ALBERT

Description: We present analyses of the 2D seismic structure beneath Source Physics Experiments (SPE) geophone lines that extended radially at 100 m spacing from 100 to 2000 m from the source borehole. With seismic sources at only one end of the geophone lines, standard refraction profiling methods cannot resolve seismic velocity structures unambiguously. In previous work, we demonstrated overall agreement between body-wave refraction modeling and Rg dispersion curves for the least complex of the five lines. A more detailed inspection supports a 2D reinterpretation of the structure. We obtained Rg phase velocity measurements in both the time and frequency domains, then used iterative adjustment of the initial 1D body-wave model to predict Rg dispersion curves to fit the observed values. Our method applied to the most topographically severe of the geophone lines is supplemented with a 2D ray-tracing approach, whose application to P -wave arrivals supports the Rg analysis. In addition, midline sources will allow us to refine our characterization in future work.

Description: Surface wave magnitude M s for a compilation of 72 nuclear tests detonated in hard rock media for which yields and burial depths have been reported in the literature is shown to scale with yield W as a + b x log[ W ], where a = 2.50 ± 0.08 and b = 0.80 ± 0.05. While the exponent b is consistent with an M s scaling model for fully coupled, normal containment-depth explosions, the intercept a is offset 0.45 magnitude units lower than the model. The cause of offset is important to understand in terms of the explosion source. Hard rock explosions conducted in extensional and compressional stress regimes show similar offsets, an indication that the tectonic setting in which an explosion occurs plays no role causing the offset. The scaling model accounts for the effects of source medium material properties on the generation of 20-s period Rayleigh wave amplitudes. Aided by thorough characterizations of the explosion and tectonic release sources, an extensive analysis of the 1963 October 26 Shoal nuclear test detonated in granite 27 miles southeast of Fallon NV shows that the offset is consistent with the predictions of a material damage source model related to non-linear stress wave interactions with the free surface. This source emits Rayleigh waves with polarity opposite to waves emitted by the explosion. The Shoal results were extended to analyse surface waves from the 1962 February 15 Hardhat nuclear test, the 1988 September 14 Soviet Joint Verification Experiment, and the anomalous 1979 August 18 northeast Balapan explosion which exhibits opposite polarity, azimuth-independent source component U 1 compared to an explosion. Modelling these tests shows that Rayleigh wave amplitudes generated by the damage source are nearly as large as or larger than amplitudes from the explosion. As such, destructive interference can be drastic, introducing metastable conditions due to the sensitivity of reduced amplitudes to Rayleigh wave initial phase angles of the explosion and damage sources. This meta-stability is a likely source of scatter in M s -yield scaling observations. The agreement of observed scaling exponent b with the model suggests that the damage source strength does not vary much with yield, in contrast to explosions conducted in weak media where M s scaling rates are greater than the model predicts, and the yield dependence of the damage source strength is significant. This difference in scaling behaviour is a consequence of source medium material properties.

Description: Seismic moments for the first four chemical tests making up phase I of the Source Physics Experiments (SPE) are estimated from 6-Hz Rg waves recorded along a single radial line of geophones under the assumption that the tests are pure explosions. These apparent explosion moments are compared with moments determined from the reduced displacement potential method applied to free-field data. Light detection and ranging (lidar) observations, strong ground motions on the free surface in the vicinity of ground zero, and moment tensor inversion results are evidence that the fourth test SPE-4P is a pure explosion, and the moments show good agreement, 8 x 10 10 N·m for free-field data versus 9 x 10 10 N·m for Rg waves. In stark contrast, apparent moments for the first three tests are smaller than near-field moments by factors of 3–4. Relative amplitudes for the three tests determined from Rg interferometry using SPE-4P as an empirical Green’s function indicate that radiation patterns are cylindrically symmetric within a factor of 1.25 (25%). This fact assures that the apparent moments are reliable even though they were measured on just one azimuth. Spallation occurred on the first three tests, and ground-based lidar detected permanent deformations. As such, the source medium suffered late-time damage. Destructive interference between Rg waves radiated by explosion and damage sources will reduce amplitudes and explain why apparent moments are smaller than near-field moments based on compressional energy emitted directly from the source.

Description: The precision of M s -yield-scaling results is exploited to place tighter constraints on the volumetric moment due to source-medium damage than previously estimated for Pahute Mesa explosions on the Nevada Test Site (NTS). Results for two coupling scenarios, one based on P waves to set a lower bound and one based on Rayleigh waves to set an upper bound, bracket the predictions of a model based on moment tensor theory for an explosion monopole and the accompanying damage. This study confirms that the apparent explosion moment M I is a consequence of direct effects of the energy release with a volumetric moment M t due to cavity formation and the effects due to source-medium damage. The source model predicts that M I = M t ( K +2)/3, where K is a damage index and a value of 1 means no permanent deformation due to damage. Excess moment due to dilation of the source medium ( K 〉1) is quantified and shown to be a factor increasing the apparent yield ( W ) scaling of M s from 0.80log[ W ] for a pure explosion with cube-root containment practice and uniform coupling to ~1.0log[ W ], a scaling commonly accepted by the explosion community. Scaling observations are related to the source model by establishing the equivalence between network M s and the theoretical Rayleigh-wave radiation for an azimuthal-independent source component. This equivalence motivates a physical basis for transporting observations to other test sites. Transported M s scaling results for NTS indicate that damage is a more important source of Rayleigh waves for Balapan explosions, most likely due to better energy coupling of upgoing shock waves and stronger free-surface interactions than for NTS explosions.

