Electrical Ground Impedance Measurements in the United States Between 200and 415KHZ.

摘要

The objectives of the work described in this report were to use and evaluate new radiowave methods of measuring earth resistivity in the low-frequency (LF) (200-415 kHz) band, and to develop estimated effective ground resistivity maps in this same band for the United States including Alaska. Both airborne and ground methods were investigated by using the wavetilt and surface impedance techniques. The airborne methods discussion concentrates on the use of the wavetilt technique in the very-low-frequency (VLF) (10-30 kHz) band because LF transmitters operate at a wide range of frequencies and supply inadequate coverage. It is concluded from the VLF study that over much of the central United States VLF airborne resistivity might well approximate LF ground resistivity. The ground methods discussion concerns the surface impedance method in the LF band using a device developed by a commercial organization for the purposes of this study. Geological and physiographic effects on LF resistivity are demonstrated in several studies and the data are used later to produce an estimated effective conductivity map of the United States. It is concluded from the LF studies that the present conductivity map based on the field strength attenuation method for BCB (Broadcast Band: 550-1600 kHz) transmitters, and in current use by the FAA, is fairly accurate for BCB purposes but inapplicable to LF purposes, mainly because of the inadequate penetration of BCB radiation. The LF estimated conductivity maps for the United States, including Alaska, show patterns similar to those in previous resistivity maps but with different values. The most resistive areas are the granitic, mountainous regions.