Transportable Seismic Discriminants: The Use of Cross-Spectral Measurements to Reduce Parameter Scatter and Identification Error,
Abstract : Much progress has been made in regional seismic discrimination over the past ten years. Aided by the availability of high-quality waveforms and ground-truth databases, researchers discovered the discrimination capabilities of Lg/P and S/P spectral ratios, especially at high frequencies. However, when spectral ratios or signal parameters which perform well in one geographic area are applied to events in another, the results are typically uncertain or erroneous. This was illustrated during the identification analysis of the Dec. 31, 1992 Novaya Zemlya event: the same data were analyzed by a number of researchers who came to uncertain or conflicting conclusions. For regional discrimination to work in a CTBT context, this problem must be overcome. The most obvious factors contributing to transportability failure include geographic variability of source and path effects. However, noise and the inherent instability of spectral ratio calculations at low SNR's and restricted bandwidths are also likely factors. Regional seismic spectra rarely follow the model of a flat low-frequency spectrum with smooth high-frequency rolloff. This is especially true for quarry explosions, where long time-delay ripple firing can add harmonic distortion. Good signal processing practice dictates that measurements be made only at frequencies where there is sufficient SNR, which often means the use of cross spectral and system identification methods. This project will extend the discrimination research conducted to date to include these methods.
Extracting and Processing Signal Parameters for Regional Seismic Event Identification
This paper provides an overview of the processes of parameter extraction, parameter evaluation, and parameter processing leading to regional seismic event identification. The concepts are illustrated using two examples which utilize data from the NORESS array. Sources of error leading to mis-identifications are also given, as well as a discussion of future challenges as they relate to the procedures outlined.
The time-frequency characteristics of quarry blasts and calibration explosions recorded in Kazakhstan, USSR
SUMMARY In this paper we consider two fundamentally different processes that can be responsible for the organization of energy in seismic coda into discrete time-independent frequency bands. One process involves the resonance of energy in low velocity horizons and the other requires the interaction of time offset wavefields produced by subevents within multiple-event mine explosions (ripple-fired quarry blasts). We examined data collected by high frequency seismometers in Kazakhstan, USSR, and observed regular time-independent spectral modulations in coda resulting from events strongly suspected to be Soviet quarry blasts, but not in the coda from single event calibration explosions detonated at similar ranges. We conclude these modulations are a source effect and due to ripple-firing . This modulation is independent of the source-receiver azimuth and we infer that the spatial array of subshots in each event must be small. We demonstrate that simple linear superposition theory can be used to reproduce effectively the spectral modulation observed in real quarry blasts. On the basis of these observations we attempt to discriminate between the two types of events using a spectral pattern-based algorithm that seeks time-independent features. We consider the detrimental effect that resonant energy in low velocity horizons can have on the successful application of our algorithm.