Abstract. This study is the first trial to apply the method of filtered back projection (FBP) to reconstruct three-dimensional (3-D) bulk density images via cosmic-ray muons. We also simulated three-dimensional reconstruction image with dozens of muon radiographies for a volcano using the FBP method and evaluated its practicality. The FBP method is widely used in X-ray and CT image reconstruction but has not been used in the field of muon radiography. One of the merits of using the FBP method instead of the ordinary inversion method is that it does not require an initial model, while ordinary inversion analysis needs an initial model. We also added new approximation factors by using data on mountain topography in existing formulas to successfully reduce systematic reconstruction errors. From a volcanic perspective, lidar is commonly used to measure and analyze mountain topography. We tested the performance and applicability to a model of Omuroyama, a monogenetic scoria cone located in Shizuoka, Japan. As a result, it was revealed that the density difference between the original and reconstructed images depended on the number of observation points and the accidental error caused by muon statistics depended on the multiplication of total effective area and exposure period. Combining all of the above, we established how to evaluate an observation plan for volcanos using dozens of muon radiographies.
Abstract. One of the key challenges for muographic studies is to reveal the detailed 3D density structure of a volcano by increasing the number of observation directions. 3D density imaging by multi-directional muography requires that the individual differences in the performance of the installed muon detectors are small and that the results from each detector can be derived without any bias in the data analysis. Here we describe a pilot muographic study of the Izu–Omuroyama scoria cone in Shizuoka Prefecture, Japan, from 11 directions, using a new nuclear emulsion detector design optimized for quick installation in the field. We describe the details of the data analysis and present a validation of the results. The Izu–Omuroyama scoria cone is an ideal target for the first multi-directional muographic study, given its expected internal density structure and the topography around the cone. We optimized the design of the nuclear emulsion detector for rapid installation at multiple observation sites in the field, and installed these at 11 sites around the volcano. The images in the developed emulsion films were digitized into segmented tracks with a high-speed automated readout system. The muon tracks in each emulsion detector were then reconstructed. After the track selection, including straightness filtering, the detection efficiency of the muons was estimated. Finally, the density distributions in 2D angular space were derived for each observation site by using a muon flux and attenuation models. The observed muon flux was compared with the expected value in the free sky, and is 88 % ± 4 % in the forward direction and 92 % ± 2 % in the backward direction. The density values were validated by comparison with the values obtained from gravity measurements, and are broadly consistent, except for one site. The excess density at this one site may indicate that the density inside the cone is non-axisymmetric, which is consistent with a previous geological study.
Scoria cone is one of the basic forms of volcanoes, and clarifying its detailed internal structure and magma movement during its formation is important for understanding the nature of eruptions and also for volcanic disaster mitigation. We conducted a multi-directional muographic survey of the Omuroyama scoria cone, Japan, in order to determine the three-dimensional density structure of the scoria cone. We used a nuclear emulsion detector optimized for multi-directional muography that was installed at 11 sites around the volcano. Muon tracks recorded on emulsion films were read with a high-speed automated readout system. We obtained the three-dimensional density structure by applying a linear inversion to the muographic images, and evaluated the uncertainties. High-density zones were detected in scoria cone. Based on these observations and detailed topographic and geological constraints, we infer that high-density zones are the central, highly welded vent of the scoria cone, three-directional radial dikes extending from the central vent, respectively. We also infer that an E-W-trending dike fed a small lava flow, and that a SSE-NNW-trending dike caused a small flank eruption and produced a crater on the flank of the cone. Our results visualize the three-dimensional internal structure of volcanoes with novel resolution, and thereby demonstrate that the formation process of volcanoes can be discussed in detail. Since it has been difficult to directly detect such clear visualization of the internal structures and eruptive processes in the past, multi-directional muography has resulted in providing one hope for understanding volcanic activity.
