In a previous question What happens to the higher frequency content of the tremors associated with a volcanic eruption?, I asked about the high(er) frequency content of a volcanic eruption.
In the same vein, I'm interested to know more about the influence of the geometry of the magma column on this particular frequency content and the methodologies that researchers use to take measurements of circular vents. Assuming a uniform cylinder all the way to the magma chamber would aid in the mechanical modeling, but I'm sure this betrays quite a bit of the changes in diameter that take place along the way.
Are precise measurements of the vents/magma conduits of volcanoes currently possible, or is this too big of a risk to personnel and equipment? Which particular techniques (e.g., radar/sonar) would work without having to actually enter the chamber?