Gravity variations observed from a detailed gravity survey at the NTS cloud chamber

Abstract

As part of the Department of Energy’s Underground Structure Detection Program, personnel from Los Alamos conducted a detailed surface gravity survey over a buried cloud chamber located in Yucca Flat, Nevada Test Site (NTS). The primary purpose for conducting the gravity survey was to measure small gravity anomalies due to near-surface density variations not directly associated with the cloud chamber itself. Such density variations are considered noise sources for the purposes of this study. Few published measurements of near-surface noise over underground structures exist. Those studies that have been made (Butler, 1984) have not focused upon near-surface noise, perhaps because the overall signal-to-noise ratios in such studies were relatively high. The motivation for measuring the microgravimetric noise due to near-surface density inhomogeneities is that such sources are believed to be one of the dominant sources of microgravimetric noise. As microgravity methods are used to detect subsurface facilities or cavities, one of the limiting factors will likely be the amplitudes and wavelengths of gravity anomalies due to naturally-occurring, near-surface density changes. Thus, measurements of such effects are required in order to estimate the utility of microgravity as a practical methodology for detecting underground facilities.