Underground Chemical Explosions Research Articles

Spatially Dependent Seismic Wavefield Scattering from an Underground Chemical Explosion: Analysis of the Source Physics Experiment Dry Alluvium Geology Large-N Array

ABSTRACT Explosion sources have been observed to generate significant shear-wave energy despite their isotropic nature. To investigate this phenomenon, we conduct an analysis of the seismic data collected as part of the Source Physics Experiment (SPE): Dry Alluvium Geology (DAG) and investigate the generation of shear-wave energy via scattering. The data were produced by three underground chemical explosions and consist of three-component seismograms, which were recorded by the DAG Large-N array. Synthetic tests suggest that for the DAG experiments, small-scale stochastic heterogeneities, defined as features with correlation lengths of 10–100s of meters, are more effective than large-scale geologic structure (scales >1–10 km) at reproducing the scattering of explosion generated wavefields observed at DAG. We analyze the seismic data for spatially variable ratios between transversely and radially polarized seismic energy, and then estimate the mean free path of P and S waves. All analyses are conducted within a frequency band of 5–50 Hz. The ratio of transversely to radially polarized energy is the highest in the east and west portion of the Large-N array. In addition, the magnitude of the estimated S-wave mean free path is shorter in the eastern portion of the Large-N array. This variation indicates that the eastern area of the DAG array is where more scattering is occurring, suggesting azimuthal dependence of P-to-P and P-to-S scattering. This azimuthal dependence of P-to-S scattering can have implications for explosion discrimination based on spectral ratios of seismic wave types, because the general assumption is that explosions do not generate shear-wave energy. Synthetic tests modeling only larger-scale geologic structure had lower transversely polarized energy (only four stations showing a transversely to radially polarized energy ratio greater than 1) and fewer stations (<10) displaying shorter (<300 m) mean free paths than what was observed in the DAG data results.

Analysis on propagation law of shallow underground chemical explosion seismic waves

Introduction: Seismic waves generated by shallow underground explosions propagate differently from those generated by surface explosions. Thus, an accurate understanding of the propagation laws of seismic waves generated by explosions at various burial depths and TNT equivalent amounts is significant in assessing the destructive power of munitions and establishing guidelines for their application.Methods: In this study, we conducted several ground vibration velocity tests of shallow underground chemical explosion seismic waves for various TNT equivalent amounts and burial depths in a shooting range and analyzed the propagation of the seismic waves. Using the explosion similarity theory and dimensional analysis, we derived an equation for the estimation of the particle vibration velocity of shallow underground chemical explosion seismic waves. This equation calculation results have a very high degree of agreement with the measured data, measured data verify that the accuracy of the calculation model is better than 90.2%.Results and discussion: This equation calculation results have a very high degree of agreement with the measured data, measured data verify that the accuracy of the calculation model is better than 90.2%, which greatly improves the calculation accuracy of the shallow underground chemical explosion seismic wave particle vibration velocity, and thus provide effective theoretical support for analyzing explosion seismic waves in engineering tests.

3D Time-Lapse Electrical Resistivity Imaging of Rock Damage Patterns and Gas Flow Paths Resulting from Two Underground Chemical Explosions

Rock damage from underground nuclear explosions (UNEs) has a strong influence on sub-surface gas movement and on seismic waveform characteristics, both of which are used to detect UNEs. Although advanced numerical simulation capabilities exist to predict rock damage patterns and corresponding detection signals, those predictions are dependent on (generally) unknown properties of the host rock. For example, the effects of in-situ mechanical heterogeneities on the explosively generated damage/fractures that provide gas flow pathways to the surface are not well understood, due largely to the difficulty in accessing and characterizing the near-source region. In this paper we demonstrate the emerging use of electrical resistivity tomography (ERT) for imaging rock damage and gas flow patterns resulting from two relatively small-scale underground chemical explosions. Pre-explosion ERT and crosshole seismic imaging revealed a natural fracture zone within the test bed. Post-explosion imaging revealed that the damage zone was non-symmetric and was focused primarily within the pre-existing fracture zone, located 10 m above the first explosion and 5 m above the second explosion. Time-lapse ERT imaging of heated air injected into the detonation borehole revealed the primary gas flow paths to be within the upper margin of the same primary damage zone. These results point to the utility of ERT imaging for understanding rock damage and gas flow patterns under experimental conditions, and to the importance of understanding the effects of geologic heterogeneity on UNE detection signals, particularly gas surface breakthrough times.

