a challenge. Two different kinds of sediment were studied for this example.

Applicable measures
 

Measure

Equivalent Circular Area Diameter
Equivalent Elliptical Area Width
Equivalent Elliptical Area Length
Ellipse Aspect Ratio
Circularity
Smoothness
Range of acceptance

3 - 500 microns
3 - 500 microns
10 - 500 microns
1 to 30
0 to 1.0
0 to 1.0

The first sample is made up of very fine grains below 50 microns in size. This is more like a silt than sand grains. A typical image has a high concentration of tiny particles:

Image of fine marine sediment.

A sample of thumbnails looks like:

Typical individual sediment grains.

With a particle concentration this high, clusters of particles are sometimes counted as a single unit. The faint thumbnails seen here are such clusters. They can be bypassed in a reanalysis of the sample.

The Circle shape model is best for such small sizes. Just three measures are sufficient for this first sample: ECA diameter, Circularity and Smoothness. We add Ellipse Aspect Ratio to see if there are non-spherical objects present. The unfiltered results of this sample show an uneven distribution of sizes below 50 microns:

Sediment analysis results.

To bypass clusters, reanalyze the sample with these constraints: circularity greater than 0.35, smoothness greater than 0.4, and rectangularity greater than 0.65. With that filter, a sample of thumbnails has this appearance:

Typical individual sediment grains after applying filter.

The filtered reanalysis gives a size distribution that is mostly between 10 and 50 microns. Circularity and smoothness are considerably higher now.

Sediment analysis results with filter applied.



The second sample has slightly larger and better-defined particles. This is a typical image:

Raw image from the second sample.

Particle thumbnails show crystalline grains with an aspect ratio of about 2:1.

Typical individual sediment grains after applying filter.

Fitting ellipses to these grains gives a good measure of length and width (numbers shown are EEA length).

Typical individual sediment grains after applying filter.

Filtering is not needed for this sample. The size and shape analysis results are below. Particle lengths are between 10 and 200 microns. Circularity is centered around 0.6, reflecting the non-spherical shapes.

Sediment sample 2 analysis results.