Attenuation and Velocity Characteristics of Nano-to-micro Scale Magnetite Mixtures – Implications for Near-surface GPR

For many higher-frequency (500-2000MHz) near–surface ground-penetrating radar applications the magnetic properties of the subsurface are considered unimportant or negligible. However, if significant amounts of magnetically lossy minerals are present, such as magnetite, hematite, maghemite and/or iron in its free state, the influence of these individual components on the GPR signal attenuation or propagation velocity cannot be ignored. In an attempt to obtain an element of quantitative control on the attenuation/propagation characteristics of such materials, the results of a dielectric spectroscopy investigation into mixtures of quartz and micro-to-nano scale magnetite is presented where the relative attenuation enhancement (in decibels per metre – dB/m) and velocity retardation (in percent velocity difference) is given for a range of volume percent, ‘generic’ magnetite/quartz mixtures. The results indicate that even small amounts of magnetite can produce significant degrees of loss and velocity reduction and at the higher frequencies associated with near-surface GPR, any observed dispersion behaviour will be predominantly controlled by the magnetic loss characteristics rather than frequency-dependent variations in propagation velocity.