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  • 1
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    Explosion Shear-Wave Generation in Low-Velocity Source Media (2012)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2012-08-01
    Description: This paper investigates which mechanism of Lg generation dominates in low-velocity source media, which is important because of the central role of Lg in discrimination and yield estimation of nuclear explosions. The mechanisms investigated are surface P -to- S conversion ( pS ), generation directly by the nonspherical component of the explosion source volume, and Rg -to- S scattering. We identify and quantify observations that distinguish between mechanisms. We also specifically test the assumptions of previous work that concluded that Rg scattering is the dominant mechanism. To do so, we analyze and simulate records of adjacent, normally buried and overburied Nevada test site (NTS) explosions, and analyze deep seismic sounding (DSS) explosion Quartz 3 data. The data analyses and simulations consistently indicate that pS is the dominant source of explosion Lg in low-velocity source media, that nonspherical source components could also contribute significantly to Lg , and that scattered Rg contributes less, except possibly at very low frequencies. For NTS overburied versus normally buried explosions, we compare Lg -to- Pg spectral ratios, corner frequencies, and tangential versus vertical and radial Lg spectral nulls. We perform simulations for the NTS to compare the contributions to Lg of pS , direct S from a CLVD, and scattered Rg . Quartz 3 data show that Rg spectral nulls vary with azimuth and differ from corresponding Sg and Lg spectral nulls, counter to assumptions required by the Rg scattering hypothesis.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 2
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    A Spectrogram-Based Method of Rg Detection for Explosion Monitoring (2017)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2017-02-03
    Description: The detection of the high-frequency fundamental-mode Rayleigh-wave Rg is important for explosion monitoring efforts because it indicates a shallow source, which is most often man-made. We developed an automated Rg detector that successfully identifies Rg using the spectrogram of the seismic signal to find peaks in the characteristic Rg frequency band. We tested the detector using a network of over 200 seismometers in Wyoming, which recorded dozens of nearby coal-mining blasts and active-source tamped borehole shots. The detector finds peaks in the summed spectrogram amplitudes from 0.4 to 0.8 Hz for mining blasts and from 0.8 to 1.5 Hz for borehole shots. We achieved successful Rg detection of mining blasts across the entire array for the largest blasts, though smaller blasts had less success at distant stations, due to low signal amplitude. Similarly, Rg detection from borehole shots was successful at distances 〈50 km but was not common at farther stations. We then developed a method to estimate the probability that a null detection was indicative of a deep source and not an explosion with low signal-to-noise ratio. Finally, we compared our method with an existing Rg detection method based on finding retrograde particle motion, typical of Rg , in signal coming from the known source–receiver back azimuth. In our application, the detector worked well for more distant stations but failed at stations within the same basin as the source, where Rg has prograde particle motion. The two detectors are fundamentally different; the new detector is easier to automate but depends on Rg having larger amplitude than the pre- Rg time window, whereas the existing detector triggers based on the particle motion and the direction of arrival, independent of the amplitude. The new method, perhaps in conjunction with the existing method, can offer increased monitoring capability and higher confidence when detecting Rg . Electronic Supplement: Tables of source information and figures of example detection.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 3
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    Explosion Shear-Wave Generation in High-Velocity Source Media (2012)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2012-08-01
    Description: We evaluate the mechanisms responsible for generation of shear waves by explosions in high-velocity source media by identifying, quantifying, and modeling observations that can distinguish between commonly suggested mechanisms. We review the literature to identify regional observations that have been or can be used to distinguish between two or more mechanisms. We supplement these historical observations with new measurements of the Semipalatinsk test site (STS) event Lg and P amplitudes at Borovoye and model the observations with nonlinear source models, Rg -to- Lg upper bound calculations, and wavenumber integration synthetic seismograms for point explosions and CLVDs. Direct generation of shear waves by the nonspherical component of the source volume is consistent with the regional Lg amplitude versus yield relationship, while S * and Rg -to- Lg scattering are not. We also analyze and model a large set of Degelen explosion records from approximately 10 to 90 km. The local Sg spectral corner frequency is lower than the Pg corner frequency by approximately the source P -to- S velocity ratio, which is consistent with shear waves directly generated by the source, and inconsistent with Sg being the result of pS , S * , or Rg -to- Lg scattering. The local Sg and Rg spectra are distinctly different. Taken together, results from previous work and new observations presented here support the conclusion that explosions in high-velocity source media dominantly generate shear waves directly, through the nonspherical part of the nonlinearly deforming source volume.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 4
