Application of geophysical and hydrogeological analyses to predict water and hydrocarbon entry to the T5 Tunnel, west Iran

The Garmsiri Project, including the 4.5 km long T5 Tunnel, is under construction in western Iran. The T5 Tunnel passes through the NW-SE-trending Emam Hasan Anticline (EHA), perpendicular to the fold axis. The fold is mainly composed of marlstone and argillaceous limestone layers of Cretaceous to Miocene age, incorporating the Pabdeh-Gurpi Formation, karst limestone of the Asmari Formation, and marlstone and gypsum of the Gachsaran Formation. There was a risk of water entry into the tunnel as it was constructed below the regional groundwater table elevation. In addition, the entry of hydrocarbons, in either liquid or vapour phase, into the tunnel was possible owing to the presence of numerous active bitumen mines in the vicinity of the anticline. To predict the risk of water or hydrocarbon entry geological and hydrogeological analyses together with the audio magnetotelluric (AMT) method were applied. Based on the field work, resistivity and geological cross-sections were provided along the tunnel path. Several boreholes were drilled along the tunnel route and water-table elevation, rock quality designation (RQD) and permeability values were measured. To determine a broad range of features related to the anticline, 55 AMT stations were positioned along the tunnel route. Dimensionality analysis confirmed 2D dimensionality of the AMT transfer functions, which allowed us to apply the 2D bimodal inversion using a non-linear conjugate gradient algorithm. Integration of the geological and hydrogeological data with the resistivity model resulted in a more detailed geological section along the tunnel, including water-table elevation and identification of highly conductive zones in which bitumen migrated. It was predicted that water entry would be observed through the Asmari Formation and also that elevated H2S concentrations would be encountered during drilling in the anomalous conductive zones. Monitoring results and field observations gained during the tunnel construction were compared with the predictions.