ABSTRACT
Through successive arrangement of stations and survey lines, EH-4 can clearly depict the 2D resistivity profile of geological structure measured and even it can combine the 3D successive resistivity stereo image. With these functions this method is very suitable for the detection of geological conditions of highly weathered engineers containing a large number of weak structures. In the measurement of EH-4, the earth is assumed as horizontal medium and the electromagnetic waves emitted by geomagnetic field then can be approximated as 2D plane electromagnetic waves perpendicular to the earth. Thus a mutually orthogonal electric field component (Ex, Ey) and a mutually orthogonal magnetic field component (Hx, Hy) can be got through signal sampling system above ground. Defining the ration of geological electric field and magnetic field as geological wave impedance Z, in the condition of homogeneous earth
1 INTRODUCTION
Based on the fact tha affected by geomagnetic field, magnetic reflected waves of the formations with different properties are different (Chen 2009). It is possible to invert the geological magnetic characteristics difference between different formation lithology through the measurement of reflected magnetic fields of different depth formation underground. Electric magnetic image system can collect magnetotelluric waves to calculate resistivity or conductivity and the abnormal body in the formation underground can then be identified through processing and analysis. Based on the magnetotelluric theory, this system belongs to magnetotelluric system combining artificial magnetic field with natural magnetic field and it is currently the most advanced method of electromagnetic prospecting methods (Wang & Wang 2003). Soft formations include geological structures such as fault fracture zone, karst cave, weak zone and water-rich zone etc. In the highly weathered zone, the underground soft structure would threaten the stability of slope engineering and underground engineering. It is significant for project’s stability analysis and management to identify the geological condition of unknown soft structure in neighboring zone of lake using EH-4 electromagnetic image system.