Description: The geology near a seismic source has a major effect on seismic waves recorded at distance. This can be especially true in the case of man-made explosions, due to increased geologic heterogeneity at shallow depths and interactions with the free surface. Yucca Flat (YF), a sedimentary basin on the Nevada National Security Site, has hosted hundreds of well-recorded underground nuclear tests. As such, it should be an ideal natural laboratory for the study of shallow explosions. Unfortunately, basin-wide models of such important physical properties as compressive- and shear-wave velocity are not available with sufficient fidelity to maximize the potential of the studies. We attempt to remedy this situation by creating a new shear-wave velocity model of YF. This model was generated by inverting Rayleigh-wave phase-velocity dispersion measurements. Because no single dataset provided a dispersion curve of the necessary frequency bandwidth for shallow, intermediate, and deep basin depths simultaneously, we combined three dispersion curves with complementary bandwidths from three data sources. The datasets, in order of low frequency to high, were (1) underground nuclear tests at YF, recorded on regional seismic networks (0.14–0.4 Hz); (2) a multimode spatially averaged coherency microtremor array located on YF (0.2–20 Hz); and (3) several refraction microtremor (ReMi) linear arrays, also on YF (2.5–50 Hz). Compared to previous work, our model is characterized by slower velocities. The known geologic boundaries such as the depth of the basin and water table are prominent at reasonable locations.

Description: A new method for studying the source characteristics of regional phases including explosion-generated S waves is developed and utilizes differences between spectrograms of two closely located explosions recorded at a common station. Relative source effects of a normal-buried explosion with respect to an overburied explosion are isolated in the resultant difference spectrogram because path and receiver site effects cancel. Difference spectrograms provide a global view of the relative frequency content, while the spectral ratio method is specific to the time window selected for Fourier analysis. Difference spectrograms for Nevada test site (NTS) explosions are characterized by the presence of amplitude modulations in Pg coda, in time windows predicted for Sn and Lg, and in long-duration Lg codas. Previous studies have modeled similar features in Lg spectral ratios by invoking the Rg-to-S scattering hypothesis and the notion of Rg imprinting, where the modulations arise due to interference of Rg waves from explosion and induced tensile failure sources. We propose that spectral peaking at low frequencies in Lg spectra is related to resonances in the source medium that affect the excitation and propagation of Rg waves, adding further support to Rg imprinting and the Rg-to-S scattering hypothesis. Difference spectrograms for explosion pairs Rousanne/Techado and Baseball/Borrego are interpreted to have a spectral null related to a pulse of Rg-derived energy from a common scatterer. Using the relative timing of this null, we estimate an Rg velocity ( approximately 0.8 km/sec) and locate a candidate scattering source at the Yucca Basin boundary near Climax Stock. Difference spectrograms provide a wealth of information about the spectral content of regional phases related to source effects, which can be used to gain insights into source generation processes.

Description: The geology near a seismic source has a major effect on seismic waves recorded at distance. This can be especially true in the case of man-made explosions, due to increased geologic heterogeneity at shallow depths and interactions with the free surface. Yucca Flat (YF), a sedimentary basin on the Nevada National Security Site, has hosted hundreds of well-recorded underground nuclear tests. As such, it should be an ideal natural laboratory for the study of shallow explosions. Unfortunately, basin-wide models of such important physical properties as compressive- and shear-wave velocity are not available with sufficient fidelity to maximize the potential of the studies. We attempt to remedy this situation by creating a new shear-wave velocity model of YF. This model was generated by inverting Rayleigh-wave phase-velocity dispersion measurements. Because no single dataset provided a dispersion curve of the necessary frequency bandwidth for shallow, intermediate, and deep basin depths simultaneously, we combined three dispersion curves with complementary bandwidths from three data sources. The datasets, in order of low frequency to high, were (1) underground nuclear tests at YF, recorded on regional seismic networks (0.14-0.4 Hz); (2) a multimode spatially averaged coherency microtremor array located on YF (0.2-20 Hz); and (3) several refraction microtremor (ReMi) linear arrays, also on YF (2.5-50 Hz). Compared to previous work, our model is characterized by slower velocities. The known geologic boundaries such as the depth of the basin and water table are prominent at reasonable locations.