Blast wave effects on decoupling with axisymmetric cavities

Abstract

An expiosion is considered in an initially air-filled, underground spherical cavity t o which is connected a long, cylindrical tunnel. The cavity and tunnel size are, taken together, equivalent in volume t o a fully decoupled spherical cavity (in accordance with the Latter criterion). Calculations indicate that, when the air is at standard conditions, the shock speed in the tunnel, D , is of the same order as the sound speed in the surrounding rock medium, c L . Moreover, the blast wave is approximately quasi-steady, and can be represented by a simple analytical function. This in turn permits a closed form solution for the far-field seismic amplitude spectrum. It is found that the low-pass filtering effectiveness of the tunnel is largely diminished unless D/cL s 1. With air-filled cavities, and depending on the source-receiver orientation, the spectral amplitude can actually exceed significantly the equivalent volume-sphere result. For practical purposes, this means that unless the cavity and tunnel are initially evacuated, say with a steam ejector system, the high-frequency decoupling capability will be no better, and possibly worse, than a conventional spherical cavity of the same total volume.

Pre-shot predictions and instrumentation of the KUCHEN experiment

“…We thus find a degradation of the decoupling factor from 15 for a n aspect ratio of 1, to a value ranging from 4 to 11 in the frequency between 1 and 30 hertz for an aspect ratio of 14.5. This result, although preliminary, is consistent with the calculations of Glenn and Rial (1987) who found that for a nuclear explosion in salt, the decoupling factor was degraded by less than a factor of 2 for aspect ratios ranging from 10 to 20. Our results also complement the calculations of Stevens et al(l991) who used an ellipsoidal geometry with an aspect ratio of 4 and found little difference in the decoupling factor when compared with a spherical geometry.…”

NUCLEAR EXPLOSION SEISMOLOGY: Verification, Source Theory, Wave Propagation and Politics

“…Small aperture regional arrays have also been investigated as to their detection capabilities as exemplified by the study at NORESS [Sereno and Bratt, 1989]. Problems with detection of decoupled explosions [OTA Report; Glenn and Rial, 1987] which would require more extensive in‐country networks for monitoring to the lowest yield levels were identified.…”

Elastic Wave Radiation from a Line Source of Finite Length

“…Several investigators, using a variety of numerical, approximate analytical, and/or heuristic approaches, have demonstrated frequency-and direction-dependence of far-field radial and transverse elastic waves propagated from explosively loaded, axisymmetric cavities (Glenn, et al, 1985(Glenn, et al, , 1986Rial and Moran, 1986;Glenn and Rial, 1987;Ben-Menahem and Mikhailov, 1995;Gibson et al, 1996). For cavities with large aspect ratios (i.e., long and thin) their results are broadly consistent with the above results for the pressurized line source.…”

“…Several investigators concerned with underground nuclear explosions have examined the elastic wavefields generated by explosively loaded, axisymmetric cavities (Glenn et al, 1985(Glenn et al, , 1986Rial and Moran, 1986;Glenn and Rial, 1987;Ben-Menahem and Mikhailov, 1995;Gibson et al, 1996). These studies have established both frequency-and direction-dependent attenuation of far-field radial and transverse waves radiated from elongated cylindncal and/or ellipsoidal cavities.…”