Spectral Ratios from the Source Physics Experiment

The Source Physics Experiment (SPE) has provided a wealth of dense, local-distance (and regional-distance) observations for investigating the characteristics of over buried shallow explosions. We investigate SPE source characteristics using spectral ratios computed using observations from the nearby seismometer profiles and local-to-regional distance coda observations. Our primary focus is the effect of the seismic velocity model on the spectral ratios, particularly at high frequencies (> 5 Hz). Of special interest are deviations in the spectral ratios unexpected from simple calculations using only theoretical source time functions. In particular, several analyses have revealed a notch in the spectral ratio in the frequency range from 6-11 Hz that may originate in source complexity, scattering, and/or variations in the near-source seismic structure. Seismic velocity models govern seismic wave propagation effects, which can be effectively removed using spectral ratios, but the structure (near the source) also significantly influences seismic wave excitation. Models for the SPE near-source region include significant near-surface velocity variations and extremely low Q values that exert a strong influence on the local seismic response. We approach the problem using relatively simple seismogram calculations using point-source source models in one-dimensional media. We use wavenumber integration and mode-based synthetic seismogram calculations to develop an understanding of the observed seismograms. Our calculations show that even at distances of less than a few kilometers, the character of inter-source high-frequency spectral ratios is complex and sensitive to variations in even simple, one-dimensional velocity and attenuation structures.