Dale N. Anderson

Detection of Short Time Transients from Spectrograms Using Scan Statistics

Abstract 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, n i , by summing columnwise over frequency. For each time window we perform a hypothesis test, H 0 : 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 = ρ 1 . We assume that n i follows the binomial distribution and compute a probability of detection (represented as a p value) for a given ρ 1 . We apply the spectrogram detector to 1xa0hr of single-channel acoustic data containing a signal from a 1xa0lb chemical surface explosion recorded at 3.1xa0km 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 1xa0lb 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.

Determination of Love- and Rayleigh-Wave Magnitudes for Earthquakes and Explosions

Since the 1960s, comparing a Rayleigh-wave magnitude M s to the body-wave magnitude m b ( M s: m b) 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 ( M s: M s). To calculate the magnitudes, we apply the time-domain magnitude technique called M s(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 M s(VMAX) obtained from Love waves is greater than that estimate from Rayleigh waves. Conversely, 79 of 82 nuclear explosions analyzed (96%) had network-averaged M s(VMAX)-Rayleigh equal to or greater than the M s(VMAX)-Love. We used logistic regression to examine an ![Graphic][1] 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 ![Graphic][2] versus M s: m b using common data. We fully expect that ![Graphic][3] will contribute significantly to multivariate event identification.nn [1]: /embed/inline-graphic-1.gifn [2]: /embed/inline-graphic-2.gifn [3]: /embed/inline-graphic-3.gif

Radionuclide gas transport through nuclear explosion-generated fracture networks

Underground nuclear weapon testing produces radionuclide gases which may seep to the surface. Barometric pumping of gas through explosion-fractured rock is investigated using a new sequentially-coupled hydrodynamic rock damage/gas transport model. Fracture networks are produced for two rock types (granite and tuff) and three depths of burial. The fracture networks are integrated into a flow and transport numerical model driven by surface pressure signals of differing amplitude and variability. There are major differences between predictions using a realistic fracture network and prior results that used a simplified geometry. Matrix porosity and maximum fracture aperture have the greatest impact on gas breakthrough time and window of opportunity for detection, with different effects between granite and tuff simulations highlighting the importance of accurately simulating the fracture network. In particular, maximum fracture aperture has an opposite effect on tuff and granite, due to different damage patterns and their effect on the barometric pumping process. From stochastic simulations using randomly generated hydrogeologic parameters, normalized detection curves are presented to show differences in optimal sampling time for granite and tuff simulations. Seasonal and location-based effects on breakthrough, which occur due to differences in barometric forcing, are stronger where the barometric signal is highly variable.

Sources of Error and the Statistical Formulation of M S : m b Seismic Event Screening Analysis

The Comprehensive Nuclear-Test-Ban Treaty (CTBT), a global ban on nuclear explosions, is currently in a ratification phase. Under the CTBT, an International Monitoring System (IMS) of seismic, hydroacoustic, infrasonic and radionuclide sensors is operational, and the data from the IMS is analysed by the International Data Centre (IDC). The IDC provides CTBT signatories basic seismic event parameters and a screening analysis indicating whether an event exhibits explosion characteristics (for example, shallow depth). An important component of the screening analysis is a statistical test of the null hypothesis H0: explosion characteristics using empirical measurements of seismic energy (magnitudes). The established magnitude used for event size is the body-wave magnitude (denoted mb) computed from the initial segment of a seismic waveform. IDC screening analysis is applied to events with mb greater than 3.5. The Rayleigh wave magnitude (denoted MS) is a measure of later arriving surface wave energy. Magnitudes are measurements of seismic energy that include adjustments (physical correction model) for path and distance effects between event and station. Relative to mb,xa0earthquakes generally have a larger MS magnitude than explosions. This article proposes a hypothesis test (screening analysis) using MS and mb that expressly accounts for physical correction model inadequacy in the standard error of the test statistic. With this hypothesis test formulation, the 2009 Democratic Peoples Republic of Korea announced nuclear weapon test fails to reject the null hypothesis H0: explosion characteristics.

Development of a matched filter detector for acoustic signals at local distances from small explosions

A method for acoustic detection of small explosions at local distances is presented combining a matched filter with a p-value representing the conditional probability of detection. Because the physics of signal generation and propagation for small, locally recorded acoustic signals from small explosions is well understood, the single hypothesis to be tested is a signal corrupted by additive noise. A simple analytical signal representation is used where a known signal is assumed with parameters to be determined. The advantage of the approach is that the detector can be combined with other detectors that measure different signal characteristics all under the same unifying hypothesis.

Sources of Error Model and Progress Metrics for Acoustic/Infrasonic Analysis: Location Estimation

How well can we locate events using infrasound? This question has obvious implications for the use of infrasound within the context of nuclear explosion monitoring, and can be used to inform decision makers on the capability and limitations of infrasound as a sensing modality. This paper attempts to answer this question in the context of regional networks by quantifying current capability and estimating future capability using an example regional network in Utah. This example is contrasted with a sparse network over a large geographical region (representative of the IMS network). As a metric, we utilize the location precision, a measure of the total geographic area in which an event may occur at a 95xa0% confidence level. Our results highlight the relative importance of backazimuth and arrival time constraints under different scenarios (dense vs. sparse networks), and quantify the precision capability of the Utah network under different scenarios. The final section of this paper outlines the research and development required to achieve the estimated future location precision capability.

Combined Rayleigh‐ and Love‐Wave Magnitudes for Seismic Event Discrimination and Screening Analysis

Abstract Love waves have the potential to aid in discrimination for anomalous explosion events. We develop a calibrated mathematical formulation for an explosion discriminant that combines Rayleigh‐ and Love‐wave magnitude values and employs an error model that correctly partitions variances among events and stations separately. The discriminant is calibrated using a global data set of 124 earthquakes and 26 nuclear explosions and applied to the May 2009 Democratic Republic of North Korea (DPRK) announced nuclear test, as well as the calibration data set. All 26 explosions were correctly identified; only 6 earthquakes were incorrectly identified as explosions. Compared to an analogous treatment using only Rayleigh data, the combined discriminant improves the DPRK event p ‐value only nominally but reduces the number of false positives in the calibration data set by 70%, with no additional false negatives. While not dramatically improving the discrimination power for anomalous events, such as the 2009 DPRK test, the combined discriminant proposed here offers improved screening capabilities for typical events. Online Material: Earthquake and explosion calibration data set.

Immobile Pore-Water Storage Enhancement and Retardation of Gas Transport in Fractured Rock

The effect of immobile pore-water on gas transport in fractured rock has implications for numerical modeling of soil vapor extraction, methane leakage, gaseous

A Bayesian framework for infrasound location

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