GIGeoscientific Instrumentation, Methods and Data SystemsGIGeosci. Instrum. Method. Data Syst.2193-0864Copernicus GmbHGöttingen, Germany10.5194/gi-4-215-2015Development of the very long-range cosmic-ray muon radiographic imaging
technique to explore the internal structure of an erupting volcano,
Shinmoe-dake, JapanKusagayaT.kusagaya@eri.u-tokyo.ac.jpTanakaH. K. M.Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi,
Bunkyo, Tokyo 113-0032, JapanT. Kusagaya (kusagaya@eri.u-tokyo.ac.jp)24November20154221522627April201529July201522October20153November2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://gi.copernicus.org/articles/4/215/2015/gi-4-215-2015.htmlThe full text article is available as a PDF file from https://gi.copernicus.org/articles/4/215/2015/gi-4-215-2015.pdf
Muography offers us a tool to observe hazardous erupting volcanoes remotely.
However, practical muographic observations of volcanoes from a distance are
difficult; therefore, various observations have been performed in the
vicinity (< 1.5 km) of volcano peaks to suppress background noise
and enhance images. In this study, we created a muographic image directly
beneath the caldera floor of the erupting Shinmoe-dake volcano in Japan by
locating our muography telescope 5 km from the peak. The Shinmoe-dake
volcano began to erupt on 19 January 2011 and, in less than 1 month, the
ejected lava almost completely filled the caldera and completely changed the
topography of the caldera floor. The resultant image shows a low-density
region underneath the western part of the newly created caldera floor, which
indicates the existence of a void there. After the volcano became less active
in February 2011, infrequent eruptions might have left a void beneath the
caldera floor, which may trigger a collapse in the future. We anticipate that
our novel muography will be a practical tool for monitoring and predicting
eruption sequences in the near future.
Introduction
To date, one of the keys to successful volcano muography that
directly contributes to the understanding of the eruption dynamics has been
to shorten the time required for capturing a practical radiographic image of
an erupting volcano (Tanaka, 2014). For example, Tanaka et al. (2014) reduced
electromagnetic background events by adding redundant detectors and radiation
shields to a conventional muography telescope (e.g., Tanaka and Yokoyama,
2013), thereby improving the time resolution. They successfully visualized
magma movements with a time resolution of 3 days. The reduction of background
events is known to be essential for shortening the measurement time (Tanaka
and Yokoyama, 2013).
Another important benefit of muography is that it allows us to observe
hazardous volcanoes from a long distance. However, the telescopes have needed
to be placed in the vicinity (0.5–1.6 km) of the volcano craters
(Tanaka et al., 2014, 2010b, 2009a, b, 2007a, b; De Lellis et al., 2014;
Tanaka and Yokoyama, 2013; Carbone et al., 2013; Kusagaya et al., 2013;
Lesparre et al., 2012; Hernández et al., 2013; Miyamoto et al., 2012)
(Fig. 1). Although Cârloganu et al. (2013) located their telescope at a
distance of 2 km from the peak of the Puy de Dôme volcano, they
reported that the background noise dominated the muon flux when the rock
thickness exceeded 1 km. Such short-range muography is practical when
the target volcanoes are dormant or less active.
Visual comparison with prior works. The past muography observation
sites are plotted as distance from the peak of the volcano.
Thus far, there are two examples of muography measurements that targeted
erupting volcanoes. In one of the examples, the telescope was buried
underground prior to the eruption (Tanaka et al., 2009b), and in the second
example the eruption was non-explosive (Tanaka et al., 2014). In the latter
case, the volcanic eruption index (VEI) was zero.
In this work, we investigated possibilities of using very long-range
muography (VLRM) as an alternative remote sensing tool for volcano
monitoring. In conjunction with a newly developed analysis method, we used a
low-background muography telescope to image the density distribution beneath
the caldera floor of the Shinmoe-dake volcano in Japan.
