Tanaka Hiroyuki K M
Earthquake Research Institute, The University of Tokyo, 113-0032 Tokyo.
Sci Rep. 2015 Feb 9;5:8305. doi: 10.1038/srep08305.
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.
虽然确定陆地块体的 bulk density 分布的好处显而易见，但依赖重力测量的现有实验技术无法唯一确定地下密度分布。我们利用遍布全国的交通隧道来解决这个问题。在隧道中测量宇宙射线μ子通量，以确定每个岩石覆盖层的平均密度。以日本三浦、南房总半岛和南伊豆半岛 146 个观测点收集的数据为例进行分析后，我们绘制出了面积为 1340 km² 的区域的浅层密度分布图。我们发现μ子成像确定的密度分布与现有地质研究中描述的地质特征之间有很好的一致性。每个半岛下方的平均浅层密度分布测定精度很高（小于±0.8%）。我们还观察到沿断层线密度有显著降低，并将其解释为是由于反复地震事件期间机械应力导致的多条裂缝的存在。我们表明，这种新型的μ子成像技术可用于估算地形密度和孔隙度分布，从而确定更精确的布格校正密度。
