Detection of Shallow Underground Cavities by Seismic Methods: Physical Modelling Approach

Abstract

In some previous papers1,2, we have pointed out the difficulties of the detection, by seismic methods, of the shallow (8-10m) underground tunnels appearing in the North of France, which are the remains of old chalk pits now unexploited: the difficulties result from the impossibility of the resolution of the potential tunnel reflections from the background of groundroll and refraction signals. The real structure is shown Fig.la. Fig. lb presents an example of the seismic section obtained by walkaway-type test3 (source gather), when the source (hammer type) was placed at position x = 0 and receivers (accelerometers) regularly spaced (0.66m) along the x-axis. The complexity of the seismograms is obvious: apart the direct P-arrival P0, the refraction Pi, and the primary reflection in the first layer Ri which are clearly visible, the rest of the section is composed essentially of a mixture of multiples of the refraction, multiples of the first layer reflections and the groundrolls G. In particular, no sign of the tunnel reflection that should appear as a hyperbole (discontinuous curve on Fig. lb) is observed.