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    Using P/S Amplitude Ratios for Seismic Discrimination at Local Distances (2016)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2016-10-08
    Description: We used a dense seismic network to test the P / S amplitude ratio discriminant for earthquakes and explosions at distances from 20 to 200 km in a complex tectonic setting. We used data from several hundred seismic stations in northern Wyoming to observe 22 earthquakes, 37 mining blasts, and 24 borehole shots and compared their P / S amplitude ratios. Frequencies of 6–8 Hz and above achieved the best discrimination results for single-shot borehole explosions, whereas 6–8 and 8–10 Hz passbands were best for mining blasts. The P / S method appears valid for 50–200 km but fails at distances 〈50 km. Geologic heterogeneities and path effects have a large effect on amplitude measurements, even after magnitude and distance amplitude corrections are applied. Geologic boundaries between source and receiver that cause sharp lateral discontinuities in seismic velocity can alter the measured P / S ratio, to the point that an event could be misidentified. The source geology is a factor as well, with noticeable amplitude differences between shots in bedrock and shots in sediment. We tested the discrimination performance of the average log( P / S ) values of a network of seismometers, which should reduce the discrimination errors caused by these heterogeneities relative to a single station, by bootstrapping the single-station measurements over varying sample sizes. We found that averaging three or four stations achieved significantly better discrimination than just one or two stations. Finally, we corrected the signal amplitude to remove prephase noise to improve P / S discrimination results. This shows promise for improving event discrimination with only simple corrections. Online Material: Table of event information and figure showing waveforms overlaid with amplitude windows.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 5
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    Lg Group Velocity: A Depth Discriminant Revisited (2004)
    Seismological Society of America
    Details
    Publication Date: 2004-04-01
    Description: Lg group velocities are slower for very shallow events than for deeper crustal events in both southern California and Israel. An Lg group velocity depth discriminant applied to data below 1 Hz was proposed and then abandoned two decades ago, but a more precise measure of the energy distribution within Lg leads to the conclusion that higher frequency Lg group velocity may identify event depth. Because of its practical importance to nuclear monitoring and its implications for the generation and propagation of Lg, we quantify and model the delay of shallow-event Lg. We consider two mechanisms. First, shallower, slower modes are more strongly excited by shallow events, while deeper events excite faster modes. Second, Rg-to-Lg scattering for shallow events could introduce a time delay into Lg relative to Lg generated directly by the source. Further, since Rg propagates close to the surface, Rg scattering would also preferentially scatter into the shallower, slower modes. The depth dependence of modal excitation is therefore important for either mechanism, but Rg scattering could increase the time delays. Mode and finite-difference simulations demonstrate the feasibility of both hypotheses. We consider the implications of related observations for the source of the Lg delay, including predictions and observations of the frequency dependence of Lg delays. We conclude that Lg group velocity variation with event depth can be explained by preferential modal excitation and may have the potential to identify very shallow events and events too deep to be man-made. Practical application of the procedure will require regionalization to account for variations in Earth structure.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 6
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    Freezing Laboratory Materials for Plant Science (1949)
    American Association for the Advancement of Science
    Details
    Publication Date: 1949-05-20
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science
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  • 7
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    NUCLEAR BEHAVIOR IN RELATION TO CULTURE METHODS FOR PENICILLIUM NOTATUM WESTLING (1944)
    American Association for the Advancement of Science
    Details
    Publication Date: 1944-05-26
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science
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  • 8