The latest eruption of the Shinmoe-dake volcano took place on 19 January
2011. An eruption column reached 3000 m above sea level on 26 January. A
small lava dome (10 m in diameter) was created on 28 January and
continued to grow inside the caldera. On 30 January it almost completely
filled the caldera. The Miyazaki Prefecture reported that 1150 people had been
evacuated from the area on the same day. At the same time, the area within
3 km of the mountain was designated as a forbidden zone. On
2 February a volcanic bomb reached beyond this restriction line. It reached
3.2 km from the peak of the mountain (Japan Meteorological Agency,
2013). The amount of magma discharged between 19 January and 2 February was
estimated to be 2.7–3.7×106m3 (VEI 2). Since 2011, the
Japan Meteorological Agency (JMA) volcano eruption levels remained high and
were at Level 2 in 2014. People are forbidden to approach the volcano any closer than
approximately 1 km from the crater when the JMA volcano eruption level
is at Level 2.
In this work, we installed our muon telescope at a distance of 5 km,
south of the center of the caldera, for the purpose of investigating a 2011
Shinmoe-dake eruption. We report our successful VLRM, which exhibits a
low-density region below the Shinmoe-dake caldera floor that might cause the
caldera to collapse in the future.
PrincipleMuography
In this section, we briefly introduce the principle of muography. To measure
the internal density distribution of a volcano, the integrated flux of
cosmic-ray muons before and after passing through the volcano is compared.
Primary cosmic rays, which predominantly consist of protons, arrive at the
top of Earth's atmosphere and generate muons as a consequence of
interaction with atmospheric nuclei. A number of past experiments have
measured the cosmic-ray muon energy spectrum at sea level (e.g., Achard et
al., 2004; Haino et al., 2004; Allkofer et al., 1985; Jokisch et al., 1979).
Although there are systematic differences between them, the average energy of
muons tends to be higher at larger zenith angles. For instance, while the
integrated vertical muon flux (0∘ from zenith) is 7×10-3
and 5×10-8cm-2sr-1s-1 above 1 GeV
and 1 TeV, respectively (Beringer et al., 2012), the horizontal flux
(85∘ from zenith) is 1×10-4 and 1.6×10-7cm-2sr-1s-1 above 1 GeV and
1 TeV, respectively (Allkofer et al., 1985). For the purpose of
interpolating the measured flux, several groups have formulated approximate
equations of the muon energy spectrum as a function of energy and zenith
angle (e.g., Gaisser, 2002; Matsuno et al., 1984; Smith and Duller, 1959).
However, the systematic difference between them ranges from 10 to 100 %
(Lesparre et al., 2010).
The interaction of muons with matter is also well known. The energy loss of
muons through matter is given as a function of density length
(density × path length along the muon path) and several approximation
methods have been attempted (e.g., Barrett et al., 1952). Monte Carlo (MC)
simulations offer more accurate results (Groom et al., 2001). The minimum
energy of muons that can penetrate a given thickness of rock can be derived
based on these calculations. For example, 1 TeV muons can penetrate matter
with a thickness of 2.5 km water equivalent. Therefore, the integrated muon
flux after passing through a mountain can be calculated by integrating the
energy flux over the range between this minimum energy and infinity
(Fig. 2). While there is a general tendency for muons to be more strongly
attenuated as the rock becomes thicker, the vertical muons are attenuated
more strongly than the horizontal muons. Because the target's path length
information can be exploited from a topographic map, the average density can
be derived along the muon paths. Muography offers images of volcano interiors
with a higher spatial resolution than possible with conventional geophysical
measurements.
Elevation angle dependence on muon flux after passing through a given
rock thickness. The rock thickness is given in units of meter water
equivalent (m.w.e.).
Very long-range muography
When performing VLRM, background sources that are unique to this method must
be considered.
Muon decay
A muon decays into an electron (a positron) and two neutrinos within a
lifetime (τμ) of 2.2 µs. The decay length (L) of
relativistic muons is given by
L=γτμc,
where γ is the Lorentz gamma factor, and c is the speed of light. As
can be calculated in Eq. (1), the muon survival probability tends to be lower
when muons travel longer. For example, 15 % of muons with an energy of
1 GeV decay after traveling 1 km in a vacuum but 56 % decay
after traveling 5 km. The secondary electrons (positrons) due to muon
decay are called decay electrons (decay positrons).
