Zoltan A. Der

Worldwide variations in the attenuative properties of the upper mantle as determined from spectral studies of short-period body waves

Abstract A worldwide study of short-period teleseismic body wave spectra shows that the high frequency falloff rates of spectra are correlated with the tectonic type of the source and receiver regions and with source depth. The data indicate, in a consistent manner, that the main cause for such variations is the lateral variation of Q in the upper mantle as well as change of Q with depth. Using the internal consistency checks provided by redundancies in the data set other effects such as crustal, site dependent distortion of the spectra, source effects and instrument non-linearity can be ruled out as significant factors influencing the t∗ estimates obtained. The results indicate high attenuation in the upper mantle under tectonic regions and new oceans. Long-period regional attenuation studies indicate similar variations in mantle Q among the types of regions mentioned but yield significantly lower Q estimates in all areas. The short- and long-period attenuation results can be reconciled only by assuming a frequency dependent Q that increases with frequency along all types of paths, such that the relative differences in Q along various types of paths retain the same sign over the short- and long-period bands.

Deconvolution of Multiple Time Series

The problem of estimating simultaneously two convolved vector time series corrupted by additive noise is considered. By regarding one of the series as being stochastic and the other as fixed, it is shown that the fixed component can be estimated by maximizing a frequency domain approximation to the likelihood. The stochastic series is estimated by using an approximation to the conditional mean evaluated at the current maximum likelihood estimators. An example involving a multiple deconvolution of seismic source and receiver functions is given.

Phase onset time estimation at regional distances using the CUSUM algorithm

Abstract A simple approach for the estimation of onset times of regional arrivals was tested. The method consists of the computation of the cumulative sum (CUSUM) of a test statistic, in this case, the absolute value of the trace amplitude, addition of a linear trend to the result and finding minima of the resulting function by various methods. The minima are located at the times where sudden increases in the trace amplitudes occur, i.e., at the phase onsets. The method requires frequency prefiltering in order to enhance the amplitude changes at the phase onsets. It was found that this algorithm performed comparably to a human analyst in finding onset times for Pn arrivals with varying S/N ratios. Moreover, when applied to the rest of the seismogram, it was effective in finding the onset times of the rest of the regional arrivals as long as these were clearly visible. In the rest of the cases, where there are many poorly defined arrivals, an analyst would also have difficulties in uniquely defining onset times. The method also shares with the human analyst the tendency to pick the onset times late when an emergent arrival is buried in noise.

A cepstral F statistic for detecting delay-fired seismic signals

Time- and frequency-domain approaches to detecting a consistent pattern of reflections on an ensemble of seismic recordings are developed. Such patterns are characteristic of mining bursts and not of nuclear explosions or earthquakes so that detecting a ripple delay structure can serve as one component for discrimination. In the frequency-domain approach, a generalization of cepstral analysis is used to derive an F statistic for detecting delay-fired events observed on an array. Detrended log-spectra are considered as realizations of stationary processes whose periodic components are frequencies, with periods proportional to delay-time differences. An F statistic is derived that is proportional to the stacked or summed cepstra and the spectrum of the stacked log-spectra. Advantages of the cepstral F statistic accrue from better resolution and the fact that statistical significance can be established for delay peaks. It is also easily incorporated into automatic detection systems. The frequency-domain appr...

Monitoring the comprehensive nuclear-test-ban treaty : data processing and infrasound

Data Processing.- Estimation of Background Noise for International Monitoring System Seismic Stations.- GIS as a Tool for Seismological Data Processing.- Optimized Seismic Threshold Monitoring - Part 1: Regional Processing.- Optimized Seismic Threshold Monitoring - Part 2: Teleseismic Processing.- A Scheme for Initial Beam Deployment for the International Monitoring System Arrays.- Adaptive Training of Neural Networks for Automatic Seismic Phase Identification.- Infrasound.- Constraints on Infrasound Scaling and Attenuation Relations from Soviet Explosion Data.- Infrasound Detection of Large Mining Blasts in Kazakstan.- Infrasonic Signal Detection and Source Location at the Prototype International Data Centre.- Surveying Infrasonic Noise on Oceanic Islands.- Seismic Precursors to Space Shuttle Shock Fronts.

Source Directivity, Signal Decorrelation, Spectral Modulation and Analysis of Spatio-temporal Patterns of Multiple Explosions

Abstract — Spectral modulation is used as a criterion for discriminating multiple mining explosions from earthquakes and single blasts. Features in the spectra of regional arrivals from quarry blasts may be explained by the spatio-temporal configuration of shot patterns and are explored via a model based on spatial waveform decorrelation and propagation delay (directivity) effects. The phenomenon of decreasing modulation with decreasing average phase velocities of the seismic wave arrivals can be attributed to a trend of decreasing similarities of waveforms with increasing distance between individual explosions making up the shot pattern for the respective arrivals. Rescaling of the modulation patterns with respect to frequency can be explained by the Doppler effect.¶The postulated relationship between shot pattern layouts and sizes, together with their firing sequences, on one hand, and the similarities and differences in modulation characteristics among the various regional arrivals, on the other hand, can be used to derive information about the mining practices in a region that is otherwise not accessible. Moreover, such analyses could be utilized for detecting unusual activities in the CTBT monitoring system.

Applications of Higher-Order Spectral Analyses to Detection and Identification of Seismic and Acoustic Signals Generated by Machinery.

Machinery typically generates mechanical vibrations at multiple, harmonically related frequencies which arise from various mechanically coupled moving components of machines or characteristic nonlinearities in their operational loads. These mechanical vibrations propagate from their origin through the air as acoustic waves and through the earth as various types of seismic waves. Of the two modes of propagation the seismic mode of propagation is the more complicated since the same harmonic may propagate simultaneously in various wave types (compressional waves, shear waves and various surface wave types) with differing propagation vehicles. Moreover, air-to-ground coupling has been shown to occur in some cases. The consequence of this multi-mode propagation is that standing wave interference patterns are set up over the terrain surrounding the sources which complicates the frequency-wavenumber analysis and identification of the signals. Since the set of harmonics omitted from a given type of machinery tend to be phase- coupled, higher order spectral analysis offers means for detecting and separating such coupled sets and reducing much of the Gaussian background noise and uncoupled sinusoidal noise components. In this paper we utilize sections through bispectral estimates obtained from continuous signals from various types of machinery with durations exceeding a minute.