Ultrasonic physical modeling of seismic-wave propagation from a graben-like structure: a preliminary report. Scientific report No. 1, 19 February 1985-30 September 1986

Ultrasonic experiments help clarify the problem of seismic-wave propagation in cases where sources are excited in a region with significantly different properties from those of the surrounding propagation medium. Such a case exists, for example, when nuclear explosions are detonated at Yucca Flat. 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. Measurements are made setting the source off on the half space (made of fine-grained gabbro, with Vp=6.2 km/s), and with a cylindrical graben of 13mm diameter and 2mm depth. The graben was filled with either crystal wax (Vp=2.407) or HPAL3 (an aluminum-filled resin with Vp=3.287). Rayleigh waves for frequency 100 KHz in the model are roughly analogous to 20 s in the Earth. 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 apparent if there were no such source region. The presence of such a source region appears to result in a relative amplification of the high-frequency part of the signal. The frequencies analogous to 3 or 4 s in the Earthmore » appear to be amplified relative to lower frequencies. Although the pencil-lead source used in this study is not exactly similar to an explosion, this result may still be significant. When the source is set off in the garben in an off-center position, a radiation pattern is established, with amplitude varying by a factor of 2 or more. Material effects appear to be accentuated when the source is excited off center.« less