ESEM/SEM Bubble Size Analysis and Distribution
The size distribution of bubbles within fine lapilli to ash size tephra particles may be determined through analysis of ESEM or SEM images using readily available software like ImageJ. The method includes the use of an ImageJ macro designed in order to analyze all the images contained in one folder.
Acquiring good images from the samples is a good start and avoids wasting of time on later processing and helps to clearly discriminate the bubbles from the background. In order to do this there should be sufficient contrast between the bubbles and the grains in which they are enclosed, for this reason the sample preparation and the choice of imaging parameters are important.
Thin sections were made from the samples and then carbon coated. BSE images were taken at the magnification and resolution required to discriminate the majority of the bubbles. The magnification was chosen taking into account the size of the volcanic particles and relative vesicles' dimension in order to have the largest number of vesicles visible in the images. At least 500 bubbles per samples have been collected.
On the microscope, brightness and contrast were then set in order to maximize the signal. The images produced are of sufficient contrast to ensure a good thresholding during the image processing, but they don't usually have enough quality to be published.
ImageJ for BSD
A macro was designed in order to automatically process all the images captured with the ESEM or SEM and included in the same folder. In order to adjust the image aspect ration it will be necessary to previously calculate the micronsXpixel ration and the scalar unit according to the characteristics of the captured images and give these parameters as input to the macro.
The result is an image showing the bubble outlines overlaying the resized original image. It also produces a comma separated values report listing shape parameters, Feret's diameters, fitted ellipse dimensions and volume for every particle found; this report is a .cvs file and it can be opened and further analyzed by Microsoft Excel or OpenOffice.calc. The macro will create a uniquely named set of images and report each time it is run if it finds previous results in the folder. This allows easier side-by-side comparison of results from changes in user-selectable analysis parameters. All the results are saved in the same folder as was selected for the analysis.
The macro presents the user with a dialog box having user-selectable values used during the analysis. Default values are those found to have worked best for the research the macro was written for and may not produce suitable results with your sample images.
Following is the step-by-step methodology used by the macro to count bubbles.
- Reduce the y dimension by the amount required to produce a square pixel having square micron dimensions.
- Invert the image.
- Threshold the image using first an autothreshold method in order to give the possibility to calculate an average value and then run again the macro with that value that seems to represent the best threshold process.
- Fill holes.
- Analyze particles including or excluding bubbles intersecting the edge of the image.
- Bin results per a comma separated list of bins (optional).
- Create and save an image composite of the resized original image overlain by the particle outlines.
- Compile results in a single .csv file in the same folder as selected for analysis.
Macro availability and usability
The macro is available as .zip file in here. Download the file, exapnd and copy it into the macro folder in the ImageJ folder. Install or run the macro under the ImageJ Plugins menu item.