Measurements and interpretation of lateral electromagnetic waves on the surface of low conductivity media

Abstract

Measurements of electromagnetic fields are used for a wide range of applications. Often such measurements provide a non-destructive and non-intrusive way of obtaining information about the internal structure and other properties of the medium. Observations may be conducted across a very broad frequency range. Depending on the application domain, the frequency bands may range from the UHF band in biomedical engineering to the ULF band in geophysics. This thesis reports on several experiments where the electric and magnetic fields were measured on the surface of different media for the purpose of determining the electric properties of the material (electric conductivity and relative permittivity). A series of interference nulls were observed while measuring the electric field at UHF frequencies on the surface of a large, uniform sand dune. The source of the field was a transmitter buried in sand. The same pattern was observed after substituting the active transmitter with a passive scatterer exposed to a surface located electromagnetic transmitter. The scatterer then became a source of the electromagnetic field producing the interference pattern identical to the pattern observed in case of the active transmitter. Modelling of the surface electric field was done using known closed-form solutions for a uniform halfspace and by using an FDTD EM solver. Both approaches showed a close match with the measured fields. By varying the resistivity and the electric permittivity in the constructed models, it is demonstrated that the observed pattern (position of the nulls, nulls depth) depend on these electric properties. The inverse problem is posed with the goal of determining the electric properties of the material using the observed field. The inverse problem was solved, and the conductivity and relative permittivity of sand were determined. Two experiments conducted in different conditions showed the following results: conductivity 1×10-6 S/m and relative permittivity of 2.13 in dry conditions and conductivity 3×10-3 S/m and relative permittivity of 2.86 in wet conditions. Another experiment reports the measurements of the surface electric field on the top surface of a small three-dimensional concrete block at microwave frequencies (1 – 5 GHz). A copper rod was placed in concrete during sample manufacturing to serve as a scatterer. The FDTD solver was used for modelling and determining the inverse problem solution. At VLF frequencies, several experiments were conducted using surface impedance measurement equipment TranSIM. TranSIM allows registering amplitude of the electric and magnetic field in the time domain in the frequency range between 100 Hz – 50 kHz. The equipment was used to measure the EM fields produced by lightning strikes. Data collection was conducted on a site where the geology is known. The short-time Fourier transform was used to obtain the spectra of the electric and magnetic fields and the surface impedance was determined in the frequency range 0.5-20 kHz. Frequency sections of apparent resistivity were constructed along the measurement profiles and can be interpreted in terms of the geological sections. These experiments and subsequent data interpretation techniques represent a novel approach of determining electric properties of the material by observing the electromagnetic field on the surface of the media. Experiments conducted at UHF and microwave (S-band) frequencies utilize the innovative approach of using the field reradiated by a passive scatterer embedded in the material. Potential applications allow using that approach for non-intrusive data collection when a passive conductor is already present or can be put in the material during manufacturing. Experiments conducted at VLF frequency range utilize the data collection equipment developed at Griffith University by Mogensen and Thiel. Novel data collection and data processing approach of registering solitary short samples of electric and magnetic fields produced by lightning strikes is discussed. It is shown that the EM field observed in that way can be used consistently for creating a reliable geoelectric model of the media.