Muographic data analysis method for medium-sized rock overburden inspections

Hiroyuki K M Tanaka,Michinori Ohshiro

doi:10.5194/gi-5-427-2016

Hiroyuki K M Tanaka, Michinori Ohshiro

Open Access

https://doi.org/10.5194/gi-5-427-2016

Copy DOI

Abstract

Abstract. Muographic measurements of rock overburdens are of particular interest because they can be applied to natural resources and undiscovered cave explorations, and even to searching for hidden chambers in historic architectural structures. In order to derive the absolute density distribution of the overburden, we conventionally needed to know accurate information about the measurement conditions, e.g., the detector's geometrical acceptance, detection efficiency, and measurement time, in order to derive the absolute value of the transmitted muon flux. However, in many cases, it is not a simple task to accurately gauge such conditions. Open-sky muon data taken with the same detector are useful as reference data to cancel these factors; however, if the detector is not transportable, this data taking method is not feasible. In this work, we found that the transmitted muon flux will follow a simple function of the areal density along the muon path as long as the incident muon energies are below a few hundred GeV. Based on this finding, we proposed a simple analysis method that does not require detailed knowledge of the detector's conditions by combining the independently measured density information for the partial volume of the target. We anticipate that this simple method is applicable to future muographic measurements of rock overburdens.

Highlights

Earth’s subsurface density structures have been extensively measured with muography in the last decade
The pyramids of Giza are not geological products. Their sizes are remarkably large for man-made architecture, and the observational configuration of muography performed by Alvarez et al (1970) was essentially the same as other types of muography performed inside tunnels to measure the rock overburdens
The Pyramid of Chephren offers us a unique target volume to test our technique for the following reasons: (A) the geometrical shape of the pyramid is much simpler than regular geological targets and its topographic features have been studied with aerial surveys (Alvarez et al, 1970); (B) Gerald Lynch did a direct measurement of a rock piece exploited from the surface of the Chephron Pyramid in 1968, and derived a density of 1.8 g cm−3 (Alvarez, 1987); (C) the subsurface structure can be directly observed in the partial cross section located near the top of the Pyramid of Chephren; and (D) the Pyramid of Chephren is the only pyramid in which muography surveys were performed by Alvarez et al in 1970

Summary

Introduction

Earth’s subsurface density structures have been extensively measured with muography in the last decade. After analyzing data collected during several months of operation, they concluded that areal density of the pyramid was measured with a precision of 2 % along the muon paths that penetrated over 100 m through limestone in the pyramid. This pioneering experiment was a crucial step that eventually led to recent muographic experiments that explored inside volcanoes (Tanaka et al, 2007, 2008, 2009; Lesparre et al, 2012; Cârloganu et al, 2012; Carbone et al, 2013; Kusagaya and Tanaka, 2015a, b), industrial plants (Tanaka, 2013; Ambrosino et al, 2015), seismic faults (Tanaka et al, 2011; Tanaka, 2015), and caves (Caffau et al, 1997; Barnaföldi et al, 2012; Oláh et al, 2012)

Results

Discussion

Conclusion