Semi-automated estimates of area of grow in circles

I was wondering whether there is an automated (or semi-automated) process by which I can estimate area of “concentric” circles of growth on an agar plate? The image shows growth as outlined in color by fine-tip marker, each color representing a different time. I can trace the circles to get area as well as make X-Y axes from the center to get diameter. Any suggestions for automated measures? In using threshold, I found that the width of the fine marker adds a second circle due to this new edge; I would need to eliminate those new lines, too.
I checked topics already but they seem to be for “set areas,” not ones where the circles are not consistent.

Can you provide a sample image?

That’s embarrassing. I meant to attach an image.

Thanks for asking.


Could you explain a bit more about how you tried thresholding them? That would have been the first step in my suggestion.

You may be interested in I would also recommend working on the image acquisition a bit since I can see glare and reflection in the petridish.