  This study concerned conebeam travel-time tomography. The focus was on a
sparse distribution of signal sources that can be necessary in a challenging in
situ environment such as in asteroid tomography. The goal was to approximate
the minimum number of source positions needed for robust detection of
refractive anomalies, e.g., voids within an asteroid or a casting defects in
concrete. Experimental ultrasonic data were recorded utilizing as a target a
150 mm plastic cast cube containing three stones with diameter between 22 and
41 mm. A signal frequency of 55 kHz (35 mm wavelength) was used. Source counts
from one to six were tested for different placements. Based on our statistical
inversion approach and analysis of the results, three or four sources were
found to lead to reliable inversion. The source configurations investigated
were also ranked according to their performance. Our results can be used, for
example, in the planning of planetary missions as well as in material testing.
