Detecting Underground Mines by Seismic Noise Autocorrelation and Geophysical Methods

Seismic noise correlation is one of the most used tools to know the earth's structure in the last decade. In this study, we used autocorrelation to determine the presence of underground mines by extracting the normal seismic response in transmission between the ground surface and the cavity roof. The experiments are carried out in the urban environment of the Mexico City western zone, where a high risk of mines collapse subsists. For this, we use ambient noise recorded for 30 min in vertical 4.5 Hz geophone arrays. We obtain zero offset sections of power spectra density from the stacking of autocorrelations in 4 s time windows. The results are compared with GPR, ERT, and seismic refraction studies. We observe that surface cavities such as drainpipe systems are present at frequencies greater than 30 Hz. Between 10 and 30 Hz, the seismic response is produced by resonances associated with cavities that can be delimited laterally by spectral maxima and whose presence agrees with discontinuities on radargrams. The mine roof depth is related to half-wavelength and the compression wave velocity of the surface layer determined by seismic refraction. The autocorrelation method does not determine the shape or vertical extent of the cavity, which is well resolved by the high resistivity values of the ERT method. However, low spectral amplitudes are observed on saturated materials where the electromagnetic wave is noisy and low resistivity values are resolved.