Publications

Identification and characterization of underground events from surface or remote data requires a thorough understanding of the rock material properties. However, material properties usually come from borehole data, which is expensive and not always available. A potential alternative is to use topographic characteristics to approximate the strength, but this has never been done before quantitatively. Here we present the results from the first steps towards this goal. We have found that there are strong correlations between compressive and tensile strengths and slopes, but these correlations vary depending on data analysis details. Rugosity may be better correlated to strength than slope values. More comprehensive analyses are needed to fully understand the best method of predicting strength from topography for this area. We also found that misalignment of multiple GIS datasets can have a large influence on the ability to make interpretations. Lastly, these results will require further study in a variety of climatic conditions before being applicable to other sites.

Geophysical techniques are often implemented as quick and inexpensive ways to locate and characterize fractures in the subsurface, which is important for a number of geoscience fields. Seismic velocities are the most widely used proxies for identification of fractures, but the correlation is not always well-defined. In this study we present material property data: unconfined compressive strength (UCS), bulk density (ρ), Young's modulus (E), Poisson's ratio (ν), P wave velocity (V_p), and S wave velocity (V_s), in conjunction with microfracture densities measured on samples of granite collected before and after underground chemical explosions. Results indicate the relationship between fractures and material properties is complex, even in this single-lithology environment. We interpret that this complexity arises from varying fracture mechanisms (e.g. dilation-inducing fractures vs compression-inducing fractures) in different parts of the core, due to differences in stress conditions. Additional complexity may result from chemical interactions between the fresh fractures and the fluids in the area. Water content appears to have a significant, if not dominant, role in the unconfined compressive strength (UCS) of the samples. We suggest caution when using elastic property measurements as a proxy for fracturing in areas of explosion-induced damage, or in other areas where a variety of mechanisms induce fracturing.

The understanding of subsurface events that cannot be directly observed is dependent on the ability to relate surface-based observations to subsurface processes. This is particularly important for nuclear explosion monitoring, as any future clandestine tests will likely be underground. We collected ground-based lidar and optical imagery using remote, very-low-altitude unmanned aerial system platforms, before and after several underground high explosive experiments. For the lidar collections, we used a terrestrial lidar scanner to obtain high-resolution point clouds and create digital elevation models (DEMs). For the imagery collections, we used structure-from-motion photogrammetry techniques and a dense grid of surveyed ground control points to create high-resolution DEMs. Comparisons between the pre- and post-experiment DEMs indicate changes in surface topography that vary between explosive experiments with varying yield and depth parameters. Our work shows that the relationship between explosive yield and the extent of observable surface change differs from the standard scaled-depth-of-burial model. This suggests that the surface morphological change from underground high explosive experiments can help constrain the experiments' yield and depth, and may impact how such activities are monitored and verified.

Accurate event locations and replicability of location analyses are essential for assessing the nature of an event, its context, ambient site conditions, and proximity to relevant facilities and infrastructure. Additionally, accurate event locations provide valuable information that reduce uncertainties, improve confidence in event analyses, and inform in-field verification activities. However, event location/relocation and replicability are difficult due to a number of factors, including spatially-sparse network coverage in some areas of the globe and variability in seismic data processing. This team proposed that the incorporation of high-fidelity imagery as a data backbone to the analytical assessment of a suspected underground explosion and/or an advanced seismic event bulletin produced by the International Data Centre (IDC) of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO PrepCom) could reduce uncertainties and improve confidence in analyses. Specifically, temporally-separated images can reduce uncertainty by identifying areas where change has occurred (e.g., building construction or demolition, road or facilities improvements). The primary goal of this project was to develop an automated geospatial processing script for imagery change detection to better reflect needs of the technical community (including the IDC) and to make the use of such a tool accessible in a variety of settings across platforms. Technical experts at Los Alamos National Laboratory successfully built GAIA: the Geospatial Automated Imagery Analysis tool, to fill this need. GAIA combines five tool components to produce orthorectified time-separated imagery and imagery change detection maps. Our toolkit (1) reduces error by providing a standardized workflow for image analyses and (2) significantly reduces processing time from between 7 and 24+ hours to approximately 5 minutes. Technical experts at Sandia National Laboratories supported GAIA via beta-testing and by introducing a web-based system approach for increased applicability. To test the function, performance, broad application, and ease-of-use of GAIA, we applied it to four separate test cases. The results of this preliminary investigation show promise in reducing uncertainty in seismic event locations: if satellite imagery can show regions where operations that produce seismic activity likely occurred, then pursuing imagery to locate epicenters of seismic nuclear events could reduce the time needed to find the true epicenter location.

