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Arrowsmith S.J.,Los Alamos National Laboratory | Taylor S.R.,Rocky Mountain Geophysics, Inc.
Journal of the Acoustical Society of America | Year: 2013

A methodology for the combined acoustic detection and discrimination of explosions, which uses three discriminants, is developed for the purpose of identifying weak explosion signals embedded in complex background noise. By utilizing physical models for simple explosions that are formulated as statistical hypothesis tests, the detection/discrimination approach does not require a model for the background noise, which can be highly complex and variable in practice. Fisher's Combined Probability Test is used to combine the p-values from all multivariate discriminants. This framework is applied to acoustic data from a 400 g explosion conducted at Los Alamos National Laboratory. © 2013 U.S. Government.

Taylor S.R.,Rocky Mountain Geophysics, Inc. | Arrowsmith S.J.,Los Alamos National Laboratory | Anderson D.N.,Los Alamos National Laboratory
Bulletin of the Seismological Society of America | Year: 2010

We present a methodology for the detection of small, impulsive signal transients using time-frequency spectrograms closely related to the emerging field of scan statistics. In local monitoring situations, single-channel detection of small explosions can be difficult due to the complicated nature of the local noise field. Small, impulsive signals are manifest as vertical stripes on spectrograms and are enhanced on grayscale representations using vertical detection masks. Bitmap images are formed where pixels above a defined threshold are set to one. A short-duration large bandwidth signal will have a large number of illuminated bits in the column corresponding to its arrival time. We form the marginal distribution of bit counts as a function of time, ni, by summing columnwise over frequency. For each time window we perform a hypothesis test, H0: signal + noise, by defining a probability model expected when a signal is present. This model is Bernoulli for signal versus no signal with probability of signal = p1. We assume that ni follows the binomial distribution and compute a probability of detection (represented as a p value) for a given p1.We apply the spectrogram detector to 1 hr of single-channel acoustic data containing a signal from a 1 lb chemical surface explosion recorded at 3.1 km distance and compare performance with a short-term average to long-term average (STA/LTA) detector. Both detectors are optimized through grid search and successfully detect the acoustic arrival from the 1 lb explosion. However, 70% more false detections are observed for STA/LTA than for the spectrogram detector. At great range, attenuation properties of the earth reduce the effectiveness of the spectrogram detector relative to STA/LTA. Data fusion techniques using multiple channels from a network are shown to reduce the number of false detections.

Patton H.J.,Los Alamos National Laboratory | Taylor S.R.,Rocky Mountain Geophysics, Inc.
Journal of Geophysical Research: Solid Earth | Year: 2011

Classical explosion source theory relates isotropic seismic moment to the steady state level of the reduced displacement potential. The theoretical isotropic moment for an incompressible source region Mt is proportional to cavity volume Vc created by pressurization of materials around the point of energy release. Source medium damage due to nonlinear deformations caused by the explosion will also induce volume change Vd and radiate seismic waves as volumetric, double-couple, and compensated linear vector dipole (CLVD) body force systems. A new source model is presented where K is a relative measure of moment MCLVD with respect to the net moment from volumetric sources Vc and V d. K values from moment tensor inversions steadily decrease from ∼2.5 at lower yields to ∼1.0 for the highest-yield shots on Pahute Mesa. A value of 1.0 implies MCLVD = 0 and, by inference, small V d. We hypothesize that the extent to which damage adds (or subtracts) volumetric moment is controlled by material properties and dynamics of stress wave rebound, shock wave interactions with the free surface, gravitational unloading, and slapdown of spalled near-surface layers. This hypothesis is tested by comparing measurements of isotropic moment M̂I with estimates of Mt based on Vc scaling relationships and velocity-density models. The results support the hypothesis and the conclusion that M̂I represents the "apparent explosion moment" since it has contributions from direct effects due to cavity formation and indirect effects due to material damage. Implications for yield estimation using M̂I are discussed in general and for the North Korean tests. Copyright 2011 by the American Geophysical Union.

Taylor S.R.,Rocky Mountain Geophysics, Inc. | Patton H.J.,Los Alamos National Laboratory
Geophysical Research Letters | Year: 2013

Effects of rock damage on teleseismic mb are investigated with P wave synthetic seismograms using a moment dipole Mzz as the equivalent elastic model for damage around buried explosions. Two manifestations of late-time damage, cavity rebound and bulking from block rotations, are represented by model decompositions into compensated linear vector dipole and monopole sources, respectively. For high-velocity media, P waves from damage destructively interfere with those from the explosion. This interference reduces the rate at which mb yield scales for a pure monopole source and provides a physical basis for observed scaling in hard rock, mb~0.75 log [yield]. For over-buried explosions, such as the North Korean tests, P waves from damage are weaker, and higher scaling rates are expected than explosions conducted under standard containment conditions. Our results highlight a cautionary note of transporting the same mb-log[yield] relation between test sites to estimate yield when source phenomenology is likely to be very different. © 2013. American Geophysical Union. All Rights Reserved.

Taylor S.R.,Rocky Mountain Geophysics, Inc.
Bulletin of the Seismological Society of America | Year: 2011

This paper describes a statistical methodology for earthquake/explosion discrimination using two-dimensional (2D) grids (frequency of S against frequency of P) of regional P/S ratios. A method similar to that of scan statistics is developed by applying a counting rule on an N-length bitmap image of the 2D plots. An average density of bits is computed for explosions in the training set; it is assumed that each bitmap cell represents an independent sample taken from a Bernoulli process. A hypothesis test HO: explosion is designed, and a p value (a statistical measure) indicating the degree of membership of a new event to the explosion population is computed. A statistical method is presented that allows construction of p values under the null hypothesis that the event in question is an explosion. The field of view is the lower-right triangular corner plot for P/S ratios closely related to the high-frequency P to low-frequency S discriminant. The plot is converted to a bit map, and bits are summed and treated as a Bernoulli random variable. Using a set of calibration data, the background density of bits for explosions can be estimated and used to compute p values for new events. Importantly, the p values from the 2D P/S ratios can be naturally combined with other p values from other discriminants constructed under the same hypothesis to form a multivariate discriminant as in Anderson et al. (2007).

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