Scattering of decay electrons in the atmosphere
As the distance between the target and muography observation site becomes
longer, the atmospheric density length increases and thus decay electrons
are scattered more often and lose their initial directions. The typical
length in which the Bremsstrahlung process takes place is called the radiation
length (X0), and X0 for an electron is 303.9 m in a 1 atm
atmosphere. When the distance that a muon has traveled is much longer than
its non-relativistic decay length (660 m) and X0 for an electron,
the muon will decay, and its decay electron will experience many
Bremsstrahlung processes and will deviate from the original muon's path.
Because muons are 207 times heavier than electrons, muons do not experience
Bremsstrahlung processes in the atmosphere within this distance scale.
Figure 3 shows the results of extended air-shower Monte Carlo (MC)
simulations. A MC code called Cosmos (Kasahara and Cohen, 2008) was used to
reproduce the energy spectrum of the electromagnetic (EM) components
(electron and positron) arriving from the zenithal angle region between 50
and 90∘ at sea level. In these simulations, 9.5×108 primary
particles with an energy range between 260 MeV and 260 TeV
(Beringer et al., 2012) were injected at the top of Earth's atmosphere. The
top five elements (H, He, C, N, and O) were selected as primary particles. These
particles were injected into the atmosphere isotropically within an angle
range of 0–95∘ from zenith.
In Fig. 3, the differential flux is plotted within an energy range between
10 MeV and 1 TeV. Electrons with energies above
10 MeV can penetrate a 5 cm thick plastic scintillator; thus,
these components could be background sources when thin plastic scintillators
(e.g., Lesparre et al., 2012) or gaseous detectors (e.g., Cârloganu et
al., 2013; Oláh et al., 2012) are used for muography observations. If the
angular resolution of a muography telescope is not enough to distinguish
these components from muons, these electrons and positrons scatter in the
air, change direction, and become a possible source of background events. The
EM fluxes integrated over the energy range above 10 MeV and
1 GeV are 9.4×10-5 and 1.7×10-6cm-2sr-1s-1, respectively. These values are
equivalent to the muon flux (arriving at 85∘ from zenith) after
passing through 100 and 1200 m of water, respectively.
Differential energy spectrum of electromagnetic components (electron
and positron) of secondary cosmic rays at sea level. The arriving angles
range between 50 and 90∘ from zenith.
MethodsMuography telescope
Our muography telescope is described in detail in a separate paper (Tanaka et
al., 2014); therefore, we briefly introduce our apparatus in this section.
The telescope had an active area of 1.5×1.5m2. A position
sensitive plane (PSP) was comprised of adjacent horizontal and vertical
scintillation counters with 10 cm wide plastic scintillator strips (ELJEN
EJ-200) and photomultiplier tubes (PMT) (Hamamatsu R7724), which formed a
segmented plane. Simultaneous signal outputs from the horizontal and vertical
scintillation counters indicated where the charged particle traversed the
PSP. Our muography telescope consisted of six PSPs. The first and sixth PSPs
were separated by 3.0 m, so that an angular interval of
33 mrad was achieved. Herein, we defined the axes that were vertical
to the ground and perpendicular to the PSP as the y and z axes,
respectively, and the axis that was perpendicular to both the y and
z axes as the x axis.
The background noise by fake tracks is created by the accidental coincidence
of vertical EM components (Tanaka and Yokoyama, 2013). To reduce such events,
a telescope with six PSPs was designed in this work. Even if EM components
hit all PSPs, the vertex points are expected to form a random alignment.
Thus, such events are expected to be removed by selecting linear trajectory
for reducing the background by fake tracks.
As seen in Fig. 3, shielding electrons with energies below 100 GeV is
essentially equivalent to removing the entire horizontal EM background. For
this reason, five 10 cm thick lead plates (50 cm in total) were
inserted between each of the six PSPs. Because the radiation length of lead
for an electron is 5.6 mm (Beringer et al., 2012), the size, weight
and cost of the radiation shield can be minimized. However, because the lead
plates were deformable, we concealed each plate in a 3 cm thick stainless
steel case (Fig. 4).
To test if our telescope could practically remove all EM components, the
number of electrons that stopped inside a lead block was investigated using
the Geant4 MC code (Agostinelli et al., 2003) for particle propagation.
