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.
Types of Seismic Events and Their Source Descriptions
The seismic source characteristics of underground nuclear explosions have now been extensively documented as a result of the testing programs which have been carried out by the various nuclear states over the past 30 years. In particular, the variations in seismic source coupling as functions of the geologic emplacement medium, yield and depth of burial of the explosion have been well-defined experimentally; and first order theoretical source models, such as the Mueller/Murphy model, have been formulated which have been shown to account reasonably well for these observed variations. The most notable feature of the nuclear source seismic coupling in the short-period band is that it is observed to be essentially the same for tamped explosions in virtually all known hardrock and water-saturated media. The only known exceptions to this uniformity are for explosions in clay and water, for which the average mb values for a given yield are higher than those in hardrock by about 0.50 ± 0.25 magnitude units, and for explosions in dry, porous media, such as dry alluvium or tuff, for which the average mb values for a given yield are lower than those in hardrock by about 0.50 ± 0.25 magnitude units. Larger changes in seismic coupling efficiency can occur if the explosion is not fully tamped, as in the cavity decoupling evasion scenario. Thus, it has been experimentally demonstrated that it is possible to reduce the mb value at a given yield by nearly two full magnitude units by detonating the explosion in a suitably large air-filled cavity. This potential variability in explosive source coupling needs to be carefully considered in the assessment of the capabilities of any proposed seismic monitoring systems.