Seismic Techniques for Surveying the Underground of Shallow Foundations

For civil structures founded on shallow foundations, the ground underneath the foundation often holds the greatest risks of the total structure. This can be due to of a very soft soil layer, an inhomogeneous subsurface or a hidden dangerous object. It would be most favorable when a cheap and quick kind of seismic “tap-and-listen” technique can be used to detect those risks. The problem is however that an applied pulse or blast always creates a combination of compression-, shear- and surface waves. These types of waves have different wave velocities and will return therefore at different time intervals. For a shallow subsurface technique, all these waves will overlap, which makes the interpretation very hard. Both the single pulse technique and the single-frequency, multiple wave technique (constant vibration) have been studied, but both techniques have their limitations. It can be concluded from finite element calculations that it will be difficult or even impossible to design good seismic techniques for surveying the underground of shallow foundations for hidden shallow objects like water pipelines, undetonated bombs, dead bodies, coffins, potholes, etc.. The main reason is that the relative amount of reflected energy is simply too low in comparison to the energy of the still present original wave.