A critical component of the Underground Nuclear Explosion Signatures Experiment (UNESE) program is a realistic understanding of the post-detonation processes and changes in the environment that produce observable physical and radio-chemical signatures. Rock and fracture properties are essential parameters for modeling underground nuclear explosions. In response to the need for accurate simulations of physical and radio-chemical signatures, an experimental program to determine porosity, hydrostatic and triaxial compression, and Brazilian disc tension properties of P-Tunnel core was developed and executed. This report presents the results from the experimental program. Dry porosity for P-Tunnel core ranged from 8.7%-55%. Based on hydrostatic testing, bulk modulus was shown to increase with increasing confining pressure and ranged from 1.3GPa-42.3GPa. Compressional failure envelopes, derived from wet samples, are presented for P-Tunnel lithologies. Brazilian disc tension tests were conducted on wet samples and, along with triaxial tests, are compared with dry tests from the first UNESE test bed, Barnwell. P-Tunnel core disc tension test strength varied nearly two orders of magnitude between lithologies (0.03MPa-2.77MPa). Material tested in both tension and compression is weaker wet than dry with the exception of Strongly Welded Tuff in compression which is nearly identical in compressive strength for confining pressures of OMPa and 1 OOMPa. In addition to the inherent material properties of the rocks, fractures within the samples were quantified and characterized, in order to identify differences that might be caused by the explosion-induced damage. Finally, material property determinations are linked to optical microscopy observations. The work presented here is part of a broader material characterization effort; reports are referenced within.

Fractures within the earth control rock strength and fluid flow, but their dynamic nature is not well understood. As part of a series of underground chemical explosions in granite in Nevada, we collected and analyzed microfracture density data sets prior to, and following, individual explosions. Our work shows an ~4-fold increase in both open and filled microfractures following the explosions. Based on the timing of core retrieval, filling of some new fractures occurs in as little as 6 wk after fracture opening under shallow (<100 m) crustal conditions. These results suggest that near-surface fractures may fill quite rapidly, potentially changing permeability on time scales relevant to oil, gas, and geothermal energy production; carbon sequestration; seismic cycles; and radionuclide migration from nuclear waste storage and underground nuclear explosions.

Direct Shear (DS) and Triaxial Shear (FCT) tests from Core holes U-15n and U-15n#10 are part of a larger material characterization effort for the Source Physics Experiment (SPE) project. This larger effort encompasses characterizing a granite body from Nevada both before and after each SPE shot. Core hole U-15n is the vertically oriented source hole for all SPE shots; pre shot core was taken from this hole for DS and FCT testing. After two SPE shots were executed, an inclined core hole (U-15n#10) was drilled; both DS and FCT tests were conducted from this core hole. The first shot (SPE-1) conducted on May 3, 2011 was a calibration shot. SPE-1 was an order of magnitude smaller than the second shot (SPE-2). After SPE-2 was conducted on October 25, 2011 the aforementioned inclined core hole (U-15n#10) was drilled. At its bottom, the inclined core hole intersects the source hole. The third shot (SPE-3) occurred on July 24, 2012. Vertical and inclined core holes were drilled post SPE-3 and specimens will soon be selected for geomechanical characterization. At the time of this writing, work is ongoing at Nevada in preparation for the fourth SPE shot (SPE-4).