PNNL

Abstract

Muon radiography is a well-established method for imaging the density structure of a number of geophysical targets. We can leverage measurements of the attenuation of cosmic-ray muons, given an appropriate geometry with the detector and the target, to build a density image of shallow geological structure, and with multiple detector locations providing sufficient crossing muon trajectories, we can build a 3D tomographic image of the sampled target. Such images have previously been obtained using low-angle muons traversing the edifice of a volcano, but we present the first such experiment conducted underground with the aim of recovering the density structure of overburden and air/ground interface. The LANL Mini Muon Tracker was operated at four locations inside a tunnel under the Los Alamos town site mesa between November 2015 and February 2016. The detector measured the flux of incoming cosmic ray muons attenuated by the tunnel overburden. In this paper we describe the data taking and analysis procedure and show the result, a three-dimensional tomography of the mesa obtained performing an unconstrained inversion of the muon data. We recover the spatial variation in the rock-air interface (the largest density contrast in the inversion), and compare it with a priori knowledge of void space. The densities obtained exhibit good agreement with preliminary results of a co-located differential gravity study, which will be developed in future work, and are compatible with values reported in the literature. The modeled rock-air interface matches the actual one within 4 m, our resolution, over a large portion of the volume considered. The success of this experiment demonstrates the potential of future borehole muon trackers to image shallow geophysical objects.