No abstract

Very low‐frequency (VLF) electric field wavetilt measurements were made continuously over a period of four months at a fixed location. Daytime and nighttime values of the wavetilt were found to be relatively constant. During sunrise and sunset the wavetilt decreased to 20 percent of the daytime value, making wavetilt measurements meaningless for two three‐hour intervals during a 24 hour period. A mechanism involving modal interference is postulated.This phenomenon may be a problem in other passive electromagnetic (EM) prospecting systems including audio‐frequency magnetic and audiomagnetotelluric.

S U M M A R YT h e natural signal magnetotelluric (MT) and audiomagnetotelluric (AMT) geophysical prospecting methods utilize the spectra of associated time-varying horizontal electric and magnetic fields at the Earth's surface to determine a frequencydependent impedance tensor. Most present methods of analysis determine the spectra using variations o n the Fourier transform and therefore mus't assume either that the signals under analysis are stationary over the record length or that any distortion in the spectral estimations d u e t o non-stationarity will occur in an equivalent manner in the spectra of both the electric and magnetic fields and thus not effect the impedance estimates. W e use time-frequency distribution (TFD) analysis techniques t o show that this is not the case, and that the impedance tensor is affected by non-stationary source field fluctuations. W e further show that TFD analysis techniques can b e used t o overcome these problems and obtain stable and reliable estimates of the impedance tensor. These techniques are compared with previous techniques by using a standard data set (EMSLAB).

Ultra-wide-band returns from surface-scattered and buried land mines give distinctive echoes that depend on the target type and soil environment. These echoes remain relatively stable for land mines with non-metallic casings but vary with the soil environment for land mines with metallic casings. This fact indicates that the signature resulting from the non-metallic-cased land mine is due to internal structure, whereas the signature ofthe metallic-cased target is due to interaction of the illuminating pulse with the metal case and the surrounding medium.The shape of these echoes can be described by a series of damped sinusoids. Certain parameters of the functions are believed to be stable for the same target in a variety of environments. Thus the echo can theoretically be used to identify targets with non-metallic casings by determining the position ofthe fixed poles that describe the exponential damped sinusoids associated with a particular target type. This paper examines extraction ofthese poles, by the Prony method, for different types of nonmetallic land mines and land-mine-like targets.

In his discussion of our paper Wait refers to the effect of an elliptically polarized radiation source on both electric field wavetilt and surface impedance measurements. If such measurements are made at the surface of a uniform isotropic half‐space, then both the horizontal electric and horizontal magnetic field strengths measured will depend upon the characteristics of the polarization ellipse and the orientation of the antennas whereas the vertical electric field strength is independent.