Abstract
AC and telluric interference on pipelines are both due to electromagnetic induction by external sources. In the case of AC interference, the source is a nearby power line, tens of meters (typically 30 feet or more) above the ground, and with currents oscillating at a frequency of 50 or 60 Hertz (Hz). Telluric currents are caused by electric currents in the ionosphere, 100 kilometers (km) above the earth’s surface, varying on time scales from seconds to days. Although the heights and frequencies of the currents are different in these two situations, the underlying physical process of electromagnetic induction is the same in each case. To model these processes we can use the distributed source transmission line (DSTL) theory to look at the network response and the multi-layered cylinder (MLC) model to determine the frequency response of pipeline sections. We review the work that has been done using these methods to model both AC interference and telluric current interference. Many of the results for AC and telluric interference, although sometimes presented in different ways, are shown to be essentially the same. We also examine what characteristics of AC and telluric interference, such as the skin depth effects, lead to differences in the response of pipelines to AC and telluric currents.