Figure 5 shows the number of secondary electrons generated inside a lead
shield as a function of depth from the surface. In total, 100 electrons with
energies of 100 GeV were injected. The number of secondary electrons
increased quickly but eventually decreased at a depth of 50 mm from
the surface, and they almost disappeared deeper than 450 mm from the
surface. A total of 4-2+3 electrons remained within a depth region
between 400 and 450 mm, and they completely disappeared at greater
depths than this region.
Schematic of the muography telescope used for the present work.
Distribution of the electrons stopped inside a lead
block. The incident energy is
100 GeV. The stopping probability per 1 mm at different
depths (red) and the corresponding cumulative probability (blue) are shown.
Caldera deformation captured with airborne synthetic aperture radar
(SAR) measurements. The black dotted and blue solid lines, respectively, show
the topography of the caldera floor before (March 2009) and after (February
2011) the 2011 eruption for the west–east cross section (a) and
south–north cross section (b).
Geometrical configuration of the experimental setup. The left panel
shows the location of the Shinmoe-dake volcano. The right panel shows an
enlarged map in the vicinity of the Shinmoe-dake volcano. The muography
telescope (Mu) was located 5 km south–southwest from the center of
the Shinmoe-dake volcano caldera. The topographic map does not reflect the
land deformation during the 2011 eruption.
Path length distribution of volcanoes viewed from different
distances. The solid angle when viewing a larger volcano (Shinmoe-dake)
(a) is smaller than that when viewing a smaller volcano
(Satsuma–Iwo Jima) (b) if the distance to the
target is farther.
We muographically imaged the Shinmoe-dake volcano, located in the Kagoshima
and Miyazaki prefectures in Japan. The altitudes at our muography telescope
and the caldera floor before and after the 2011 eruption were 610, 1240, and
1350 m, respectively (Fig. 6, Shimono et al., 2011). Subsequent to
the eruption, no significant changes to the topography of the caldera floor
have been reported. We installed our muography telescope 5 km south of the
center of the Shinmoe-dake caldera (Fig. 7). Therefore, the elevation angles
of the caldera floor before and after the eruption were 128 and
150 mrad, respectively. We defined an azimuth angle of 0 mrad
in the direction of the center of the caldera. The angular interval (Tanaka
et al., 2010a) of the telescope was ±33 mrad; therefore, the
telescope could capture the magma deposit beneath the present caldera floor
within a 131±33 mrad range of elevation angles. Figure 8 shows the path
length distribution of Shinmoe-dake as a function of elevation and azimuth
angles. As seen in this figure, path lengths exceeded 5000 m in the
region below the 100 mrad elevation angle. It was difficult for us to image
this region with our present telescope. The measurements began on 10 October
2014 and continued until 4 November 2014. The observational duration was
64 days.
AnalysisMuon tracking
The directions of the incident particles in the telescope can be determined
by connecting two vertex points in the first and the last position sensitive
planes (PSP #1 and #6). For muon tracking, we selected straight
trajectories for incident events according to the following process.
When an event was detected in all six PSPs within a time window of
40 ns, the vertex points in PSP #1 and #6 were recorded.
When two or more events were detected in a single PSP within a time
window of 40 ns, these events were not recorded (Tanaka et
al., 2001).
When the vertex points in the redundant PSPs, i.e., PSPs #2–5, deviated within ±N segments (N value) from the
straight line created by connecting the vertex points in (a), event tracks
were generated. Smaller N values gave better tracking linearity.
Figure 9 illustrates our tracking scheme for N=1. Figure 10 shows the
number of recorded open-sky event tracks that survived for different N
values. Because we could not identify the location of the vertex points
within the scintillator strip, the telescope failed to track some events if
only linear trajectories were selected (N=0). The figure also shows that
the number of muons scattered at large angles in the telescope was negligible
because there are no significant variations for N>2.
Figure 11 shows the maximum deviations of the Geant4-simulated vertex points
in 4 redundant PSPs (i.e., PSPs #2–5) from a linear
trajectory drawn by connecting the vertex points in PSPs #1 and #6.
For these simulations, muons were injected into the virtual telescope, which
is identical to the actual telescope, with an energy spectrum of 70∘
from zenith. The plot in Fig. 11 is consistent with the N dependence on the
recorded number of tracks (Fig. 10), and we therefore conclude that the event
tracks recorded with our telescope using the just-discussed tracking scheme
were muon tracks. We specified N=1, because large N values blur the
resultant image.
