Geoelectrical and EMI Studies at an Urban Site in Buenos Aires, Argentina, for Localizing an Old Tunnel.

BONGIOVANNI M. V.; GRÜNHUT V.; MARTINELLI H. P. ; DE LA VEGA M.; BONOMO, N.

Oporto

Congreso; 24º European Meeting of Environmental and Engineering Geophysics; 2018

EAGE

The Avellaneda Park is a historical site in Buenos Aires, Argentina. It is one of the largest greenspaces in the city of Buenos Aires. It occupies the old residence of DomingoOlivera, built in the early 1800s, named ?La Casona de los Olivera?. This houseis the only head of a farm that the city of Buenos Aires preserves. Historicalrecords indicate the presence of a few tunnels that come out from thisresidence. In particular, we were interested in a tunnel that links ?La Casonade los Olivera? with ?Villa Ambato?, another old residence that nowadays is atechnical school. No more details about its localization were reported. Themystery of the tunnels is explained as a solution to the harshness of thewinters of the Pampas and as a means to hide or run away from the aborigines ifnecessary. This particular tunnel served as a secret path between these tworesidences that were of the few existing at that time in the zone. The maingoal of the geophysical study was to localize it. Detecting tunnels isgenerally difficult, even more when they are placed in urban sites. GPR, Geoelectricsand EMI have been used for this task, alone or sometimes in combination, withvaried effectiveness, depending on the characteristics of the soil and thecontrast with the materials forming the tunnel, its depth and state ofconservation, and the environmental conditions at the surface (Beetham &Hutchinson, 2009; Martinez-Lorenzoet al., 2011; Tawfik et al., 2011; Osella etal., 2015). Considering the conductive features of the subsoil, mainly formedby clays and silt, the complex characteristics of the buried structure, and theurban localization of the study area with its consequent high level ofelectromagnetic noise, we performed prefeasibility studies to determine theusefulness of these geophysical methods.  Although ground penetrating radar (GPR) is one of the geophysical methodsmost frequently applied for urban archaeological studies due to its resolution(e.g. Leucci & Negri, 2006; Bonomo et al., 2012), we did not apply itbecause, due to the silty clay nature of the soil, its penetration depth wouldbe much smaller than the minimum required of about 2 m. Wefirst applied the EMI method then the electricalresistivity tomography (ERT) method. Next, we describe the methodology used fordata acquisition, the obtained results, and the corresponding interpretation.