Ultrasonic Physical Modeling of Seismic Wave Propagation from Graben-Like Structures
Abstract
This report describes ultrasonic experiments intended to help clarify the problem of seismic wave propagation in cases where sources are excited in a graben-like structure with significantly different material properties from those of the surrounding propagation medium. Ultrasonic waves are produced using a breaking pencil lead as a source (step unloading of the surface) and a true- displacement conical transducer as a receiver. We have made measurements by exciting the source on the half space (made of fine grained gabbro, with V sub p = 6.2 km/s) and within two graben-like structures: a cylindrical plug graben of 13 mm diameter and 2 mm depth; and a scale model (1 mm = 1 km) of Yucca Flat. In the case of cylindrical graben, the presence of a source region with significantly slower velocities than the surrounding region appears to lead to a more complex signal, with more ringing than would be apparant if there were no such region. The presence of such a source appears to result in a relative amplification of the high frequency part (200-400 KHz) of the signal. If we scale the results so that they correspond to a cylindrical basin roughly the size of Yucca Flat, this means that frequencies analogous to 3-10 s in the Earth appear to be amplified relative to lower frequencies. Experiments on the scale model of Yucca Flat yield, in general, less dramatic results. The loss of symmetry apparently leads to less opportunity for focussing, and amplifications are not as great. Waveforms do not vary as much azimuthally in character or shape as they do in the cylindrical case, nor do they vary as much with source position within the graben.
Document Details
- Document Type
- Technical Report
- Publication Date
- Nov 01, 1987
- Accession Number
- ADA189066
Entities
People
- B. R. Tittmann
- M. S. Vassiliou
- Monem Abdel-gawad