Near surface investigations over a disposal site for blast-furnace slag using Rayleigh surface waves

Dirk Orlowsky, Christoph Witte, and Bodo Lehmann, DMT, Germany describe a relatively simple use of near-surface seismic to identify potential instability at a building site formerly used for blast furnace slag. Surface and channel waves have been used for investigating lateral subsurface variations in the Earth for many years. Surface waves propagate along the Earth’s surface and their amplitudes decay with depth. Due to this amplitude decay in one space dimension, the propagation of surface waves is restricted to two space dimensions and the energy is effectively confined to the Earth’s surface. This leads to signal signatures with strong amplitudes at the surface and surface waves are therefore very sensitive to changes of the near-surface structure. Two different kinds of surface waves are observed in elastic media, Love waves and Rayleigh waves, characterized by different particle polarization. Channel waves, which occur in low velocity layers, are very similar to surface waves. Knowledge about surface and channel waves in the past four decades has led to the development of the in-seam seismic method (ISS-method) (Dresden & Ruter, 1994) for the detection of seam disturbances. In seismology, surface waves have been used over many decades for the investigation of the Earth’s crust and the upper mantel structure (Seidl & Muller, 1997; Keilis-Borok, 1989; Nakanishi, 1993). However, just a few years ago, geophysicists started to adapt in seam seismic methods and to develop new routine techniques (Park, Miller, & Xia, 1999) for the purpose of investigating the near-surface structure of the Earth with surface waves. This has led to the development of the surface wave seismic method (SWS-method) for near-surface investigations. In this context, the expression ‘near-surface’ denotes a range from the surface to a maximum depth of about 20 m.