To obtain an internal S-wave velocity structure, we conducted a passive seismic campaign with 21 1-Hz seismometers on and around the Showa-Shinzan lava dome, which emerged during the 1943–1945 eruption of Usu Volcano, Japan. Before the campaign, we calibrated seismometers and found slight phase-response differences between seismometers of less than 1–2 degrees. After the campaign, we extracted seismic wavefield by taking cross-correlations of vertical-component ambient noise records between seismic sites. We developed a new method to measure phase-velocities of the Rayleigh wave automatically by assuming layered structure and finally obtained one-dimensional S-wave velocity models in summit, roof and base regions. The obtained S-wave velocity right beneath the intruded lava dome is higher than that in surrounding areas by a few tens of percent down to a few hundred meters below sea level, indicating narrow but deep existence of the root of the lava dome. The obtained S-wave velocity at depths shallower than ~50 m inside the lava dome in the summit area was ~1 km/s, significantly lower than that predicted from the density of ~2.3 x 10^3 kg/m estimated in previous muon-radiography studies and a conventional scaling, indicating the effect of cracking in the lava dome.
Summary To obtain an internal S-wave velocity structure, we conducted a passive seismic campaign with 21 1-Hz seismometers on and around the Showa-Shinzan lava dome, which emerged during the 1943–1945 eruption of Usu Volcano, Japan. Before the campaign, we calibrated seismometers and found slight phase-response differences between seismometers of less than 1–2 degrees. After the campaign, we extracted seismic wavefield by taking cross-correlations of vertical-component ambient noise records between seismic sites. We developed a new method to measure phase velocities of the Rayleigh wave automatically by assuming layered structure and finally obtained one-dimensional S-wave velocity models in summit, roof and base regions. The obtained S-wave velocity right beneath the intruded lava dome is higher than that in surrounding areas by a few tens of percent down to a few hundred meters below sea level, indicating narrow but deep existence of the root of the lava dome. The obtained S-wave velocity at depths shallower than ∼50 m inside the lava dome in the summit area was ∼1 km/s, significantly lower than that predicted from the density of 2.3 × 103 kg/m3 estimated in previous muon-radiography studies and a conventional scaling, indicating the effect of cracking in the lava dome.
Abstract. One of the key challenges for muographic studies is to reveal the detailed 3D density structure of a volcano by increasing the number of observation directions. 3D density imaging by multi-directional muography requires that the individual differences in the performance of the installed muon detectors are small and that the results from each detector can be derived without any bias in the data analysis. Here we describe a pilot muographic study of the Izu–Omuroyama scoria cone in Shizuoka Prefecture, Japan, from 11 directions, using a new nuclear emulsion detector design optimized for quick installation in the field. We describe the details of the data analysis and present a validation of the results. The Izu–Omuroyama scoria cone is an ideal target for the first multi-directional muographic study, given its expected internal density structure and the topography around the cone. We optimized the design of the nuclear emulsion detector for rapid installation at multiple observation sites in the field, and installed these at 11 sites around the volcano. The images in the developed emulsion films were digitized into segmented tracks with a high-speed automated readout system. The muon tracks in each emulsion detector were then reconstructed. After the track selection, including straightness filtering, the detection efficiency of the muons was estimated. Finally, the density distributions in 2D angular space were derived for each observation site by using a muon flux and attenuation models. The observed muon flux was compared with the expected value in the free sky, and is 88 % ± 4 % in the forward direction and 92 % ± 2 % in the backward direction. The density values were validated by comparison with the values obtained from gravity measurements, and are broadly consistent, except for one site. The excess density at this one site may indicate that the density inside the cone is non-axisymmetric, which is consistent with a previous geological study.
Abstract. This study is the first trial to apply the method of filtered backprojection (FBP) method to reconstruct three-dimensional (3D) bulk density images via cosmic-ray muons, We also simulated three-dimensional reconstruction image with dozens of muon radiographies using FBP method for a volcano and evaluated its practicality. FBP method is widely used in X-ray and CT image reconstruction but has not been used in the field of muon radiography. One of the merits to use FBP method instead of ordinary inversion method is that it doesn't require an initial model, while ordinary inversion analysis need an initial model. We also added new approximation factors by using data on mountain topography into existing formulas to successfully reduce systematic reconstruction errors. From a volcanic perspective, airborne radar is commonly used to measure and analyze mountain topography. We tested the performance and applicability to the model of Omuroyama, a monogenetic scoria cone located in Shizuoka, Japan. As a result, it was revealed that the density difference between the original and reconstructed images depended on the number of observation points and the accidental error caused by muon statistics depended on the multiplication of total effective area and exposure period. Combining above all things, we established how to evaluate an observation plan for volcano using dozens of muon radiographies.
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