3D Electrical Resistivity Image of the Morelia City Cathedral Subsoil

The historical Morelia City (Michoacán State’s capital) downtown was declared Cultural Mankind Heritage in 2004 by UNESCO. The 17th century Morelia City’s Cathedral is found within the heart of the Colonial Centre. Oral traditions speak about the existence of underground tunnels connecting the main historical buildings with the Cathedral. A geophysical study was carried out to characterize the Cathedral’s subsurface and to find evidences of such tunnels. A 3D Electrical Resistivity Tomography (ERT) was applied employing 98 electrodes at an interval of 3 m inserted on the ground surrounding the temple. The survey was designed in an L-shaped profile switching automatically electrical lines as current electrodes and as potential electrodes. 2,480 resistive data coming from 20 levels of information were recorded. A series of low resistive anomalies were found at an average depth of 8 m related to the cathedral foundations defining a complex subsoil. On the other hand, four deep interesting anomalies were outlined towards the four sides of the Cathedral edifice at depth. Their computed geometry suggests the presence of tunnels carved in the original rock. Such structures also coincide with the Cathedral main entrances at depth