On various mechanisms of shear wave generation from underground chemical explosions in hard rocks

SUMMARY Significant shear waves observed during underground explosions can make them look similar to natural earthquakes. In the near field, they can be caused by the source geometry and emplacement conditions, heterogeneous and anisotropic properties of the rock mass, relaxation of a non-uniform in situ stress around the source after the departure of the shock wave as well as by activation of nearby faults. The stress relaxation mechanism is important at depth when the difference in main principal stresses becomes significant. Another mechanism of shear motion generation is due to explosive gas flow from the source cavity into the cracks initiated by the shock wave and joints connected to the cavity. Also, the shock wave from the explosion can cause weakening of the rock either due to permanent damage or shock-induced liquefaction at saturated joints. Numerical modelling of underground chemical explosions in hard rock at various confinement stresses has been conducted to compare various mechanisms of shear wave generation and their seismic characteristics with that of a simple fault rupture process observed during an earthquake.

Using Dense Array Waveform Correlations to Build a Velocity Model with Stochastic Variability

ABSTRACTIn an ongoing effort to improve 3D seismic-wave propagation modeling for frequencies up to 10 Hz, we used cross correlations between vertical-component waveforms from an underground chemical explosion to estimate the statistical properties of small-scale velocity heterogeneities. The waveforms were recorded by a dense 2D seismic array deployed during the Source Physics Experiments for event number 5 (SPE-5) in a series of six underground chemical explosions, conducted at the Nevada National Security Site. The array consisted of 996 geophones with a 50–100 m grid spacing, deployed at the SPE site at the north end of the Yucca Flat basin. The SPE were conducted to investigate the generation and propagation of seismic and acoustic waves from underground explosions.Comparisons of decay rates of waveform cross correlations as function of interstation distance, computed for observed and synthetic seismograms from the SPE-5 chemical explosion, were used to constrain statistical properties of correlated stochastic velocity perturbations representing small-scale heterogeneities added to a geology-based velocity model of the Yucca Flat basin. Using comparisons between recorded and simulated waveform cross correlations, we were able to recover sets of statistical properties of small-scale velocity perturbations in the velocity model that produce the best-fit between the recorded and simulated ground motion. The stochastic velocity fluctuations in the velocity model that produced the smallest misfits have a horizontal correlation distance of between 400 and 800 m, a vertical correlation distance between 100 and 200 m, and a standard deviation of 10% from the nominal model velocity in the alluvium basin layers. They also have a horizontal correlation distance of 1000 m, a vertical correlation distance of 250 m, and a standard deviation of 6% in the underlying and consolidated sedimentary layers, up to a depth of 4 km.Comparisons between observed and simulated wavefields were used to assess the proposed small-scale heterogeneity enhancements to the Yucca Flat basin model. We found that adding a depth-resolved stochastic variability to the geology-based velocity model improves the overall performance of ground-motion simulations of an SPE-5 explosion in the modeled frequency range up to 10 Hz. The results may be applicable to other similar basins.

Comparison of Surface Phenomena Created by Underground Chemical Explosions in Dry Alluvium and Granite Geology From Fully Polarimetric VideoSAR Data

Phase I of the Source Physics Experiment (SPE) series involved six underground chemical explosions, all of which were conducted at the same experimental pad. Research from the sixth explosion of the series (SPE-6) demonstrated that polarimetric synthetic aperture radar (PolSAR) is a viable technology for monitoring an underground chemical explosion when the geologic structure is Cretaceous granitic intrusive. It was shown that a durable signal is measurable by the H/A/α polarimetric decomposition parameters. After the SPE-6 experiment, the SPE program moved to the Phase II location, which is composed of dry alluvium geology (DAG). The loss of wavefront energy is greater through dry alluvium than through granite. In this article, we compare the SPE-6 analysis to the second DAG (DAG-2) experiment. We hypothesize that despite the geology at the DAG site being more challenging than at the Phase I location, combined with the DAG-2 experiment having a 3.37 times deeper scaled depth of burial than the SPE-6, a durable nonprompt signal is still measurable by a PolSAR sensor. We compare the PolSAR time-series measures from videoSAR frames, from the SPE-6 and DAG-2 experiments, with accelerometer data. We show which PolSAR measures are invariant to the two types of geology and which are geology dependent. We compare a coherent change detection (CCD) map from the DAG-2 experiment with the data from a fiber-optic distributed acoustic sensor to show the connection between the spatial extent of coherence loss in CCD maps and spallation caused by the explosion. Finally, we also analyze the spatial extent of the PolSAR measures from both explosions.