Example of muon tracking for N=1. If a signal (a red square) is
generated at (x,y)=(5,10) in PSP #1 (a) and at (x,y)=(15,1) in PSP #6 (f) simultaneously, the signal generation
points (red squares) in PSPs #2–5 (b–e) are used for tracking.
Number of event tracks as a function of N. Error bars indicate the
range of integration.
Muon scattering inside the telescope. The event was normalized to
the total number of events.
Background estimation
To analyze long-range muography data, we developed an alternative analysis
method to solve the following problems that were not addressed in prior
works.
As described in Sect. 2.2, when a muon travels
long distances, the muon decay process cannot be neglected. Because the
radiation lengths of decay electrons/positrons are short (309 m),
there is a high probability that they will experience multiple Bremsstrahlung
processes and that the muon's initial direction therefore will be lost. We
confirmed that a sufficiently thick lead shield (50 cm) would
eliminate those EM backgrounds.
Muons scatter inside the telescope. We must consider two different types
of muon sources regarding their incident directions (because we do not
distinguish forward-directed muons from backward-directed muons): (i) muons
arriving from above the ridge of the mountain (the forward direction) and
(ii) those from the opposite side of the mountain (the backward direction).
For long-range muography (LRM), a mountain tends to be smaller in
angular space. For example, as shown in Fig. 8, while the elevation angle of
a peak (700 m relative height) is 500 mrad for short-range
(1.2 km) muography (SRM) (Tanaka et al., 2014), the elevation of a
taller mountain's peak (750 m relative height) is only 150 mrad for
long-range (5 km) muography (the present work). This means that
near-horizontal muons are utilized more with LRM.
When we consider factors (b) and (c), a tracking problem unique to LRM
arises. Firstly for (i), in general, relativistic muon scattering is very
small and negligible for SRM. For example, muons that scatter more than
200 mrad after passing through the radiation shield are 0.001 %
of the total; thus, scattered open-sky muons will not contaminate tracks
from the region of interest. However, for LRM, this effect becomes serious.
Muons that scatter more than 100 mrad after passing through the
radiation shield are 0.1 % of the total. Likewise for (ii), as long as
the region of interest is located at high elevations, the scattering of
backward-directed muons is negligible. However, for LRM, because the region
of interest is usually located at low elevations, this effect also becomes
serious.
To estimate muon scatterings inside the telescope, MC simulations (Geant4)
were performed. In these simulations, 500 000 muons were injected into the
modeled radiation shields (50 cm thick lead and 30 cm thick stainless steel
plates) according to an energy spectrum measured within an angle range
between 60 and 70∘ from zenith (Achard et al., 2004; Tsuji et
al., 1998; Abdel-Monem et al., 1975). The lowest muon energy considered in
the simulation was 1 GeV, which corresponded to the cutoff energy
derived from the detectors and the shields. Figure 12 shows the scattering
angle distribution of near-horizontal muons after they passed through the
radiation shields. While approximately 4 % of the muons scattered at
angles greater than 33 mrad, only 0.7 % of the muons scattered at
angles greater than 100 mrad. These numbers correspond to scattered
muon fluxes of ∼10-6cm-2sr-1s-1 if we
assume that the arriving angle of the muons was 80∘. These are likely
orders of magnitude estimates.
To estimate the background level from muon scattering inside the telescope, we
calculated the flux of muons that scattered towards the telescope's
y direction. The elevation angle dependence on the number of background
events NBG(θ) measured within the elevation angle range from
θ to θ+Δθ is
NBGθ=∫NθP(θ)dθ,
where Δθ is an angular interval of our detector, and Nθ is the elevation angle dependence on the number of muons
measured within an elevation angle range from θ to θ+Δθ.
Pθ is given by
Pθ=∫θθ+Δθϕθdθ∫0∞ϕθdθ,
where ϕθ is the muon scattering probability
distribution function shown in Fig. 12.
The elevation angle dependence on the forward-directed and
backward-directed background event flux is plotted in Fig. 13 by calculating
Eq. (2). Because low energy muons tend to scatter inside the telescope, the
recorded background events were reduced by selecting near-linear
trajectories, depending on the N value. Figure 13 also shows the background
event flux for a selection criterion of N=1.
