Limits...
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.


Inversed reduction factor as a function of the average thickness of the tunnel overburden.The error bars associated with the data points (blue dots) show the 1σ upper and lower limits. Theoretical curves were calculated by assuming uniform overburden densities of 2.0 g/cm3 (red), 2.5 g/cm3 (gray), and 3.0 g/cm3 (yellow).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4321185&req=5

f4: Inversed reduction factor as a function of the average thickness of the tunnel overburden.The error bars associated with the data points (blue dots) show the 1σ upper and lower limits. Theoretical curves were calculated by assuming uniform overburden densities of 2.0 g/cm3 (red), 2.5 g/cm3 (gray), and 3.0 g/cm3 (yellow).

Mentions: Fig. 3 plots the inverse of the observational reduction factor (Nμ /N0) as a function of averaged overburden thickness. The data collected from all of three peninsulas were used to plot this graph along with theoretically predicted reduction factors for different assumed densities (see Method section). By comparing the data points with these theoretical predictions, the density averaged over these three regions was deduced to be 2.51 ± 0.02 g/cm3. This muographically deduced density was compared with independent results from gravity studies. Nawa et al. (1997)3 reported the average density of tertiary sedimentary and volcanic rocks above sea level in Southwest Japan to be 2.54 ± 0.15 g/cm3 and 2.53 ± 0.13 g/cm3, respectively. These values are in agreement with our results. As shown in Fig. 4, one data point (between 30 and 40 m) shows density (2.35 g/cm3) much lower than 2.51 g/cm3. This is because the average density was affected by several low density points measured along the active fault lines as described more in detail in the following subsections and the discussion section. Since our detector is moving inside the tunnel, and cannot distinguish muons with different angles of incidence, the thickness in Fig. 4 is presented as “the average reduction factor”, which is the reduction factor calculated for a mixture of various rock thicknesses. Thus, the average reduction factor tends to be systematically lower than the reduction factor associated to the same rock thickness.


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

Tanaka HK - Sci Rep (2015)

Inversed reduction factor as a function of the average thickness of the tunnel overburden.The error bars associated with the data points (blue dots) show the 1σ upper and lower limits. Theoretical curves were calculated by assuming uniform overburden densities of 2.0 g/cm3 (red), 2.5 g/cm3 (gray), and 3.0 g/cm3 (yellow).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Inversed reduction factor as a function of the average thickness of the tunnel overburden.The error bars associated with the data points (blue dots) show the 1σ upper and lower limits. Theoretical curves were calculated by assuming uniform overburden densities of 2.0 g/cm3 (red), 2.5 g/cm3 (gray), and 3.0 g/cm3 (yellow).
Mentions: Fig. 3 plots the inverse of the observational reduction factor (Nμ /N0) as a function of averaged overburden thickness. The data collected from all of three peninsulas were used to plot this graph along with theoretically predicted reduction factors for different assumed densities (see Method section). By comparing the data points with these theoretical predictions, the density averaged over these three regions was deduced to be 2.51 ± 0.02 g/cm3. This muographically deduced density was compared with independent results from gravity studies. Nawa et al. (1997)3 reported the average density of tertiary sedimentary and volcanic rocks above sea level in Southwest Japan to be 2.54 ± 0.15 g/cm3 and 2.53 ± 0.13 g/cm3, respectively. These values are in agreement with our results. As shown in Fig. 4, one data point (between 30 and 40 m) shows density (2.35 g/cm3) much lower than 2.51 g/cm3. This is because the average density was affected by several low density points measured along the active fault lines as described more in detail in the following subsections and the discussion section. Since our detector is moving inside the tunnel, and cannot distinguish muons with different angles of incidence, the thickness in Fig. 4 is presented as “the average reduction factor”, which is the reduction factor calculated for a mixture of various rock thicknesses. Thus, the average reduction factor tends to be systematically lower than the reduction factor associated to the same rock thickness.

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.