Effect of Random 3D Correlated Velocity Perturbations on Numerical Modeling of Ground Motion from the Source Physics Experiment

ABSTRACTExplosions are traditionally discriminated from earthquakes, using the relative amplitude of compressional and shear waves at regional and teleseismic distances known as the P/S discriminant. Pyle and Walter (2019) showed this technique to be less robust at shorter distances, in detecting small-magnitude earthquakes and low-yield explosions. The disparity is largely due to ground motion from small, shallow sources being significantly impacted by near-surface structural complexities. To understand the implications of wave propagation effects in generation of shear motion and P/S ratio during underground chemical explosions, we performed simulations of the Source Physics Experiment (SPE) chemical explosions using 1D and 3D velocity models of the Yucca Flat basin. All simulations used isotropic point sources in the frequency range 0–5 Hz. We isolate the effect of large-scale geological structure and small-scale variability at shallow depth (<5 km), using a regional 3D geologic framework model (GFM) and the GFM-R model derived from the GFM, by adding correlated stochastic velocity perturbations. A parametric study of effects of small-scale velocity variations on wave propagation, computed using a reference 1D velocity model with stochastic perturbations, shows that the correlation length and depth of stochastic perturbations significantly impact wave scattering, near-surface wave conversions, and shear-wave generation. Comparisons of recorded and simulated waveforms for the SPE-5 explosion, using 3D velocity models, demonstrate that the shallow structure of the Yucca Flat basin contributes to generation of observed shear motion. The inclusion of 3D wave scattering, simulated by small-scale velocity perturbations in the 3D model, improves the fit between the simulated and recorded waveforms. In addition, a relatively low intrinsic attenuation, combined with small-scale velocity variations in our models, can confirm the observed wave trapping and its effect on duration of coda waves and the spatial variation of P/S ratio at basin sites.

The Complicated Link Between Material Properties and Microfracture Density for an Underground Explosion in Granite

AbstractGeophysical 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. Here we present material property data: unconfined compressive strength, bulk density (ρ), Young's modulus (E), Poisson's ratio (ν), P wave velocity (Vp), and S wave velocity (Vs), 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 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.

Comparison of PolSAR Surface Measurements From Underground Chemical Explosions to Recorded and Predicted Surface Ground Motion

The Source Physics Experiment (SPE) Phase I conducted six underground chemical explosions at the same experimental pad with the goal of characterizing underground explosions to enhance the United States (U.S.) ability to detect and discriminate underground nuclear explosions (UNEs). A fully polarimetric synthetic aperture RADAR (PolSAR) collected imagery in VideoSAR mode during the fifth and sixth explosions in the series (SPE-5 and SPE-6). Previously, we reported the prompt PolSAR surface changes cause by SPE-5 and SPE-6 explosions within seconds or minutes of the underground chemical explosions, including a drop of spatial coherence and polarimetric scattering changes. Therein it was hypothesized that surface changes occurred when surface particles experienced upward acceleration greater than 1 g. Because the SPE site was instrumented with surface accelerometers, we explore that hypothesis and report our findings in this article. We equate explosion-caused prompt surface expressions measured by PolSAR to the prompt surface movement measured by accelerometers. We tie these findings to UNE detection by comparing the PolSAR and accelerometer results to empirical ground motion predictions derived from accelerometer recordings of UNEs collected prior to cessation of U.S. nuclear testing. We find the single threshold greater than 1 g hypothesis is not correct for it does not explain the PolSAR results. Our findings show PolSAR surface coherence spatial extent is highly correlated with surface velocity, both measured and predicted, and the resulting surface deformation extent is corroborated by accelerometer records and the predicted lateral spall extent. PolSAR scattering changes measured during SPE-6 are created by the prompt surface displacement being larger than the spall gap.

