Near Field and Tunnel Effects Study of Controlled Source on Hallandssen Horst

In nearly all studies conducted in Sweden using the EnviroMT controlled source (CS) technique, near field or transitional field effects can be seen at frequencies below 15 kHz. In this study a profile parallel to the Hallandssen railway tunnel in southern Sweden crossing the 2D Hallandssen horst, the electromagnetic response is further influenced by the conductive armed concrete coating the tunnel walls at a depth of 150 m below the surface. The simulated CSMT data show a strong transitional effect. This effect is more visible in the parallel mode with the electric currents flowing along the tunnel, but less visible in the orthogonal mode with electric currents flowing perpendicular to the tunnel. In order to estimate a reliable 2D inversion model it is necessary to exclude the data showing transitional and tunnel effects from the inversion.