Kevin Jourde

Implementation of sub-nanosecond time-to-digital convertor in field-programmable gate array: applications to time-of-flight analysis in muon radiography

Time-of-flight (TOF) techniques are standard techniques in high energy physics to determine particles’ propagation directions. Since particle velocities are generally close to c, the speed of light, and detector typical dimensions at the metre 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 time-to-digital converter (TDC) ones, implemented in low-cost programmable logic circuits like FPGA (field programmable gate array), feature a very interesting figure of merit since a very good timing performance may be achieved with limited processing resources. This issue is relevant for applications where unmanned sensors should have the lowest possible power consumption. Actually this paper describes in detail 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 muon 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 forecasts. This paper describes a TDC implementation fulfilling the requirements of a TOF measurement applied to muon tomography.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. PACS numbers: 1315, 9440T ar X iv :1 31 0. 42 81 v1 [ ph ys ic s. in sde t] 1 6 O ct 2 01 3 FPGA-TDC in muon radiography 2

Improvement of density models of geological structures by fusion of gravity data and cosmic muon radiographies

Both muon tomography and gravimetry are geophysical methods that provide information on the density structure of the Earth’s subsurface. Muon tomography measures the natural flux of cosmic muons and its attenuation produced by the screening effect of the rock mass to image. Gravimetry generally consists in measurements of the vertical component of the local gravity field. Both methods are linearly linked to density, but their spatial sensitivity is very different. Muon tomography essentially works like medical X-ray scan and integrates density information along elongated narrow conical volumes while gravimetry measurements are linked to density by a 3-dimensional integral encompassing the whole studied domain. We develop the mathematical expressions of these integration formulas – called acquisition kernels – to express resolving kernels that act as spatial filters relating the true unknown density structure to the density distribution actually recoverable from the available data. The resolving kernels provide a tool to quantitatively describe the resolution of the density models and to evaluate the resolution improvement expected by adding new data in the inversion. The resolving kernels derived in the joined muon/gravimetry case indicate that gravity data are almost useless to constrain the density structure in regions sampled by more than two muon tomography acquisitions. Interestingly the resolution in deeper regions not sampled by muon tomography is significantly improved by joining the two techniques. Examples taken from field experiments performed on La Soufrière of Guadeloupe volcano are discussed.

Muon dynamic radiography of density changes induced by hydrothermal activity at the La Soufrière of Guadeloupe volcano

Imaging geological structures through cosmic muon radiography is a newly developed technique which shows a great potential in volcanology. Here we demonstrate that muon radiography permits to detect and characterize mass movements in shallow hydrothermal systems of low-energy active volcanoes like the La Soufrière lava dome. We present an experiment conducted on this volcano during the Summer 2014 and bring evidence that very important density changes occurred in three domains of the lava dome. Depending on their position and on the medium porosity the volumes of these domains vary from 1 × 106 m3 to 7 × 106 m3. However, the total mass budget remains approximately constant : two domains show a mass loss (Δm∈ [−0.8;−0.4] × 109 kg) and the third one a mass gain (Δm∈ [1.5; 2.5] × 109 kg). We attribute the negative mass changes to the formation of steam in shallow hydrothermal reservoir previously partly filled with liquid water. This coincides with the emergence of new fumaroles on top of the volcano. The positive mass change is synchronized with the negative mass changes indicating that liquid water probably flowed from the two reservoirs invaded by steam toward the third reservoir.

Monitoring temporal opacity fluctuations of large structures with muon tomography : a calibration experiment using a water tower tank

Usage of secondary cosmic muons to image the geological structures density distribution significantly developed during the past ten years. Recent applications demonstrate the method interest to monitor magma ascent and volcanic gas movements inside volcanoes. Muon radiography could be used to monitor density variations in aquifers and the critical zone in the near surface. However, the time resolution achievable by muon radiography monitoring remains poorly studied. It is biased by fluctuation sources exterior to the target, and statistically affected by the limited number of particles detected during the experiment. The present study documents these two issues within a simple and well constrained experimental context: a water tower. We use the data to discuss the influence of atmospheric variability that perturbs the signal, and propose correction formulas to extract the muon flux variations related to the water level changes. Statistical developments establish the feasibility domain of muon radiography monitoring as a function of target thickness (i.e. opacity). Objects with a thickness comprised between ≈50 ± 30 m water equivalent correspond to the best time resolution. Thinner objects have a degraded time resolution that strongly depends on the zenith angle, whereas thicker objects (like volcanoes) time resolution does not.

Muon tomography applied to activevolcanoes

Muon tomography is a generic imaging method using the differential absorption of cosmic muons by matter. The measured contrast in the muons flux reflects the matter density contrast as it does in conventional medical imaging. The applications to volcanology present may advantadges induced by the features of the target itself: limited access to dangerous zones, impossible use of standard boreholes information, harsh environmental conditions etc. The Diaphane project is one of the largest and leading collaboration in the field and the present article summarizes recent results collected on the Lesser Antilles, with a special emphasis on the Soufri\`ere of Guadeloupe.

Three-dimensional density structure of La Soufrière de Guadeloupe lava dome from simultaneous muon radiographies and gravity data†

Muon imaging has recently emerged as a powerful method to complement standard geophysical tools in the understanding of the Earths subsurface. Muon measurements can yield a “radiography” of the average density along the muon path, allowing to image large volumes of a geological body from a single observation point. Here we jointly invert muon data from three simultaneous telescope acquisitions together with gravity data to estimate the three-dimensional density structure of the La Soufriere de Guadeloupe lava dome. Our unique datas et allows us to achieve an unprecedented spatial resolution with this novel technique. The retrieved density model reveals an extensive, low-density anomaly where the most active part of the volcanic hydrothermal system is located, supporting previous studies that indicate this region as the most likely to be involved in a partial edifice collapse.