The Effects of Atmospheric Models on the Estimation of Infrasonic Source Functions at the Source Physics Experiment

ABSTRACTWe invert infrasound signals for an equivalent seismoacoustic source function using different atmospheric models to produce the necessary Green’s functions. The infrasound signals were produced by a series of underground chemical explosions as part of the Source Physics Experiment (SPE). In a previous study, we inverted the infrasound data using so-called predictive atmospheric models, which were based on historic, regional-scaled, publicly available weather observations interpolated onto a 3D grid. For the work presented here, we invert the same infrasound data, but using atmospheric models based on weather data collected in a time window that includes the approximate time of the explosion experiments, which we term postdictive models. We build two versions of the postdictive models for each SPE event: one that is based solely on the regional scaled observations, and one that is based on both regional scaled observations combined with on-site observations obtained by a weather sonde released at the time of the SPE. We then invert the observed data set three times, once for each atmospheric model type. We find that the estimated seismoacoustic source functions are relatively similar in waveform shape regardless of which atmospheric model that we used to construct the Green’s functions. However, we find that the amplitude of the estimated source functions is systematically dependent on the atmospheric model type: using the predictive atmospheric models to invert the data generally yields estimated source functions that are larger in amplitude than those estimated using the postdictive models.

Acoustic Overpressure Signals from Fully Confined and Vented Chemical Explosions

ABSTRACT We analyzed acoustic overpressure signals generated by overburied underground chemical explosions conducted in hard rock in New Hampshire in 2018. The explosions had comparable yields between 62.8 and 82.6 kg trinitrotoluene equivalent and were buried at depths between 12 and 13 m. Two explosions resulted in crater formation and gas venting, whereas the remaining explosions were fully confined and did not result in ground failure. Acoustic signals from the confined explosions were produced by the ground shock near ground zero. Acoustic signals from cratered explosions represent a combination of a ground shock signal and a time-delayed high-amplitude signal generated by gas venting. The cratering and venting occurred during the free-fall phase observed on the near-source accelerograms. We argue that the main reason for the cratering in this experiment is the low-rock porosity, preventing postexplosion pressure relief in the cavity and promoting long fracture formation during the unloading phase and subsequent containment failure. The ground-shock-induced signals were modeled using the Rayleigh integral of the near-source ground acceleration. The equations of nonlinear acoustics were used to model the observed gas venting signals produced by the gas flow from the explosion cavity to the surface. By comparing the near-source signals produced by venting to theoretical signals from surface blasts we have shown that the venting signals have significantly lower peak pressures and longer signal durations compared to surface blasts of the equivalent impulse. The observed amplitudes of the acoustic signals produced by the venting are significantly higher than the ground-shock related signals expected from overburied explosions. This is important to consider because higher acoustic amplitudes may potentially lead to errors in yield estimation.

Rapid clay precipitation in explosion-induced fractures

Abstract 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.

Monitoring Surface Phenomena Created by an Underground Chemical Explosion Using Fully Polarimetric VideoSAR

Sandia National Laboratories flew its Facility for Advanced RF and Algorithm Development X-Band (9.6-GHz center frequency), fully polarimetric synthetic aperture radar (PolSAR) in VideoSAR mode to collect complex-valued SAR imagery before, during, and after the sixth Source Physics Experiment’s (SPE-6) underground explosion. The VideoSAR products generated from the data sets include “movies” of single-and quad-polarization coherence maps, magnitude imagery, and polarimetric decompositions. Residual defocus, due to platform motion during data acquisition, was corrected with a digital elevation model-based autofocus algorithm. We generated and exploited the VideoSAR image products to characterize the surface movement effects caused by the underground explosion. Unlike seismic sensors, which measure local area seismic waves using sparse spacing and subterranean positioning, these VideoSAR products captured high-spatial resolution, 2-D, time-varying surface movement. The results from the fifth SPE (SPE-5) used single-polarimetric VideoSAR data. In this paper, we present single-polarimetric and fully polarimetric VideoSAR results while monitoring the SPE-6 underground chemical explosion. We show that fully polarimetric VideoSAR imaging provides a unique, coherent, time-varying measure of the surface expression of the SPE-6 underground chemical explosion. We include new surface characterization results from the measured PolSAR SPE-6 data via $H/A/\alpha $ polarimetric decomposition.

