ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉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 〈span〉P〈/span〉 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 〈span〉P〈/span〉‐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.〈/span〉

Description: In this article, we investigate seismic amplitude reduction for the chemical explosions conducted in low-coupling fractured rocks. The explosions used in this study were detonated in the fracture zones left by earlier explosions (repeat shots) in granitic rocks in Barre, Vermont. The seismic amplitudes from the repeat shots were compared with the amplitudes from the same-yield explosion conducted in the intact rock in the same area. The results of the experiments show seismic amplitude reduction for the repeat shots by a factor of 1.5–2 in the entire frequency range. The amplitude reduction observed for the fractured rock explosions can be explained by compaction due to pore collapse, other inelastic losses, and by reduced elastic moduli in a localized damage zone around the charge. We used the Mueller and Murphy (1971) model to calculate spectra of the first and repeat explosions. To account for the reduction of the elastic moduli in the damage zone, we propose a hybrid medium model with the intact medium outside the elastic radius and the medium with reduced elastic moduli inside the elastic radius. The contribution from the pore collapse and other inelastic losses can be modeled by changing the compaction parameter d . Both models (higher compaction and the hybrid elastic moduli) predict reduced amplitudes at all frequencies and qualitatively agree with the observations. Because of the complexities in the explosion processes and the uncertainties in the damage zone configuration, the proposed model may not exactly reproduce the explosion source; however, using the models with a combination of the parameters can provide bounds on the resulting spectral ratios.

Description: We studied seismic-wave generation from five small (60–122 kg) fully contained explosions detonated in Barre Granite as a part of the New England Damage Experiment (NEDE). The explosions were conducted using three types of explosives with different velocities of detonation (VOD): black powder, ammonium nitrate fuel oil (ANFO) emulsion, and composition B (COMP-B). Empirical evidence suggests that the low VOD explosives produce more shear-wave energy than high VOD explosives. The proposed mechanisms to explain this effect include: (a) inhibition of gas-driven fracture propagation by thicker pulverized zone for high VOD explosions, and (b) fracture toughness increase at higher loading rate. The main objective of the experiment was to study differences in shear-wave generation between different types of explosives, and to determine the likely mechanism responsible for these differences. Seismic amplitude analysis revealed that COMP-B releases more energy and larger amplitude P waves for the same weight of explosives, while producing smaller amplitude S waves. Furthermore, large radial cracks were observed on the surface after the ANFO and black powder shots, while there was no surface fracturing after the COMP-B shots. Thus, longer fractures correlate with higher S -wave amplitudes. However, drilling into the source region indicates that high VOD explosions may actually produce a smaller pulverized zone, which means that fracture inhibition is a less plausible explanation. Therefore we hypothesize that increase in loading rate, in combination with shorter impulse duration for high VOD explosives, inhibits fracture processes, and subsequently reduced S -wave radiation.

Description: Improved understanding of the seismic radiation generated by explosions in low coupling (damaged/fractured) media is extremely important for nuclear monitoring, as source coupling affects both detection and yield estimation. Some empirical evidence for seismic amplitude reductions have been noted for nuclear and chemical explosions detonated in fractured media (e.g., Sokolova, 2008 ). In order to define the physical mechanism responsible for the amplitude reduction and quantify the degree of the amplitude reduction in fractured rocks, we conducted Phase I of a multi-phase explosion experiment in central New Hampshire. The experiment involved conducting explosions of various yields, including a 46.3-kg explosion in the damage/fracture zone of a 231.8-kg explosion and a 46.8-kg shot in nearby undamaged rock. Our analysis confirms a seismic amplitude reduction in damaged rock by a factor of 2–3. The amplitude differences are frequency dependent, with the explosion in the undamaged rock having a higher corner frequency than the explosion in the damaged zone. The overshoot parameter for the virgin/undamaged rock shots is higher than that for the damaged rock shot. We found that the corner frequency correlates with the overshoot parameter, and only weakly correlates with the yield. Additional experiments will be conducted in the near future to further quantify seismic-wave characteristics as a function of the depth of burial, type of explosives, and other factors. Online Material: Movies of explosions.

