From Ampere’s law, it is known that a current (i.e. a moving electrical charge) generates
a magnetic field. The inverse corollary to this is that a magnetic field can also influence a
moving electrical charge. Thus, an oscillating EM field will be influenced to varying
degrees by a magnetically susceptible material.
Conductivity
is an intrinsic property of a microscopic volume of material. Apparent
conductivity is a volume average of a heterogeneous half-space except that the averaging is
not mathematical but dependent on each instrument. Only when the earth is a homogenous
half-space is the apparent conductivity the same as the true conductivity. The main advantage
of the electromagnetic conductivity method is that contact with the sample is not required. If
the conductive material is moved near to the measurement coil, new elementary electric
circuits are created. The sample will then behave like small secondary coils and influence the
magnetic flux through the measurement coil caused by mutual inductance. Naturally, this
leads to a change in the frequency. In general, the amplitude of the voltage signal decreases if
the conductivity increases and vice versa. Our method is based on the analysis of this change.
1.2.2 Operating principle
The KT-10 v2 utilizes a 10 kHz LC oscillator with an inductive coil to measure the magnetic
susceptibility and conductivity. Magnetic susceptibility is calculated from the frequency
difference between the sample and free air measurements, while conductivity is calculated
from the difference in amplitude between the two. It also takes into account geometric
corrections to determine the true susceptibility. The frequency of the oscillator is extremely
sensitive to temperature deviations. Any temperature instability is propagated in frequency
deviations and has a direct impact on maximum sensitivity. To minimize these effects, the
KT-10 v2 takes multiple measurements in free air before measuring the sample, and then
multiple free air measurements are taken afterwards. Then using a sophisticated algorithm,
the negative impact of temperature shift is minimized.
The sequence required to obtain a measurement is:
Step 1: The frequency and amplitude of the oscillator is determined in free
air.
Step 2: The oscillator frequency and amplitude is then measured when the
coil is placed on a rock sample, drill core, or outcrop.
Step 3: The frequency and amplitude of the oscillator is then measured
again, in free air, and then the results are displayed.
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