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Muographic mapping of the subsurface density structures in Miura, Boso and Izu peninsulas, Japan.

Tanaka HK - Sci Rep (2015)

Bottom Line: While the benefits of determining the bulk density distribution of a landmass are evident, established experimental techniques reliant on gravity measurements cannot uniquely determine the underground density distribution.We also observed a significant reduction in density along fault lines and interpreted that as due to the presence of multiple cracks caused by mechanical stress during recurrent seismic events.We show that this new type of muography technique can be applied to estimate the terrain density and porosity distribution, thus determining more precise Bouguer reduction densities.

View Article: PubMed Central - PubMed

Affiliation: Earthquake Research Institute, The University of Tokyo, 113-0032 Tokyo.

ABSTRACT
While the benefits of determining the bulk density distribution of a landmass are evident, established experimental techniques reliant on gravity measurements cannot uniquely determine the underground density distribution. We address this problem by taking advantage of traffic tunnels densely distributed throughout the country. Cosmic ray muon flux is measured in the tunnels to determine the average density of each rock overburden. After analyzing the data collected from 146 observation points in Miura, South-Boso and South-Izu Peninsula, Japan as an example, we mapped out the shallow density distribution of an area of 1340 km(2). We find a good agreement between muographically determined density distribution and geologic features as described in existing geological studies. The average shallow density distribution below each peninsula was determined with a great accuracy (less than ±0.8%). We also observed a significant reduction in density along fault lines and interpreted that as due to the presence of multiple cracks caused by mechanical stress during recurrent seismic events. We show that this new type of muography technique can be applied to estimate the terrain density and porosity distribution, thus determining more precise Bouguer reduction densities.

No MeSH data available.


Vertical muon flux after passing through rock.Vertical flux as a function of rock thickness (in units of meter water equivalent (m.w.e.)) (a) was derived by integrating the differential vertical flux over the range between the minimum energy (Emin) and infinity. The minimum energy that the muon can penetrate the target rock with a given thickness is shown in (b).
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f11: Vertical muon flux after passing through rock.Vertical flux as a function of rock thickness (in units of meter water equivalent (m.w.e.)) (a) was derived by integrating the differential vertical flux over the range between the minimum energy (Emin) and infinity. The minimum energy that the muon can penetrate the target rock with a given thickness is shown in (b).

Mentions: Although vertical muon flux has been measured by several groups, there are large systematic discrepancy among the measurements, thus we utilized the vertical muon flux reported by the BESS Collaboration16. The muon flux was measured in Tsukuba, Japan, which is located near our observation sites, and the integrated open-air vertical flux is 68 m−2sr−1s−1. Since the BESS detector's angular acceptance is ±12°, we extrapolated the flux to apply the BESS flux to our measurement according to the law I ∝ cos2(θ), where θ is the arriving angle from zenith. The error in the integrated flux due to this extrapolation process was estimated to be less than 1%. Fig. 11 shows the vertical muon intensity after passing through rock with a given thickness. The total number of muon counts (Nμ) recorded by the detector moving linearly along the z-axis inside a tunnel with a uniform velocity V is given by the following equation: where I (θ, ϕ, z) is the muon intensity after passing through the rock overburden and where its thickness X is a function of zenith (θ), azimuth (ϕ) angles and the location (z) inside the tunnel. Here, since the velocity of the detector is uniform, z is a linear function of t: Since the critical energy (Ec) can be calculated once the thickness X ( = ρ × L) is given17, I (θ, ϕ, z) is derived by integrating the vertical muon flux16 over the energy region between Ec and infinity. X (θ, ϕ, z) is derived by referring to the 1:25000 topographic map distributed by Geographical Survey Institute (GSI), Japan. Since X (θ, ϕ, z) varies over the distance inside the tunnel, I (z) is integrated over the distance inside the tunnel as shown in Eq. (3). If the tunnel is not linear, Eq. (3) is calculated along the tunnel. A differential vertical muon flux is assumed for the spectrum shape, but the intensity is assumed to be proportional to cos2(θ). Our muography detection system was installed in a car, and moved inside the tunnel with a uniform velocity as shown in the lower panel of Fig. 12. The data collected at the both ends of the tunnel (typically 20–30 m from the end) was discarded in order to remove the erroneous data from overburdens that were not sufficiently thick. The beginning and ending times of the measurement were recorded, and Eq. (3) was integrated over the measurement time range for a given V and various density values in order to compare with the muon counts recorded inside the tunnel.


