Shear Waves by an Explosive Point‐source: The Earth Surface as a Generator of Converted P‐s Waves*

Abstract

FERTIG, J. 1984, Shear Waves by an Explosive Point-Source: The Earth Surface as a Generator of Converted P-S Waves, Geophysical Prospecting 32, 1-17.The most common source of seismic energy is an explosion at some depth in a borehole.The radiated waves are reflected not only at the subsurface layers but also at the free surface.The earth's surface acts as a generator of both P-and S-waves.If the source depth is much less than the dominant wavelength the reflected waves resemble closely the waves generated by a single force. Theoretical seismograms were computed with different methods to look for the relevance of the surface-reflected waves. The numerical experiments show reflected shear waves even for small shotpoint-receiver distances. Due to their polarization these waves can be detected most easily on in-line horizontal geophones. The existence of these waves was examined during a conventional survey in Northern Germany. Conventional data analysis shows a large variability in the up/u, ratio. The method used here produced a shear-wave section with a rather good signal-to-noise ratio down to 4 s S-wave reflection time., I i I N T R O D U C T I O NThe general formulation for the displacements in an elastic medium leads to very complicated formulae. The general solution of these equations is unknown. Only simplifications of the problem or approximations of the medium lead to equations which we can solve analytically, or at least approximately. We have the most complete knowledge for waves propagating in a homogeneous, infinite, isotropic, and ideally elastic medium. In a medium with so many contraints we have a wavepropagation of two independent kinds of waves. The most important one is the P-wave which is faster than the S-wave. Both propagation velocities depend on the elastic parameters of the medium. These velocities describe the kinematical aspects of wave-propagations. The causes of these waves can be described by single forces.

A Numerical Study of Lamb's Problem*

“…The synthetic seismograms we produced, including the records shown in figs 9a, 9b and 9c, suggest in all cases that the hodochrone associated with this arrival is linear and that it extrapolates to the epicenter. This observation is in accord with observations mentioned in recent literature by several authors, notably Fertig (1984) and Gutowski et al (1984), and also with computed seismograms presented by Daley and Hron (1983). In our case, the compressional point source is at a depth of 10 m, roughly 1/4.…”

Investigations ofSnandPnphases in the area of the upper Rhine Graben and northern Switzerland

“…Concerning the different S , behaviour in earthquake data However, a pure explosive source generates no S-wave energy; only indirectly is S-wave energy created by conversion of P to S waves at interfaces, especially at the free surface (Fertig 1984). From synthetic seismograms, not shown here, it can be seen that the relation of S-to P-wave energy for earthquakes is much larger than for explosive sources.…”

A new constraint on the composition of the topmost continental mantle-anomalously different depth increases ofPandSvelocity

“…From true-amplitude plots we determined that underwater explosions of the FENNOLORA experiment with charges of up to 3 tons generated SMS amplitudes 10 times higher than the observed amplitudes in SW Germany where borehole shots of up to 400 kg were used. P-to-S conversion at the ocean floor is a possible explanation for the generation of S-waves (see, e.g., Fertig 1984); however, the amplitudes compared to P-waves and the occurence of S arrivals on the transverse component (not shown in this paper) with amplitudes comparable to the radial component are surprising. We will not discuss in detail how the S-wave arrivals were produced by the seashots, but emphazise that crustal P-and S-waves, e.g., PMP and SMS appear with about the same magnitude on true-amplitude record sections of vertical and radial components ( Fig.…”