MANUAL – REFLEX EZ-TRAC™ | 13
used in an electromagnet, and an alternating current is run through
the coil, the magnetic polarity of the coil changes directions back and
forth. The saturation occurs symmetrically in each half of the cycle.
However, the saturation depends on the total magnetic intensity. If
the electromagnet is placed in an existing magnetic field, the total
magnetic intensity will vary. In one half of the cycle, the magnetisation
due to the current in the coil will be added to the existing magnetic
field. In the other half, the existing magnetic field strength will be
reduced by the magnetisation. Hence, in half of the cycle the
saturation will occur a bit earlier, in the other half it will happen
somewhat later. This asymmetry is possible to register electronically.
Now, if a fluxgate magnetometer, which is based on the operation
described above, is placed in the borehole, it will be capable of
register the Earth’s magnetic field. Three magnetometers placed at
right angles are needed to fully determine the strength and direction
of the field.
4.3 Gravity
Gravity is a force that attracts objects together. On Earth, gravity pulls
objects towards the centre of the Earth.
4.3.1 Accelerometers
The instrument probe contains three MEMS capacitive
accelerometers aligned in orthogonal directions. The accelerometers
provide the vertical component (dip or inclination) and the rotation
relative gravity. The accelerometer readings are also used to
compensate for the rotation of the instrument as it moves in the
borehole.
MEMS is an abbreviation for Micro-Electro-Mechanical Systems.
MEMS technology allows the creation of small functional components
via semiconductor-like manufacturing. A MEMS accelerometer is a
fully self-contained sensor, with all electronics integrated into a
component of small dimensions.
A capacitive accelerometer senses the change in electrical
capacitance with respect to acceleration. The sensing element
consists of two parallel plate capacitors, with a seismic mass in
between. Under acceleration, the inertia makes the mass move
between the plates, which changes the values of the capacitors. This
differential variation is captured by a detecting circuit, which
translates the signal into a calibrated voltage output.
4.4 Temperature
The instrument probe contains an electronic temperature senor. This
sensor is used to measure the temperature for internal stabilization.
The temperature values are also displayed in the control unit.
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