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Borehole Muography. 1 Earthquake Research Institute, the University of Tokyo, Japan 2 National Research Institute for Earth Science and Disaster Prevention, Japan. p. Purpose. Atmosphere. μ. μ. Fault. Typical surface detector cannot measure underground structures - PowerPoint PPT Presentation
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Borehole Muography
1Earthquake Research Institute, the University of Tokyo, Japan2National Research Institute for Earth Science and Disaster Prevention, Japan
1
Extension to the underground of MUOGRAPHY
Target : Fault zone structure
-position, strike, dip ,width, and density→Prediction on seismic intensity
BOREHOLE MUOGRAPHY
Purpose
2
There are boreholes near the fault
Fault
Detector
Typical surface detector cannot measure underground structures because muons only come from the sky
How do we measure underground structure?→Put a detector into underground
Atmosphere
p
μμ
Development of a new method & detector
• Traditional detector1m×1m×1m→CANNOT be put into the borehole good angular resolution ~ 1°
MuonScintillator
3
1m
1m
~ 10 cm
Borehole
・ New detector4cm×7cm×70cmAngular resolution ~30°Need scanning in boreholes (rotation and up-down)
Zenith Z
X
Y
PMT
Only 2 coupled scintillators are used
7𝑐𝑚We count when muons come common section
3
70𝑐𝑚
4𝑐𝑚
Sensitivity of the detector
Detector
4
Zenith angle
Using this detector as a probe, we count muons as a function of depth and azimuthal angle
Two factors of sensitivity・ Shape of detectorMost sensitive from perpendicular to the detector’s section・ Angular distribution of muonsMost of muons come from vertical directionsHorizontal muons are absorbed by the ground
Perpendicular to the section
36 °
𝒁
𝑿
𝒀
𝒁
𝑿
𝑿
𝒀
Azimuth angle
Detector
4
Muons
Perpendicular to the section
Borehole
Change depth
Change angle
Test observation at Yayoi well (borehole)
5
A B
C
D
Does the detector have an ability to measure the density deficit of the low density area(Cavity) ?
20cm
Yayoi well, located in U Tokyo, was dug by Earthquake Research Institute in 1897.
~ 70m in depth
Well
Observation
6
Interval : 10m
4 directions
Detector
Period : March ~ April, 2013Points : Depth=10m ~ 60m at 10m intervals4 directions at 45°intervals Time : about 24h at each point
Rods for directional control
Wire
C
AD
B
200cm
12cm6
ResultDoes detector know the existence of cavity?
Cavity exists in the direction A from well
Counting rate A > Counting rate B
7
Layer2.0g/cc
Cavity0.3g/cc
A
B Cavity is detected!
Depth
Ratio
Depth
3.0
2.5
2.0
1.5
1.0
0.5
0
Analysis Result : DensityEstimating density of layers and cavity simultaneously
8
10m
10m
10m
50m
60°
Layers
Cavity(60°)
Cavity
0 10 20 30 40 50 60
Dens
ity )
Layer 2
𝜒2 (𝝆 )=∑𝑖
(𝑁 𝑖❑
𝑐𝑎𝑙 (𝝆 )−𝑁 𝑖❑𝑜𝑏𝑠
√𝑁 𝑖❑𝑜𝑏𝑠
)2
300° 360° 360° 360° 360° 360°
Layer 1
Layer 3
Using all 24 data points
Summary
• We developed a new method and a detectorto measure the seismic fault zone from boreholes
• Test observation at Yayoi well– We measured density deficit• Existence of cavity• Density of cavity and layers
9
Treatment of observation data
Data got by observation: Muon + noise counting rate(/s)
→ True muon’s counting rate (/s)
Process1 Process2 Cut noise by energy loss revise detection efficiency in 100 %in the detector
10
MuonNoise
Coun
ts
Muo
n Ev
ents
)
Depth Pulse height
100%
Existence of cavity → Density of cavity
11
𝜌𝑐𝑎𝑣𝑖𝑡𝑦
Layer-Cavity Model
𝜒2 (𝝆 )=∑𝑖
(𝑁 𝑖❑
𝑐𝑎𝑙 (𝝆 )−𝑁 𝑖❑𝑜𝑏𝑠
√𝑁 𝑖❑𝑜𝑏𝑠
)2
Observation point according to depth and directionCounting rate (Simulation) in the point under the density model : Counting rate (Observation) in the point
is optimum solution
ρ𝑙𝑎𝑦𝑒𝑟 1ρ𝑙𝑎𝑦𝑒𝑟 2ρ𝑙𝑎𝑦𝑒𝑟 3
・・・