Powerline contamination in time-domain electromagnetic data: experiment, theory, and basic building blocks

Abstract

Time-domain Electromagnetics (TEM) is a powerful tool to image the conductivity structure of the subsurface. These systems can be ground-based or can be mounted on an airborne platform, which allows for greater data density and resolution. Airborne electromagnetic (AEM) surveys are continually being pushed to regions with increasing culture; regions such as producing oil-fields with a need of ground-water contaminant monitoring, populated agricultural regions for water resource management, and/or mineral exploration around existing developments. Culture and cultural noise are proportional, and powerlines are a common and persistent source of cultural noise. Powerlines present a unique signature in TEM data. AEM data from Iowa, contracted by the USGS, exemplify these effects. The affected data are smoothly elevated and contamination can be seen as far as 400+ m away from a powerline. Standard data processing techniques simply cull the affected data from the dataset yielding a data loss in excess of 25%. While the problem is well documented in the literature, the basic building blocks of the physics behind the problem are not well understood. This thesis is an attempt to characterize and understand the powerline coupling problem, both theoretically and experimentally. A critical component of this work is the conceptual idea of a Skywire – a vertical loop of wire, simulating powerline grounding structures and their earth return. This concept was tested experimentally – by building a Skywire and performing TEM surveys around it, and studied theoretically – through modeling the coupling of a Skywire to an idealized TEM system. Both approaches show that powerline grounding structures are indeed the culprit, but also suggest that the mutual inductance between the earth and the Skywire plays a large role in the contamination. While the modeling does not fully capture the amplitudes and spatial extent of the contamination observed in AEM or ground-based data, many aspects of the problem were investigated. These aspects include Skywire resistance, Skywire self-inductance, Skywire–earth mutual-inductance, and differing geometries between airborne- and ground-based systems and Skywires. A methodology to repair affected TEM data and recover the earth structure has yet to be developed, however, I present a modeling algorithm and a way to quantify contamination levels in TEM data.