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    An Upper Bound on Rg to Lg Scattering Using Modal Energy Conservation (2009)
    Seismological Society of America
    Details
    Publication Date: 2009-04-01
    Description: We use conservation of energy to place an upper bound on Lg generated by explosions through Rg scattering. We assume that all energy in Rg scatters very rapidly into the higher mode surface waves that make up Lg. We consider Rg generated directly by the explosion source and by a collocated but possibly shallower compensated linear vector dipole (CLVD) source. Rg scattering may be a viable mechanism for generating Lg at low frequencies (〈1 Hz), but at higher frequencies Lg generated directly by the CLVD or explosion source is comparable to or greater than the upper bound on Lg generated by Rg scattering. For shallow sources at frequencies up to 2-3 Hz the direct CLVD waves may be smaller than the direct explosion or the upper bound limit on the scattered waves. Either the direct waves or scattering from the spherical explosion Rg will dominate over scattering from the CLVD generated Rg at all frequencies.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America
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  • 9
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    Reply to "Comment on 'An Upper Bound on Rg to Lg Scattering Using Modal Energy Conservation' by Jeffry L. Stevens, Heming Xu, and G. Eli Baker" by Howard Patton and Indra N. Gupta (2010)
    Seismological Society of America
    Details
    Publication Date: 2010-01-27
    Description: Rg scattering has often been suggested as the source of the regional Lg phase generated by explosions (e.g., Patton and Taylor, 1995; Gupta and Wagner, 1998). The main point of our paper (Stevens et al., 2009) was that it is very difficult to get enough energy out of Rg to generate the observed Lg and that direct generation of S by the source or pS conversion are likely to be stronger sources of Lg than Rg scattering. To compare the contributions of each mechanism, we derived an upper bound on the possible size of Rg-to-Lg scattering by assuming that 100% of Rg is scattered very quickly into Lg with no energy loss or conversion to other phases. This is almost certainly much more Rg-to-Lg scattering than actually occurs, and we showed that even this upper bound is only comparable to or less than direct generation of Lg or of Lg generated by P-to-S conversion at the free surface. Patton and Gupta (2009) disagreed with this analysis and made five specific points, which we subsequently address. We have summarized each of their points, followed by our response. 1. Stevens et al.’s (2009) equations (19) and (21) for the spectrum of converted higher modes can be expressed as a convolution between Rg source excitation and higher-mode response for a surface load, mediated by an energy transfer function (ETF) for plane-layered velocity models. Yes, it can. Patton and Gupta’s (2009) equation 5 is identical to Stevens et al.’s (2009) equation 19 for a step function explosion source (Patton and Gupta [2009] also include a Mueller–Murphy source spectral shape, so results will differ slightly for larger events). It should be remembered, however, that this is an upper bound based on 100% energy conversion, not a prediction for the actual …
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 10
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    Explosion Shear-Wave Generation in High-Velocity Source Media (2012)
    Seismological Society of America
    Details
    Publication Date: 2012-08-01
    Description: We evaluate the mechanisms responsible for generation of shear waves by explosions in high-velocity source media by identifying, quantifying, and modeling observations that can distinguish between commonly suggested mechanisms. We review the literature to identify regional observations that have been or can be used to distinguish between two or more mechanisms. We supplement these historical observations with new measurements of the Semipalatinsk test site (STS) event Lg and P amplitudes at Borovoye and model the observations with nonlinear source models, Rg-to-Lg upper bound calculations, and wavenumber integration synthetic seismograms for point explosions and CLVDs. Direct generation of shear waves by the nonspherical component of the source volume is consistent with the regional Lg amplitude versus yield relationship, while S (super *) and Rg-to-Lg scattering are not. We also analyze and model a large set of Degelen explosion records from approximately 10 to 90 km. The local Sg spectral corner frequency is lower than the Pg corner frequency by approximately the source P-to-S velocity ratio, which is consistent with shear waves directly generated by the source, and inconsistent with Sg being the result of pS, S (super *) , or Rg-to-Lg scattering. The local Sg and Rg spectra are distinctly different. Taken together, results from previous work and new observations presented here support the conclusion that explosions in high-velocity source media dominantly generate shear waves directly, through the nonspherical part of the nonlinearly deforming source volume.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America
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