Scattering angle distribution of near-horizontal muons. The events
are normalized by the total number of muons that were injected.
Elevation angle dependence on background event flux. The
background levels are expected to be measured for different elevation angles
with the telescope and are plotted for the events arriving from the direction
of the target mountain (a) and from the side opposite to the
mountain (b). The plot also shows the background event flux before
(red squares) and after (blue diamonds) selecting linear trajectories from
these events with a selection criterion of N=1.
Elevation angle dependence on averaged event flux observed at
Shinmoe-dake and Satsuma–Iwo Jima. The flux was averaged between azimuth
ranges from -430 to 430 mrad for Shinmoe-dake and from 0 to
400 mrad for Satsuma–Iwo Jima. The backward-directed (open sky) and
forward-directed (mountain side) flux is shown for Shinmoe-dake and
Satsuma–Iwo Jima with blue and red triangles and blue and red circles,
respectively.
ResultsComparison between different muography ranges
To study the effect of the muography range, we compared muography data collected
at different distances from volcanoes. For comparison, data collected at
the Satsuma–Iwo Jima volcano were reanalyzed because the data were collected
using the same type of the detector 1.4 km from the peak. Figure 14
shows the dependence of the event flux on elevation angle for Satsuma–Iwo Jima
(blue dots in Fig. 14) and Shinmoe-dake (red dots in Fig. 14). Firstly, we
found that the elevation angle dependencies on the backward-directed event
fluxes were comparable. Because there was mostly open sky opposite to the
mountains at both locations, we expect that these data will be similar.
Secondly, we found that the muon counts 1.5-0.2+0.3×10-7cm-2sr-1s-1 recorded at Satsuma–Iwo Jima did
not show the minimum value at an elevation angle of 33 mrad, even
though the target thickness was a maximum (> 2000 m) in this
direction. The expected muon flux is 1×10-8cm-2sr-1s-1 there. Likewise, the observed flux
at an elevation angle of 66 mrad was 6.5-0.9+2.8×10-8cm-2sr-1s-1, while the expected muon flux is
2×10-8cm-2sr-1s-1. This deviation is caused
by the backward-directed horizontal muons that were scattered in our
radiation shields, as described in Sect. 3.2.2. As shown in Fig. 15, the
theoretically estimated NBGθ for N=1
(Fig. 13b) explains the excess of forward-directed event fluxes at low
elevation angles (33 and 66 mrad).
On the other hand, while the Shinmoe-dake and Iwo Jima fluxes were similar at
elevation angles of 33 and 66 mrad, the Shinmoe-dake flux was
systematically higher than the Iwo Jima flux at elevation angles larger than
100 mrad. There are two possible reasons for this deviation: (a) the
thickness of the rock at Shinmoe-dake is thinner than at Satsuma–Iwo Jima or
(b) background events offset the data. The path lengths of the muons at
Shinmoe-dake ranged from 5000 to 7000 m within the angular region. The
corresponding flux was ∼10-10cm-2sr-1s-1;
therefore, (a) can be rejected. Because the ridge of Shinmoe-dake is located
at an elevation angle of approximately only 200 mrad, even small muon
scatterings inside the telescope could appear to be more horizontal muon
events from the direction below the ridge. Therefore, the possible background
sources were the sum of the (a) forward-directed and (b) backward-directed
muon scatterings inside the detector. Figure 15 compares the elevation angle
dependence on the Shinmoe-dake flux with the sum of the forward- and
backward-directed background event fluxes NBGθ
for N=1. As seen in this figure, the excess of the Shinmoe-dake flux at
lower elevation angles (33 and 66 mrad) can be explained by these
backgrounds. The difference between the Shinmoe-dake flux and NBGθ at elevation angles larger than 100 mrad was
recognized from muons after passing through the
volcano.
Comparison of the measured and theoretical background fluxes. The
blue and red circles show the elevation angle dependence on the Shinmoe-dake and
Iwo Jima fluxes, and the purple crosses and green circles show the estimated
background from backward-directed muons and both forward and
backward-directed muons, respectively.
