Scattering from tunnels near sources and their effect on seismic signatures of explosions

Explosions often take place in complexes with chambers, shafts, and tunnels used for access and instrumentation. Cavities and tunnels have a very larg e impedance contrast relative to the surrounding material and might act as strong scatterers of seismic waves. They represent secondary sources with d ifferent radiation patterns and different distance dependencies than the pr imary waves. The characterization of the primary events is made more diffic ult by these secondary signals. We used the 2D multiple multipole (MRP) method to calculate seismograms and radiation patterns for linearized, effective sources near heterogeneities such as cavities. We examined the effects of distance between source and a scatterer, the effect of a regular arrangement of cavities and the location of the source therein, and the effect of different source mechanisms for a :particular :L-shaped geometry representing a large tunnel with a source in a side-drift. Depending on the 2D configuration of cavities, we obtained p eak-amplitude ratios of S to P waves ranging from 5 to 70%. Seismograms recorded during the nonproliferation experiment (NPE) show stro ng azimuthal variations in waveforms and amplitudes. Near the NPE source, t here was a large complex of tunnels and cavities. For an idealized 2D geome try, we calculated synthetic seismograms, The early arrivals on the synthet ics compared fairly well to the observations, especially if near-receiver h eterogeneities are included to couple radial (P) and transverse (S) compone nts. Around the arrival of near-source generated S waves, the envelopes are roughly similar. Because our models are oversimplified, that is, 2D, we do not conclude that near-source scattering causes the near-source generated S waves. Rather, we believe that one cannot exclude near-source heterogenei ties, for example, cavities, as a possible cause of near-source generated S waves.