Analysis of Salmon Near-Field Data for Nonlinear Attenuation
Abstract
In order to assess the existence and impact of mild nonlinear contributions to the attenuation of seismic signals from underground explosions, free-field motion data from underground 5.3 kT nuclear test Salmon have been examined. These data, which were taken at ranges from 166 to 660 meters, show moderate strains (10-3 to .0001) which may provide nonlinear attenuation. The attenuation over an order of magnitude in peak amplitude can be described approximately by an attenuation function Q of a bit less than 10; however, the resulting waveform is noticeably wider than the data. A linear but frequency dependent Q which decreases with decreasing frequency gives a reasonable fit to much of the waveform change as well as the peak amplitude decay with range. There remains a higher speed precursor which precedes the main pulse in the data which cannot be described by this linear Q. With a spherical finite difference calculation driven by the 166 meter. Salmon pulse, it is found that a rapid shear modulus decrease at a .0001 strain threshold can reproduce the observed precursor and other features of the pulses at greater ranges when a linear absorption band Q approx. 10 is also added. The attenuation of the Salmon pulse is thus partly attributable to a nonlinear effect of material failure as well as a conventional linear mechanism. Keywords: Salmon shot.
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 01, 1989
- Accession Number
- ADA219367
Entities
People
- G. D. Mccartor
- W. R. Wortman