Muographic mapping of the subsurface density structures in Miura, Boso and Izu peninsulas, Japan.

Tanaka HK - Sci Rep (2015)

Vertical muon flux after passing through rock.Vertical flux as a function of rock thickness (in units of meter water equivalent (m.w.e.)) (a) was derived by integrating the differential vertical flux over the range between the minimum energy (Emin) and infinity. The minimum energy that the muon can penetrate the target rock with a given thickness is shown in (b).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4321185&req=5

f11: Vertical muon flux after passing through rock.Vertical flux as a function of rock thickness (in units of meter water equivalent (m.w.e.)) (a) was derived by integrating the differential vertical flux over the range between the minimum energy (Emin) and infinity. The minimum energy that the muon can penetrate the target rock with a given thickness is shown in (b).
Mentions: Although vertical muon flux has been measured by several groups, there are large systematic discrepancy among the measurements, thus we utilized the vertical muon flux reported by the BESS Collaboration16. The muon flux was measured in Tsukuba, Japan, which is located near our observation sites, and the integrated open-air vertical flux is 68 m−2sr−1s−1. Since the BESS detector's angular acceptance is ±12°, we extrapolated the flux to apply the BESS flux to our measurement according to the law I ∝ cos2(θ), where θ is the arriving angle from zenith. The error in the integrated flux due to this extrapolation process was estimated to be less than 1%. Fig. 11 shows the vertical muon intensity after passing through rock with a given thickness. The total number of muon counts (Nμ) recorded by the detector moving linearly along the z-axis inside a tunnel with a uniform velocity V is given by the following equation: where I (θ, ϕ, z) is the muon intensity after passing through the rock overburden and where its thickness X is a function of zenith (θ), azimuth (ϕ) angles and the location (z) inside the tunnel. Here, since the velocity of the detector is uniform, z is a linear function of t: Since the critical energy (Ec) can be calculated once the thickness X ( = ρ × L) is given17, I (θ, ϕ, z) is derived by integrating the vertical muon flux16 over the energy region between Ec and infinity. X (θ, ϕ, z) is derived by referring to the 1:25000 topographic map distributed by Geographical Survey Institute (GSI), Japan. Since X (θ, ϕ, z) varies over the distance inside the tunnel, I (z) is integrated over the distance inside the tunnel as shown in Eq. (3). If the tunnel is not linear, Eq. (3) is calculated along the tunnel. A differential vertical muon flux is assumed for the spectrum shape, but the intensity is assumed to be proportional to cos2(θ). Our muography detection system was installed in a car, and moved inside the tunnel with a uniform velocity as shown in the lower panel of Fig. 12. The data collected at the both ends of the tunnel (typically 20–30 m from the end) was discarded in order to remove the erroneous data from overburdens that were not sufficiently thick. The beginning and ending times of the measurement were recorded, and Eq. (3) was integrated over the measurement time range for a given V and various density values in order to compare with the muon counts recorded inside the tunnel.

Bottom Line: While the benefits of determining the bulk density distribution of a landmass are evident, established experimental techniques reliant on gravity measurements cannot uniquely determine the underground density distribution.We also observed a significant reduction in density along fault lines and interpreted that as due to the presence of multiple cracks caused by mechanical stress during recurrent seismic events.We show that this new type of muography technique can be applied to estimate the terrain density and porosity distribution, thus determining more precise Bouguer reduction densities.

View Article: PubMed Central - PubMed

Affiliation: Earthquake Research Institute, The University of Tokyo, 113-0032 Tokyo.

ABSTRACT
While the benefits of determining the bulk density distribution of a landmass are evident, established experimental techniques reliant on gravity measurements cannot uniquely determine the underground density distribution. We address this problem by taking advantage of traffic tunnels densely distributed throughout the country. Cosmic ray muon flux is measured in the tunnels to determine the average density of each rock overburden. After analyzing the data collected from 146 observation points in Miura, South-Boso and South-Izu Peninsula, Japan as an example, we mapped out the shallow density distribution of an area of 1340 km(2). We find a good agreement between muographically determined density distribution and geologic features as described in existing geological studies. The average shallow density distribution below each peninsula was determined with a great accuracy (less than ±0.8%). We also observed a significant reduction in density along fault lines and interpreted that as due to the presence of multiple cracks caused by mechanical stress during recurrent seismic events. We show that this new type of muography technique can be applied to estimate the terrain density and porosity distribution, thus determining more precise Bouguer reduction densities.

No MeSH data available.