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...

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