CORE ANALYSIS
the Agency toward the longer term goal of exploration and understanding of planetary bodies within the solar system.
DIELECTRIC SPECTROSCOPY
Dielectric reflectance spectroscopy is a maturing science that can be used to derive significant information about min- eralogy with little or no sample preparation. It may be used in applications when other methods would be too complicated, time consuming, or require destruction of precious samples. In this paper we propose a dielectric measurement system concept capable of being embedded in drilling tools and drill heads for use in down-hole operations on Earth and other terrestrial bodies in the solar system. The system can also be adopted for core sample analysis.
The energy in an electromagnetic wave interacts with materials through either a relaxation or resonance process. The index of refraction is determined by chemical, thermal, mechanical, and electronic mechanisms in the core material as a function of frequency. At lower frequencies, electromag- netic excitation causes physical movement and relaxation of the ionic or polar molecules within a core sample material. At higher frequencies, electromagnetic excitation causes reso- nances of the atomic nucleus or the electron cloud surrounding the nucleus. Depending on the nature of the material, the dielectric mechanism, and the excitation frequency (f), energy is either stored or lost.
THEORY OF SANSEC Square Loop Antenna Network Analyzer
Sensor
Fig. 3. RF Network Analyzer connected to Loop Antenna illuminating a SansEC Sensor
related to the sensor geometry. The sensor is coupled to the loop antenna through the magnetic near field and the induced current (total current) in the sensor will have the maximum magnitude near its resonant frequency. At resonant state, the energy radiation of the sensor reaches its maximum value and so does the energy transferred to heat by the intrinsic resistance of the sensor trace. The resonant frequency of the sensor is indicated by the minimum amplitude of the reflection coefficient at the terminals of the loop antenna. The response is measured by the network analyzer as a return loss scattering
parameter. The return loss S-parameter S11 is the reflection coefficient and is displayed on the network analyzer as a dis- tinct amplitude resonance signature as a function of frequency.
Fig. 2. A Generic Open-Circuit SansEC Sensor.
An open circuit resonant sensor has been developed for the purpose of dielectric spectroscopy of geological materials. The SansEC sensor is a planar resonant spiral or helix structure configured as an open circuit without direct (Sans) electrical connection (EC) to the material it is sensing or to the record- ing instrumentation.
The electro dynamic process of the open-circuit resonator is governed by Maxwell’s equations with zero current boundary conditions at both ends of the resonant spiral. The free elec- trons carried by the conductor are uniformly distributed along the conductive trace when no external source is applied, but when driven by an oscillating electromagnetic field the induced electromotive force (EMF) pushes the electrons carried by the conductor into the resonant state where the electrons move back and forth.
The transmitted energy from the loop antenna excites reso- nant modes in the sensor. The resonant response frequency is usually comprised of a fundamental and associated harmonics
Fig. 4. Proof-of-Concept Sensor on geologic core sample.
Depending on the nature of the rock, the permittivity, per- meability, and conductivity differ for different frequencies of the incident electromagnetic waves. For the proof-of-concept tests, sensor 1, sensor 2, and sensor 3 were placed in the exact same position on the flat smooth surface of the semi- cylindrical geologic core sample simulating a linear array of sensors scanning across the sample. This allowed for multiple interrogation frequencies to couple into the rock sample. The scattering of multi-frequency electromagnetic energy from grain surfaces or refracted through mineral particulates and layers causes dielectric dispersion. Dispersion allows for the taking in account of the molecular material properties that
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