Yield and Emplacement Depth Effects on Acoustic Signals from Buried Explosions in Hard Rock

Research Article| April 02, 2019 Yield and Emplacement Depth Effects on Acoustic Signals from Buried Explosions in Hard Rock Daniel C. Bowman Daniel C. Bowman Corresponding Author aSandia National Laboratories, 1515 Eubank SE MS 0404, Albuquerque, New Mexico 87123 U.S.A., dbowma@sandia.gov Search for other works by this author on: GSW Google Scholar Author and Article Information Daniel C. Bowman Corresponding Author aSandia National Laboratories, 1515 Eubank SE MS 0404, Albuquerque, New Mexico 87123 U.S.A., dbowma@sandia.gov Publisher: Seismological Society of America First Online: 02 Apr 2019 Online Issn: 1943-3573 Print Issn: 0037-1106 © Seismological Society of America Bulletin of the Seismological Society of America (2019) 109 (3): 944–958. https://doi.org/10.1785/0120180285 Article history First Online: 02 Apr 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Daniel C. Bowman; Yield and Emplacement Depth Effects on Acoustic Signals from Buried Explosions in Hard Rock. Bulletin of the Seismological Society of America 2019;; 109 (3): 944–958. doi: https://doi.org/10.1785/0120180285 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract Subterranean energy releases such as explosions and earthquakes may disturb the Earth‐atmosphere interface, creating acoustic waves that can travel great distances. These waves provide a record of the ground motion directly above the event. The information they encode may provide critical insight into the depth and size of underground explosions, the sequence of events immediately before volcanic eruptions, and the magnitude of strong motion resulting from earthquakes. However, the effect of event size and burial depth on the resulting acoustic wave has not been explored in detail. Here, the relationship between acoustic amplitude, frequency, and energy is investigated for a series of well‐characterized underground chemical explosions in granite. Acoustic amplitude was found to vary linearly with explosive yield divided by emplacement depth. Peak acoustic frequency appears to be a function of explosive yield alone. The ratio of radiated acoustic energy to source energy had a relatively poor fit to yield, depth, and combinations thereof. These relationships suggest that acoustic analysis can be used to determine the size and depth of a buried explosion. The results presented here have particular relevance to the nuclear monitoring community, because depth is difficult to determine with seismic methods. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Rock Damage and Seismic Radiation: A Case Study of the Chemical Explosions in New Hampshire

Research Article| October 30, 2018 Rock Damage and Seismic Radiation: A Case Study of the Chemical Explosions in New Hampshire Anastasia Stroujkova Anastasia Stroujkova aWeston Geophysical Corp., 181 Bedford Street, Suite 1, Lexington, Massachusetts 02420, ana@westongeo.com Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (2018) 108 (6): 3598–3611. https://doi.org/10.1785/0120180117 Article history first online: 30 Oct 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Anastasia Stroujkova; Rock Damage and Seismic Radiation: A Case Study of the Chemical Explosions in New Hampshire. Bulletin of the Seismological Society of America 2018;; 108 (6): 3598–3611. doi: https://doi.org/10.1785/0120180117 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract The effect of the medium damage on seismic waves generated by underground chemical explosions in hard rock is investigated. An explosion experiment conducted in New Hampshire in 2016 has demonstrated that the amount of explosive gas products released in the cavity improves seismic coupling, which is manifested in higher seismic amplitudes. It has also shown that detonating explosions in water‐filled boreholes has similar effects on the spectra as increasing the amount of the gaseous detonation products by changing the explosive type. A postexplosion well logging survey revealed that using explosives releasing higher amount of gaseous products results in an increase in length and aperture of the explosion generated macrofractures. Placing the charges into water‐filled rather than drained boreholes results in a similar increase in fracture dimensions. Thus, the extent and intensity of postexplosion macrofracturing correlates with improved seismic coupling expressed as P and Rg amplitude increase, particularly in the low‐frequency range.The radiation patterns of the P waves are different for the waveforms bandpassed in a low‐frequency range (1–15 Hz) and a high‐frequency range (15–100 Hz). The symmetry of the radiation patterns indicates the presence of nonzero terms associated with the off‐diagonal moment tensor terms (⁠Mxz and Myz⁠). The amplitude of the seismic component attributed to the off‐diagonal moment tensor elements is significant and can be as large as 15%–16% of the isotropic moment. The observed P‐wave radiation pattern is consistent with either sliding along the pre‐existing fractures and zones of weakness or shear failure along the high‐angle borehole parallel fractures during the explosions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Multi-scale fracture damage associated with underground chemical explosions

