  Time-of-flight (tof) techniques are standard techniques in high energy
physics to determine particles propagation directions. Since particles
velocities are generally close to c, the speed of light, and detectors typical
dimensions at the meter level, the state-of-the-art tof techniques should reach
sub-nanosecond timing resolution. Among the various techniques already
available, the recently developed ring oscillator TDC ones, implemented in low
cost FPGA, feature a very interesting figure of merit since a very good timing
performance may be achieved with limited processing ressources. This issue is
relevant for applications where unmanned sensors should have the lowest
possible power consumption. Actually this article describes in details the
application of this kind of tof technique to muon tomography of geological
bodies. Muon tomography aims at measuring density variations and absolute
densities through the detection of atmospheric muons flux's attenuation, due to
the presence of matter. When the measured fluxes become very low, an identified
source of noise comes from backwards propagating particles hitting the detector
in a direction pointing to the geological body. The separation between
through-going and backward-going particles, on the basis of the tof information
is therefore a key parameter for the tomography analysis and subsequent
previsions.
