Initial exploration of this idea began in August 2023.
Existing commercial products
First up, I wanted to see what other people were doing. A quick search narrowed in on "tree tomography" as a term of art; in turn, that turned up a number of very interesting YouTube videos from companies selling equipment.
The first shows how one company collects sound from an array of sensors around the tree trunk. Impacts are generated by a hammer on one nail, then collected at sensors attached to other nails around the circumference of the trunk. The operator then shifts the impact site to the next nail, and so on until all the nails have been hit. Time-of-flight data (I think) is then integrated to generate a map (or tomograph) of the inside of the trunk:
This video has a good illustration of what that final tomograph looks like:
Finally, this is a much simpler system; it just looks at time-of-flight between two points, and leaves the operator to construct their own "mental tomograph" (so to speak) of the inside of the trunk:
I thought the idea of using a section of trunk for demonstrating the principle was quite good, and I hope to get something similar to experiment with.