Magnetometers and electro-magnetomenters

Abstract

A magnetometer is used in a geophysical survey to measure magnetic susceptibility variations in earth. It is a passive sensor because it uses the ambient earth magnetic field as the source of excitation. An electromagnetic (EM) sensor operating below the radio frequency (RF), or in an EM induction (EMI) mode, is commonly used to measure electrical conductivity variations in earth. (An EM sensor operating above the RF, commonly called ground-probing radar or GPR, can measure variations in dielectric permittivity, which we do not consider here.) For this reason, an EMI sensor is often called a conductivity meter . It is an active sensor because it carries its own source of excitation. It has been common to consider that a magnetometer is the principal sensor for measuring magnetic susceptibility, and that an EMI sensor is the principal sensor for measuring electrical conductivity. Discussions of EMI in many geophysics textbooks often start with “Let us assume that the magnetic permeability is that of free space…” or similar sentences that dismiss the role of susceptibility in EMI physics. Such discussions, as well as historical limitations in the sensor technologies, have promoted the notion that magnetometers and EMI sensors have separate and distinct functions for measuring each physical property. It is well known, however, that an EMI sensor responds to both electrical conductivity and magnetic susceptibility. In fact, an EMI sensor operating at sufficiently low frequencies acts more as a magnetometer than as a conductivity meter. At the so-called “resistive limit” where the conductivity-frequency product is small, an EMI sensor responds only to magnetic susceptibility and ignores electrical conductivity. It would be a serious misnomer in this case to call an EMI sensor a conductivity meter. A correct designation would be an active magnetometer or, as we propose here, an electro-magnetometer . A properly …