Preliminary results of vibrator-aided experiments in non-linear seismology conducted at Uetze, F.R.G.

Abstract

Following acoustics and largely thanks to the pioneering work of a group of Soviet geophysicists, the study of non-linear elastic properties and their effects is becoming an active field of research in geophysics. The research so far has produced a substantial body of evidence indicating that earth materials, from soils to crystalline rocks, are much more non-linear than is commonly believed and certainly far more non-linear than the materials usually used and studied in acoustics. But although most of the evidence comes from vibrator-aided experiments, apparently best suited for investigating the non-linearity of the geophysical medium owing to the ability of modern vibrators to generate seismic signals of prescribed form, the absence of a standard experimental technique makes it difficult to estimate and compare the various results, particularly in view of the considerable non-linearity of the source (vibrator-ground system) itself. The aim of the present vibrator-aided experiments was to try to confirm certain results in non-linear seismology, particularly the effects of harmonic generation and non-linear interaction in vibrator-induced sinusoidal seismic waves, by using an experimental method designed to enable one to discriminate between source, near-field and far-field non-linear effects. In the experiments, two identical prospecting vibrators, installed on the ground surface some 16 m apart, were driven in the harmonic regime separately and simultaneously (tests on non-linear evolution and interaction), a wide range of excitation frequencies and amplitudes being covered, and the resulting ground-surface motion was picked up by an array consisting of 48 groups of 12 vertical geophones-velocimeters and recorded, in multiplex form, by a prospecting seismic station. Tests were made first for short and intermediate source-receiver distances (near field, distance range covered 0–200 m, 5-m spacing of geophone groups), then for large distances (far field, range 1.5–3.5 km, 50-m spacing). A preliminary examination of the field data, available at present for analysis in the form of traces or Fourier spectra, both without amplitude information, indicates the effects of harmonic generation (both in the near and far fields) and non-linear interference (only in the near field) as well as the phenomenon of modulational instability (only in the near field and for very low excitation levels). It seems that, whereas the near field was dominated by surface Rayleigh waves, the far field was dominated by body waves propagated through the water-saturated layer. Lastly, as expected, the baseplate-accelerometer signal, recorded for one of the vibrators, typically contains a large amount of harmonic distortion, usually decreasing with increasing excitation frequency, but somewhat startingly, sometimes decreasing with increasing excitation level.