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Balser and Wagner [1960], using a vertical antenna, have recently measured the spectrum LETTERS TO THE EDITOR 1583 gungen des Hohlraumes Erde-Luft Ionosph•ire, erregt durch Blitzentladungen, Z. angew. Phys., August 1957. Wait, J. R.. Mode theory and the propagation of ELF radio waves, NBS J. Research, 64D (4), 387, 1960. Williams, J. C., Thunderstorms and very low frequency noise, Ph.D. Thesis, Harvard

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In two previous papers, the authors discussed a frequency-domain simulation of received signals in a focused array radar system Reidy, SPIE 2747, April 1996, pp. 202-213) illuminating a swath of flat terrain with one or more small plastic mines buried a few inches below the surface and a clutter background consisting of returns from random permittivity fluctuations, aggregates of rocks and the ground surface (SPIE 3392, pp 754-765, April 1998 and "Near-field and timing effects in simulation of focused array radar signals from a mine in subsurface clutter" , SPIE, April 1999) . The second paper emphasizes generalization of the algorithm to take proper account of the fact that distances between participants in the scattering processes are within fractions of wavelengths, enhancing the importance of near field effects in the modeling.In the present work, further generalizations are made which enhance the realism of the model. In particular, we investigate the role of surface height variations in changing the apparent propagation delay of the signal from a subsurface region. Since delay is the major discriminent in the processing of received signals in this system, this has a possibly significant effect on the simulated images. Still another generalization under investigation is that of inclusion of surface reflections from the interface's underside on the effective illumination of subsurface regions.Simulated image plots will be shown and compared with some previous results in order to assess the effects of these enhancements in the model.