The characteristics of transient electromagnetic responses between two dipole antennas are investigated theoretically and experimentally for the case where one dipole antenna is located above an interface of a lossy ground half-space and another is buried underground. First, an asymptotic expression for the refracted electric fields is derived based on the saddle point method when a horizontal dipole is buried in the lossy half-space. A suitable saddle point which differs from the conventional one is selected for evaluation when the observation point is located far enough away from the buried antenna. The resultant formula indicates that the refracted electric fields can be interpreted by the sum of two waves. When the observation point is very close to the interface, one of which is a direct wave which propagates from the buried antenna toward the interface point just beneath the observation point and the other a lateral wave which propagates from the buried antenna to the interface with an angle of total reflection and then propagates along the interface. Next, the transient reception voltages of the antenna over the ground surface are measured. It is shown that the experimental results agree very well with the theoretical results, confirming the validity of the theory.

A two-dimensional quasi-exact active imaging method for detecting the conducting objects buried in a dielectric half-space is proposed. In this imaging method, an image function which is a projection of buried object to an arbitrary direction, is introduced exactly by taking account of the presence of the planar boundary. The image function is synthesized from the scattering fields which are measured by moving a transmitting antenna (a current source) and a receiving antenna (an observation point) simultaneously along the ground surface. The scattering field is generated by the physical optics current assumed on the surface of buried object. Because the effectiveness of physical optics approximation has been confirmed for this problem, this is a quasi-exact active imaging method. The validity of this imaging method is confirmed by some numerical simulations and an experiment.

This letter shows that a portable visual stimulator for MEG measurements can be realized using an optical fiber bundle and a CRT display system offering high brightness and high speed raster scanning, and that MEGs with neither magnetic contamination nor jitter can be measured by the stimulator.