The fact that ground penetrating radar (GPR) pulses propagate through dry salt with little attenuation has been exploited to probe salt formations since the mid-seventies of the last century (Unterberger and Stewart 1975; Unterberger 1976; Annan, Davis et al. 1979) Consequently, the ability of high frequency GPR to detect feature of few cm in thickness makes it a good technique for mapping anomalies in Potash mines. One particular interest in that environment is avoiding thin ‘clay’ layers, which, if cut too close to, can influence the safety and stability of the underground workings. To help in interpreting ground penetrating radar data, a clear understanding of the factor causing the reflection is essential; knowing why a reflection disappears is just as important as understanding the reflection itself. This is better understood if samples from the zones of reflections are characterized in terms of their complex permittivity and chemical and geometrical composition. In this contribution, we measure the dielectric properties of mixtures of halite with other primarily evaporite minerals taken directly from recently sampled core. We find substantial wave speed and attenuation dispersion in these mixtures that may result from the enhanced conductivity due to clay minerals.
The propagation velocity of an electromagnetic (radar) wave in a nonmagnetic medium is given by
Where c is the velocity of propagation of light in free space, and one measure of the attenuation tand: the dissipation factor is defined as the ratio between the imaginary (e?) and the real (e') permittivity respectively, which, for non-lossy medium vanishes and the velocity become a function of the real permittivity e' only.
In previous measurements on synthetic mixtures of salt with dry clays, mixtures (Aqil and Schmitt 2008), we showed that the imaginary permittivity increased substantially above that for either material by itself. This is possibly due to the juxtaposition of clay surfaces with abundant cations and increased conductivity (Aqil and Schmitt 2008). In such a case ignoring tand will produce a serious error in calculating the velocity according to Eqn. 1. Here we extend this study to the natural materials found in the mine.
An open ended coaxial probe technique was used to measure the complex dielectric constant. The measurement system is composed of a coaxial line probe (figure 1), a network analyzer and PC. The amplitude and phase of the incident and reflected electromagnetic wave at the plane of contact between the probe head and the sample are used to calculate the dielectric permittivity. The probe was calibrated with open, short circuit and deionised water. The accuracy of the measurement system was tested using a single halite and sylvite crystal and was found to be less than 5%. The measurement system operates at frequencies between 10MHz and 3GHz.
A subset of our halite/sylvite samples taken from core obtained in 2008 from a potential mine potash mine site in Saskatchewan are presented here. These priority samples were taken as they are associated with known continuous reflections observed in the underground GPR profiling.
