Surface-to-Hole FEM Modeling in Conductive Half Space

This work presents several modeling examples for frequency-domain electromagnetic (FEM) system. Measuring system consists of a large surface transmitter loop and a borehole receiver probe. In EM methods a typical target is a conductive body in a resistive host. The primary magnetic field at the borehole receiver stations is first calculated theoretically and then reduced from the measured response. This residual field is used for the interpretation. This method provides also visual interpretation, but if the surrounding rock is not very resistive, the method leads to difficulties and may cause false results. In conductive subsurface, currents inside the conductive body are induced, instead of a magnetic dipole, by more complicated electromagnetic field, known as the "smoken ring" in the time-domain EM method. On the contrary, when a conductive body is lying in a resistive environment, the induced currents are flowing and decaying inside the body. That gives information about a shape of the body, but in the conductive environment the current flows freer trough the interface of the body. Interaction between the body and the host increases and the shape of the body becomes more difficult to interpret from the data.