Impulse response of viscous remanent magnetization: laboratory measurements by a pulse induction system

Transient electromagnetic responses measured in the field or in the laboratory may bear effects of viscous remanent magnetization (VRM) associated with magnetic relaxation of ultrafine grains of ferrimagnetic minerals or superparamagnetism. The behavior of VRM can be studied in time or frequency domain, TDEM measurements being advantageous because they are done in the absence of primary field and owing to broad time range providing high accuracy of VRM parameters. Another advantage is that the rate of viscous decay measured as voltage decay does not need to be corrected for stable and/or slowly decaying viscous component of total remanence. Time-dependent transient responses of viscous decay follow the power law a ⋅ t^–b, where a is the initial emf signal (varying in a broad range) and b is the exponent approaching 1. Laboratory tests with a pulse induction coil system reveal a strong linear correlation of the parameter a with frequency-dependent magnetic susceptibility Δκ used commonly for constraining the relative abundances of superparamagnetic particles. Such systems are thus suitable for quick measurements of the large number of samples for detection of superparamagnetic (SP) particles and quantifying their contribution. The difference of b from 1, though being minor, exceeds markedly its error in estimates from measured data. Simulated TDEM responses of a superparamagnetic ground show both parameters (a and b) to depend on particle volume distribution, which is prerequisite for inversion of time-domain transients to magnetic properties of rocks and soils.