Electrical imaging for localizing historical tunnels at an urban environment

ANA OSELLA; P. MARTINELLI; GRUNHUT DUENYAS VIVIAN; M. DE LA VEGA

JOURNAL OF GEOPHYSICS AND ENGINEERING

IOP PUBLISHING LTD

Lugar: Londres; Año: 2015 vol. 12 p. 674 - 685

1742-2132

We performed a geophysical study at a historical site in Buenos Aires, Argentina,corresponding to the location of a Jesuit Mission established during the 17th century,remaining there until the 18th century. The site consisted of a church, cloisters, a school,orchards and a procurator?s office; also several tunnels were built, connecting the missionwith different public buildings in the town. In the 19th century the Faculty of Sciences of theUniversity of Buenos Aires was built in a sector of the site originally occupied by an orchard,functioning until its demolition in 1973. At present, this area is a cobbled square. With the aimof preserving and restoring the buried structures, work was carried out in this square lookingfor tunnels and remains of the basement of the old building.Considering the conductive features of the subsoil, mainly formed by clays and silt, thecomplex characteristics of the buried structures, and the urban localization of the study areawith its consequent high level of environmental electromagnetic noise, we performed prefeasibilitystudies to determine the usefulness of different geophysical methods. The bestresults were achieved from the geoelectrical method. Dipole?dipole profiles with electrodespacings of 1.5 and 3 m provided enough lateral and vertical resolution and the requiredpenetration depth. Reliable data were obtained as long as the electrodes were buried at least15 cm among the cobble stones. Nine 2D electrical resistivity tomographies were obtained byusing a robust inversion procedure to reduce the effect of possible data outliers in the resultingmodels. The effect on these models of different error estimations was also analyzed. Then,we built up a pseudo-3D model by laterally interpolating the 2D inversion results. Finally,by correlating the resulting model with the original plans, the remains of the expected mainstructures embedded in the site were characterized. In addition, an anomaly was identified thatindicates the presence of a tunnel not previously reported