Application of the Time Domain Electro-Magnetic Method to surveying of tunnels with significant overburden and swellable ground

Abstract

In geophysical exploration of tunnel routes, the seismic refraction survey is usually used. However, even when dynamite is used as a seismic source, a maximum depth of investigation is only ~200 m. The Time Domain Electro-Magnetic Method (TDEM), which is usually used in resource exploration, has a depth of exploration of several hundred meters or more. To investigate the applicability of the TDEM to surveying of tunnels beneath large overburden, the results of seismic refraction survey and TDEM obtained from two tunnels with overburden > 200 m are compared with excavation results. In swellable rocks resistivity could decrease with increasing Cation Exchange Capacity (CEC), and thus one aim of this paper is to determine the presence or absence of swellable Neogene mudstone from the resistivity values analyzed by the TDEM. Furthermore, sensitivity of the TDEM to the low resistivity layer, and the reliability of TDEM analysis, are verified by numerical simulation. The results suggest that the TDEM is effective for extracting the layer boundary for a given tunnel, which is typically difficult to verify using seismic refraction survey when overburden exceeds 200 m. The TDEM can also detect swellable low resistivity layers.