PhD Thesis Defense: Michele Maxson

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Meeting ID: 938 0823 2593
Passcode: 332811

"Early-Time/High Frequency Electromagnetic Induction Sensing for Minimal-Metal and None-Metallic Subsurface Targets"

Abstract

Conventional electromagnetic induction (EMI) systems operate within the quasi-static regime where measurements are typically dominated by conduction currents. As a result, these systems effectively detect highly conductive targets and bulk soil properties but exhibit limited sensitivity to low-conductive environments such as permafrost, composite materials and minimum-metal landmines, where relevant information resides in early-time/high-frequency electromagnetic responses. This limitation represents a mismatch between system design and the underlying physics of the target-sensing phenomenon, restricting the capability of standard EMI for resolving fine-scale non-metallic subsurface structure.

This thesis presents modeling and experimental studies of early-time/high-frequency sensitivity as a unifying design principle for extending EMI sensing beyond the quasi-static regime for geophysical investigations of low-conductivity and layered subsurface targets. First, a novel frequency-domain EMI (FDEMI) system meant for permafrost detection is presented.  This system operates in the intermediate frequency range (∼100 kHz to 1 MHz), where both conduction and displacement currents contribute to the measured response. This new FDEMI system employs a combined geometric and electronic primary-field nulling approach, enabling measurement of the secondary field at higher frequencies. Layered-media modeling and field experiment results demonstrate sensitivity to permafrost, which may improve spatial resolution relative to conventional EMI approaches.

Building on this framework, the thesis extends into the time domain by demonstrating an early-time electromagnetic induction (ETEMI) system. A custom transmitter and coil enable measurements beginning at approximately 3.5µs after transmitter shutoff, extending the observable time window of conventional time-domain systems. Integral equation techniques, such Method of Auxiliary Sources (MAS) and Method of Moment (MoM), are used to model target responses and validate measured early-time decays. Experimental results show strong agreement with modeled responses of carbon fiber sheets, which are intermediate electrically conducting non-metallic targets, robustness to environmental backgrounds, and improved sensitivity to low-conductivity and low-metal-content targets, including minimal-metal landmines.

Together, these results demonstrate that access to high-frequency or early-time electromagnetic responses recovers target information that is inaccessible to traditional EMI systems. The combined frequency and time-domain approaches establish a unified framework for EMI system design in geophysical investigations, extending potential sensing capability into regimes governed by both conductivity and permittivity.

Thesis Committee

• Fridon Shubitidze (Chair)
• Benjamin Barrowes
• Ryan Halter
• Don Perovich
• Taylor Sullivan (University of Alaska-Fairbanks; USACE ERDC CRREL)

Contact

For more information, contact Thayer Registrar at thayer.registrar@dartmouth.edu .