Effects of a Grounded Parasitic Line on Earth Conductivity Measurements

摘要

When earth conductivity measuring systems are operated near grounded conductors such as utility lines and railroad tracks, the coupling between the array and the parasitic element can lead to erroneous measurements for earth conductivity. The problem of parasitic line contamination is studied using theory and outdoor modeling with respect to three common techniques: Four Terminal Array, E/H Surface Impedance, and H/I Near Magnetic Field. Experimental measurements were found to agree well with the theory which shows that the four-terminal array and the E/H methods are far less contaminated by parasitic lines than is the H/I. Since the high H/I error falls off very quickly as the spacing to the parasitic is increased, it compares favorably with other techniques for parasitic distances larger than 0.2 delta. It is shown that if the self-impedance of the parasitic is 1000 ohms, or higher, the conductivity error will generally be negligible for both the end-grounded (or remotely grounded) and the periodically-grounded parasitics as long as the conductivity array is not near a parasitic ground point. Ungrounded parasitics should present no problem under most circumstances. It is shown that low impedance parasitics cause the highest error and require the largest separations from the parasitic to insure a negligible error. Theoretical expressions were derived which will permit calculating the degree of contamination for any of these arrays when operated near horizontal parasitic lines which are either on or above the surface of the ground. (Author)