Azimuth distribution of event flux measured at the Shinmoe-dake
volcano. The data collected at the elevation angle of 130 mrad are
plotted along with the theoretical flux calculated for different density
values (1.5, 2.0, 2.5, and 3.0 gcm-3). The data before (black
squares) and after (red squares) the background event removal are plotted.
Interpolated muographic images showing (a) mean densities,
(b)1σ, (c)2σ, and (d)3σ
upper limit densities below the Shinmoe-dake caldera floor. The east–west
cross sections of the mountain before (dotted lines) and after the 2011
eruption (February 2011) are shown. The color scale shows the average density
along the muon paths.
Very long-range muographic imaging of the Shinmoe-dake
volcano
To derive the average density along the muon paths passing through a region
right below the caldera floor, in Fig. 16 we compared the theoretical and
experimental Shinmoe-dake muon fluxes at an elevation angle of
130 mrad as a function of azimuth angle (ϕ). In this figure,
only the data points with more than 50 events were plotted. For ϕ<-350 mrad and ϕ>200mrad, the numbers of events
were less than 50 because (a) the active areas of the telescope were small
and (b) the rock became thick.
Discussion
Figure 17 shows muographic images below the Shinmoe-dake caldera. These
images were produced by interpolating the discrete data points for the
purpose of enhancing the visual effect. In this image, because we did not
record sufficient number of events below the caldera floor before the 2011
eruption, we modeled the density in this region by applying the average
density of the crater rim. In Fig. 17, an east–west cross section in the
region of the mountain peak is also plotted to show the profile of the
caldera before and after the 2011 eruption. To illustrate the statistical
significance of the density contrast, the 1σ, 2σ, and 3σ
upper limits of the density are plotted in Fig. 17b–d, respectively. As seen
in Fig. 17, the density is sensitive to variations of the sigma values at an
elevation angle of 100 mrad. This means that the statistical error is
large; therefore, there is a large uncertainty in the density values in this
region.
As seen in Fig. 17, there are two low-density regions below the present
caldera floor. The regions are localized at the eastern and western areas,
while the central area has higher densities. Figure 18 shows the location of
a crater of an explosive eruption (subplinian event) and a growing lava dome
found on the caldera floor on the morning of 28 January 2011 (Nakada et
al., 2013). A series of eruptions in 2011 at Shinmoe-dake started from this
crater.
Schematic of the Shinmoe-dake caldera after the 2011 eruption
(Nakada et al., 2013). The left and right panels, respectively, show the
caldera immediately after the eruption (28 January 2011) and after more than
a year had passed (the summer of 2012). The numbers in the plot indicate the following:
1, a vent of subplinian events; 2, a vent of continuous emission of fine ash;
3, a lava dome; 4, a steam vent; 5, the limit of lava accumulation; and
6, altered, thickly deposited ash. The vertical cross sections are plotted
along three red horizontal lines (a, b, and c) in
Fig. 19.
Vertical cross sections inside the caldera of the Shinmoe-dake volcano
along the lines (a), (b) and (c), respectively, in Fig. 18 on
6 October 2011 (red) and 26 February 2011 (blue) by airborne SAR (Pi-SAR2)
measurements (Kobayashi et al., 2013).
Figure 19 shows the results of airborne synthetic aperture radar (SAR)
measurements in February and October 2011 (Kobayashi et al., 2013). Figure 19
shows the vertical cross sections along the lines shown in Fig. 18. As seen
in this figure, the southeast part of the caldera floor collapsed some time
between February and October 2011. The size of the collapse was 100 m
in diameter and 30 m in depth; however, the depth may have been
greater. The location of the collapse coincides with that of the steam vent
shown in Fig. 18.
In the present analysis, the caldera topography measured in February 2011 was
utilized to derive the average density along the muon paths. This means that
the 100 m diameter collapse was not incorporated into our analysis, and the
muographic images shown in Fig. 17 would reflect this newly created vertical
channel. When considering that the typical path length in this region is
∼ 1000 m, the reduction in density would be ∼ 10 %.
This reduction could explain the low-density region found in the eastern part
of the caldera. However, there were large density errors in this region;
therefore, the contrast was not as apparent (the density contrast disappears
when we take the 3σ upper limit of the determined density; Fig. 17d).
