We enhance the FDTD method for simulating transient EM surveys of underground objects

in which impulse excitations are laid on the ground surface and the electromotive force (EMF) is measured along the ground surface.In the late time of impulse excitation

the displacement current can be neglected because of slow variation.The diffusion characteristic dominates the electromagnetic process under ground.In order to apply the FDTD method to treat the underground problem

we introduce an artificial displacement current term in the diffusion equation.Examples given demonstrate the efficiency of presented scheme.