Description: 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.

Description: We studied seismic body-wave generation from four fully contained explosions of approximately the same yields (68 kg of TNT equivalent, where TNT stands for trinitrotoluene) conducted in homogeneous granite in Barre, Vermont. The explosions were detonated using three types of explosives with different velocities of detonation: black powder (BP), ammonium nitrate fuel oil/emulsion (ANFO), and composition B (COMP B). The main objective of the experiment was to study differences in seismic-wave generation among different types of explosives and to determine the mechanism responsible for these differences. The explosives with slow burn rate (BP) produced lower P -wave amplitude and corner frequency, which resulted in lower seismic efficiency (0.21%) in comparison with high burn rate explosives (1.3% for ANFO and 1.9% for COMP B). The seismic efficiency estimates for ANFO and COMP B agree with previous estimates for nuclear explosions. The body-wave radiation pattern is consistent with an isotropic explosion with an added azimuthal component caused by vertical tensile fractures oriented along pre-existing microfracturing in the granite, although the complexities in the P - and S -wave radiation patterns suggest that more than one fracture orientation could be responsible for their generation. Analysis of the S / P amplitude ratios suggests that a significant fraction of the shear-wave energy can be explained by opening of the tensile fractures and spall.

Description: Since the 1960s, comparing a Rayleigh-wave magnitude Ms to the body-wave magnitude mb (Ms:mb) has been a robust tool for the discrimination of earthquakes and explosions. In this article, we apply a Rayleigh-wave formula as is to Love waves and examine the possibilities for discrimination using only surface-wave magnitudes (Ms:Ms). To calculate the magnitudes, we apply the time-domain magnitude technique called Ms(VMAX), developed by Russell (2006), to Rayleigh and Love waves from explosions and earthquakes. Our results indicate that, for the majority of the earthquakes studied (〉75%), the Ms(VMAX) obtained from Love waves is greater than that estimate from Rayleigh waves. Conversely, 79 of 82 nuclear explosions analyzed (96%) had network-averaged Ms(VMAX)-Rayleigh equal to or greater than the Ms(VMAX)-Love. We used logistic regression to examine an discriminant. Cross-validation analysis of the new discriminant correctly identifies 57 of 82 explosions and 246 of 264 earthquakes, while misidentifying 22 explosions as earthquakes and 11 earthquakes as explosions. Further comparative research is planned for versus Ms:mb using common data. We fully expect that will contribute significantly to multivariate event identification.

Description: 〈span〉〈div〉ABSTRACT〈/div〉The heterogeneous medium structure along the propagation path adds significant complexities to the far‐field 〈span〉P〈/span〉‐wave spectra. High‐frequency seismic waves radiated by explosions are strongly affected by the site conditions (i.e., subsurface geology and topography), as well as by the velocity and attenuation structure along the propagation path. Therefore, removing the propagation and site effects is particularly important for explosion monitoring. To achieve this goal, a nonparametric generalized inversion method (〈a href="https://pubs.geoscienceworld.org/srl#rf2"〉Andrews, 1986〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf3"〉Boatwright 〈span〉et al.〈/span〉, 1991〈/a〉) was implemented to simultaneously estimate the source spectra and the medium transfer functions for the six nuclear explosions conducted by North Korea. For the recovered source spectra, the estimated value of the exponent relating the corner frequency to the low‐frequency asymptote is found to be approximately 0.305±0.059. The scaling of the explosion source spectra estimated for this dataset is shown to be consistent with the explosion source model prediction (〈a href="https://pubs.geoscienceworld.org/srl#rf13"〉Mueller and Murphy, 1971〈/a〉; 〈a href="https://pubs.geoscienceworld.org/srl#rf6"〉Denny and Johnson, 1991〈/a〉). The successful application of the method to the North Korean nuclear test data shows promising results for future use in the explosion monitoring studies.〈/span〉