Unlike the eastern part, the topography of the western part of the caldera
floor seems more stable and did not change between February and October 2011
(Fig. 19). However, muographic images show a distinct low-density region
immediately below the western caldera floor (Fig. 17). As a consequence, the
following scenario can explain the sequence of the 2011 Shinmoe-dake
eruption. Lava and ash emitted from the Shinmoe-dake caldera almost filled it
over less than a month after the first eruption on 19 January 2011, which
took place in the western part of the caldera. After the volcano became less
active at the end of February, the more fragile part inside the caldera
(i.e., the layer of thickly deposited ash shown in Fig. 18) collapsed right
above a steam vent (Fig. 18) and created a 100 m diameter vertical channel.
On the other hand, a lava layer covered an initial subplinian event crater
(Fig. 18), and hotter and more fluid magma underneath the solidified surface
(that is more stable than deposited ash) was gradually drained into the
crater. As a consequence, a low-density (porous) region was created
underneath the western caldera floor.
In prior works, it was reported that a void was created beneath the caldera
floor of the Miyake-jima volcano in 2000. The day after the June 8 eruption, an
ellipsoidal region with a length of ∼ 700 m and width of
∼ 400 m collapsed ∼ 200 m in depth (Nakada et
al., 2001). The precursor of the caldera collapse was detected by gravimetric
measurements 2 days prior to the event. Furuya et al. (2001) compared the
spatiotemporal changes between 2 days and 2 years before the collapse. A
difference in gravity values between those times was -145 µgal
inside the caldera, and the data points showed that the values below
-100 µgal were located within a region 1.0–1.5 km from
the caldera center. A gravity change of -145 µgal corresponds
to an upheaval of several tens of centimeters. However, such an upheaval was
not detected using Global Positioning System (GPS) measurements. Furuya et
al. (2001) interpreted this inconsistency as a new void that formed
underneath the crater floor during the 2000 Miyake-jima eruption.
In the present work, muography measurements were carried out at a distance of
5 km from the volcano summit. This implies that VLRM can provide us
with a new tool to predict caldera collapse by mapping out the density
distribution directly beneath the caldera floor. On 8 October 2014, it was
reported that Shinmoe-dake began to deform because of an accumulation of
magma beneath the volcano (Geospatial Information Authority of Japan, 2014)
that had reached the same level of expansion that was observed right before the
2011 Shinmoe-dake eruption. In future, joint
muographic and absolute gravimetric measurements can provide more useful
information for predicting volcano eruption sequences by independently
measuring magma movements in shallow regions compared to movements in deeper
regions. Such measurements can give us new information about magma dynamics
inside volcanoes.
Conclusions
In this work, we successfully applied a calorimeter-type
muography telescope with radiation shields and redundant detectors (PSPs) to
an erupting volcano for very long-range muography (VLRM). Conventional
muography is limited to the vicinity of a volcanic crater, and it is
therefore impractical to apply conventional techniques when monitoring
erupting volcanoes. Our results show that muography has the potential to be a
practical tool for monitoring hazardous volcanic eruptions.
Entry to the summit area of Shinmoe-dake volcano is still restricted. Since
the beginning of the 2011 eruption, the surface phenomena associated with
eruption transitions have been revealed by airborne and land-based
measurements. However, it has been technically difficult to image the
internal structure of the volcano from a secure distance. Using VLRM, we
found a low-density region directly below the caldera floor, which implies
that magma dynamics occurred below the caldera floor after eruptions became
less active in February 2011. The location of the low-density region is
coincident with the crater that was formed during the initial eruption in
January 2011. The void might cause a caldera collapse.
Our present proof-of-principle (POP) work shows the capability of conducting
muographic measurements 5 km from the summit. This is considered to be a
safe distance for many volcanic eruptions. On the other hand, for muography
applied to erupting volcanoes, precise topographic information of the target
is required because the surface topography of an erupting volcano varies
with time. Such information can be obtained using repeated airborne
synthetic aperture radar (SAR) measurements. In conjunction with recent
progress in those airborne techniques, the range of application of muography
can be remarkably expanded.
Acknowledgements
We are grateful to two anonymous referees for their valuable comments, which
greatly improved this paper.
Edited by: M. Díaz-Michelena
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