Abstract
Efficient detection and high-fidelity quantification of surface changes resulting from underground activities are important national and global security efforts. In this investigation, a team performed field-based topographic characterization by gathering high-quality photographs at very low altitudes from an unmanned aerial system (UAS)-borne camera platform. The data collection occurred shortly before and after a controlled underground chemical explosion as part of the United States Department of Energy’s Source Physics Experiments (SPE-5) series. The high-resolution overlapping photographs were used to create 3D photogrammetric models of the site, which then served to map changes in the landscape down to 1-cm-scale. Separate models were created for two areas, herein referred to as the test table grid region and the nearfield grid region. The test table grid includes the region within ~40 m from surface ground zero, with photographs collected at a flight altitude of 8.5 m above ground level (AGL). The near-field grid area covered a broader area, 90–130 m from surface ground zero, and collected at a flight altitude of 22 m AGL. The photographs, processed using Agisoft Photoscan® in conjunction with 125 surveyed ground control point targets, yielded a 6-mm pixel-size digital elevation model (DEM) for the test table grid region. This provided the ≤3 cm resolution in the topographic data to map in fine detail a suite of features related to the underground explosion: uplift, subsidence, surface fractures, and morphological change detection. The near-field grid region data collection resulted in a 2-cm pixel-size DEM, enabling mapping of a broader range of features related to the explosion, including: uplift and subsidence, rock fall, and slope sloughing. This study represents one of the first works to constrain, both temporally and spatially, explosion-related surface damage using a UAS photogrammetric platform; these data will help to advance the science of underground explosion detection.
Highlights
Underground explosions can produce changes that may be observed at the surface and recorded by sensors at a range of different standoff distances
The Source Physics Experiment (SPE), a series of controlled underground chemical explosive experiments conducted under the auspices of the US Department of Energy’s National Nuclear Security Administration (NNSA), aims to advance the physical understanding of seismic wave propagation through geologic media (Snelson et al 2013)
Results of the photo data collection and subsequent digital elevation model (DEM) models are summarized in Table 1 and discussed below
Summary
Underground explosions can produce changes that may be observed at the surface and recorded by sensors at a range of different standoff distances. The ground surface damage information presented here helps validate other real-time field geologic and geophysical measurements (including surface fracture mapping, downhole accelerometer instrumentation, and seismic wave analyses recorded from near-field and farfield sensors; e.g., Snelson et al 2013; Patton 2015; Larmat et al 2017), and provides critical input into predictive models (e.g., Larmat et al 2015). This action took place approximately 800 times for each model, given the number of GCP targets and the coverage of at least 9 photos per target. The point clouds registered no data gaps due to the densely overlapping photo coverage and extension of flight paths 10–20 m beyond the desired data capture region
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