Near-field diffraction pattern by a cylindricalcavity

Abstract

A cross-borehole continuous wave electromagnetic probing is an advantageous and effective method for the detection of high contrast geological anomalies such as underground cavity. If the wavelength of the transmitting signal is comparable with the size of the cavity, the amplitude pattern of the received signal has two significant signal drop points corresponding to the top and bottom boundaries of the cavity. In this dissertation the diffraction pattern by an underground cylindrical void is analyzed to find the signal frequencies giving the strongest double dips. the measurement distance, refractive index of the background medium, and the shape of the cavity cross-section are expected to influence on the generation of these double dips. It is found for a circular cylindrical cavity that double nulls of the total field occur for the signal wavelength comparable with the dimension of the cross-section in the near-field region, while these double nulls become double dips of finite strength in the far-field region. The phase of the total field jumps 180° at the null points and varies less than 180° but abruptly about the dip points. The signal frequency providing the strongest double dips in the far-field region is shown to be inversely proportional to the relative refractive index of the background medium. Double dips dropping 25 dB from the incident field is measured by the scale-down experiment and its shape and dips are comparable with the calculations for two dimensional model. For a general shape of the cross-section of the cavity, boundary element method may be applied. It is shown for a rectangular cylindrical cavity that there also occur double nulls in the near-field region and the region where nulls exist is larger for the narrower cavity. Null pattern in the near-field region is shown to exist also for a trapezoidal cylindrical cavity and their null frequencies corresponding to the upper and the lower boundaries, respectively, are different due to t...