The resistivity method in void detection

The detection of void and/or partially filled caves has been attempted using all the surface geophysical methods. However, the microgravity and the resistivity seems to be the most suitable methods, although, nowadays, the radar has a more and more important role in this domain. The resistivity method has been used by several authors, Spiegel et al. (1980), D. L. Smith (1986), Andrade Afonso (1987), Andrade Afonso et al. (1989). Spiegel et al. (1980) considered the caves as void zones using the integral equation method. to obtain the solution for the pole-dipole array. Andrade Afonso (1987) and Andrade Afonso et al. (1989), generalized this method for the dipole-dipole array. However, the method is unstable for voids close to the terrain surface. So, we tested the hypothesis of approaching, for numerical proposals, the void zones to areas of high resistivities in relation to the surrounding environment. The results obtained in an automated electric analogie test tank (2,lx1,25x1,25 m), filled with a 26 Ohmxm copper sulphate aqueous solution, and in the field (caves with known geometry) were compared with the numerical results obtained by the mentioned method and with the results obtained using numerical techriiques (A. Dey et al,1979: N. C. Smith et al., 1984 and Sasaki, 1989) suitable for continuous media. The experiment al pseudo-sections for the pole-dipole, as well as, for the dipole-dipole arrays fit very well with the numerical ones.