Three-dimensional seismic modeling

Abstract

The development of three-dimensional seismic modeling techniques has reached the state where such experiments can now be performed on a size scale convenient for the laboratory. A very homogeneous and isotropic syntactic foam was chosen as the seismic modeling material and a very sensitive surface motion transducer based on RF capacitive techniques was developed. This transducer is capable of measuring surface displacements as small as 1 microinch. (0.0254 micrometer) with rise times as fast as 1 microsecond and should be useful in many other applications. As a first test of the entire seismic modeling system, a 2 x 2 x 1 ft foam block was instrumented with three surface motion transducers, two strain gages, and three ytterbium foil stress gages and the block was loaded by detonation of our small, spherical explosive source. Peak surface displacements of about 10 micrometers and peak surface lateral strains of about 0.0001, both with better than 1% precision were recorded. The stress gage recorded peak stresses of about 5 bars, but lacked good precision in this very low stress region. Preliminary studies were made of producing a low-amplitude (on the order of 100 bar) plane wave loading over a wide area for future seismic modeling experiments. The use of a low-energy explosive mixture of PETN powder and hollow plastic spheres, separated from the surface to be loaded by an orifice plate, shows good promise. (Author)