Quantification of m sub Lg for Small Explosions
Abstract
This report focuses on a USGS data set from refraction surveys within the State of Maine. These data contain excellent recordings of short period Rayleigh waves (Rg) generated by the explosions. The initial intent was to use these data to characterize shear wave velocity and Q in the upper 1-2 km in this environment of folded paleozoic sediments. However the data set permitted other interesting studies. First, significant lateral changes in the moveout of the Rg, S and P wave were observed. Analysis techniques were developed to successfully invert for a laterally varying earth structure and to model the Rg waveforms, both in terms of absolute amplitude and phase. Second, the shear wave velocity model derived from the surface waves, was used in a blind test to predict the S-wave arrival times for two profiles having significant S waves. An excellent fit was obtained without having used the S-wave data in the inversion for the earth model, indicating the ability of the 1-4 Hz surface wave data to resolve shear wave velocity structure to depths of 2 km in this environment. Finally, since absolute instrument gains were known, forward modeling was used to compare the observed and estimated amplitude decay in different frequency bands. This permitted an estimation of the isotropic moment for these one ton explosions. By comparing estimates made in different portions of the 1-5 Hz frequency band, we conclude that a step source time function in moment is adequate to model the surface wave observations for these one ton shots. The estimated isotropic moments are similar to other chemical yield-moment observations. However, larger industrial chemical blasts, e.g., 100 ton or greater, would be expected to show a frequency dependence in the isotropic moment estimate because of the distributed source dimensions.
Document Details
- Document Type
- Technical Report
- Publication Date
- Apr 23, 1992
- Accession Number
- ADA253915
Entities
People
- G. Al-eqabi
- K. Hutchensen
- R. B. Herrmann
Organizations
- Saint Louis University