Understanding rock damage induced by explosions is critical for a number of applications including the monitoring and verification of underground nuclear explosions, mine safety issues, and modeling fluid flow through fractured rock. We use core observations, televiewer logs, and thin section observations to investigate fracture damage associated with two successive underground chemical explosions (SPE2 and SPE3) in granitic rock at both the mesoscale and microscale. We compare the frequency and orientations of core-scale fractures, and the frequency of microfractures, between a pre-experiment core and three post-experiment cores. Natural fault zones and explosion-induced fractures in the vicinity of the explosive source are readily apparent in recovered core and in thin sections. Damage from faults and explosions is not always apparent in fracture frequency plots from televiewer logs, although orientation data from these logs suggests explosion-induced fracturing may not align with the pre-existing fracture sets. Core-scale observations indicate the extent of explosion-induced damage is 10.0 m after SPE2 and 6.8 m after SPE3, despite both a similar size and location for both explosions. At the microscale, damage is observed to a range distance of 10.2 ± 0.9 m after SPE2, and 16.6 ± 0.9 and 11.2 ± 0.6 in two different cores collected after SPE3. Additional explosion-induced damage, interpreted to be the result of spalling, is readily apparent near the surface, but only in the microfracture data. This depth extent and intensity of damage in the near-surface region also increased after an additional explosion. This study highlights the importance of evaluating structural damage at multiple scales for a more complete characterization of the damage, and particularly shows the importance of microscale observations for identifying spallation-induced damage.

Two Sources of Nonisotropic Radiation From Underground Explosions in Granite

AbstractSignificant tangential ground motion observed during underground explosions makes it difficult to distinguish them from natural earthquakes. Such motion can be generated by the source geometry and emplacement conditions, by the heterogeneous nature of the rock mass (mechanical properties may vary in space due to the presence of cracks, joints, faults, and various geologic layers) and also by the nonuniform in situ stress state. The last mechanism is increasingly important with depth when the difference in main principal stresses becomes significant. This paper is focused on the role of material strength of the rock mass in generation of nonradial motion during explosions in prestressed media. Numerical modeling of underground chemical explosions in granite at various depths has been conducted to compare two possible mechanisms of shear wave generation. The first, caused by rock mass anisotropy, is important at shallow depth. The second is related to elastic‐plastic relaxation around the cavity created by the explosion. Tangential motions for these two mechanisms have different signatures.

Near-field non-radial motion generation from underground chemical explosions in jointed granite

Near-field non-radial motion generation from underground chemical explosions in jointed granite

Detecting Surface Changes from an Underground Explosion in Granite Using Unmanned Aerial System Photogrammetry

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.

Effect of Gaseous Products of Underground Chemical Explosions on Seismic Coupling

Abstract The effect of the amount of gaseous by‐products from small chemical explosions on seismic source signatures is examined. The explosions analyzed in this article were conducted using different types of explosives (ammonium nitrate–fuel oil and Composition B) as a part of the New England Damage Experiment (NEDE) in Barre, Vermont. These explosives have different densities, burn rates, and energy contents per unit mass, and upon detonation they produce different amounts and compositions of gaseous by‐products. The amount of gas products released during the detonation depends on the chemical composition of the explosives as well as on the presence of other compounds in and around the cavity that can vaporize or chemically decompose during the explosion (e.g., water). Analysis of the NEDE data suggests that the characteristics of the P spectra from these explosions depend on the amount of noncondensable gases released in the cavity during explosive detonation. The low‐frequency amplitudes of the explosion spectra show strong correlation with the amount of gas produced by the explosions, which can be explained by higher steady‐state pressure in the cavity resulting in a larger cavity and subsequently higher static value of the reduced displacement potential. Alternatively, the increase in low‐frequency amplitudes can be due to a longer duration of the source function caused by the late time damage from gas‐driven fracturing. The amplitudes in the high‐frequency band around the corner frequencies (overshoot amplitudes) show a relationship with the trinitrotoluene (TNT)‐equivalent yield and with the heat of the detonation. Thus, based on our dataset, the increase in the low‐frequency amplitudes with the corresponding decrease in overshoot parameter can be explained by the increase of the amount of cavity gas released during the detonation. Online Material: MP4 movies of NEDE1 and NEDE2 explosions.