Spatial Filtering in Seismic Shallow Prospecting
This paper concerns the problem of detection, by means of seismic reflection1, of underground empty spaces appearing in the north of France, which are the remains of the old chalk pits. They are situated in chalk at a depth of 8–10 m and the terrain is covered by a clay layer. To explain the problem let us consider the field situation (Fig.1.a.) where the source is placed over the tunnel in position x = 0 and the signals s(x,t) are recorded along the x axis. The preliminary measurements and studies show2 that in these field conditions the P-type tunnel reflection r(x,t) is at least 10–20 dB smaller than the coherent noise signal Cn (x,t) observed at the moment of reflection arrival. The most troublesome component of the coherent noise seems to be the refraction on top of the chalk, which has a frequency spectrum ressembling the spectrum of reflection, and therefore separating them by means of classic time-domain frequency filtering may not, in fact, be effective. In Fig.2, we have shown the forms and the spectra of two signals: chosen as typical from the refraction, and recorded at the tunnel vault which can be assumed as a potential tunnel reflection. Both the signals have the similar spectrum up to 700 Hz, and apparent differences for greater frequencies seem to us to be inexploitable. Finally, to detect the tunnel the reflection/coherent noise ratio (SNR) must be improved. The way to resolve this it is to suppress the nearly horizontally running refraction by means of spatial filtering. To carry this out the multi-channel acquisition system will be applied.
KeywordsClay Attenuation Refraction Geophysics Acoustics
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