STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

912 Drilling and Well Completions
Mechanical characteristics
Length = 60 cm (24 in.)
Diameter = 3.75 cm (1.5 in.)
Mass = 2 kg (4 lb)
Alignment = f0.4”
Electrical characteristics
Scale factor = 5 V/g fl% (g = 32.2 ft/s2)
Bias = f0.005 g @ 25°C
Linearity = fO.l% full scale
Environmental characteristics
Vibrations = 1.5 cm p-p (peak-to-peak), 10 to 50 Hz
50g, 50 to 2000 Hz
Shock = 2000 g, 0.5 ms, 0.5 sine
Magnetometers. Magnetometers used in the steering tools or MWD tools are
of the flux-gate type.
The basic definition of a magnetometer is a device that detects magnetic
fields and measures their magnitude and/or direction. One of the simplest types
of magnetometers is the magnetic compass. However, due to its damping
problems more intricate designs of magnetometers have been developed. The
“Hall effect” magnetometer is the least sensitive. The “flux-gate” magnetometer
concept is based on the magnetic saturation of an iron alloy core.
If a strip of an iron alloy that is highly “permeable” and has sharp “saturation
characteristics” is placed parallel to the earth’s magnetic field, as in Figure 4227,
some of the lines of flux of the earth’s field will take a short cut through the
alloy strip, since it offers less resistance to their flow than does the air. If we
place a coil of wire around the strip, as in Figure 4-228 and pass enough
electrical current through the coil to “saturate” the strip, the lines of flux due
to earth’s field will no longer flow through the strip, since its permeability has
been greater reduced.
Therefore, the strip of iron alloy acts as a “flux gate” to the lines of flux of
the earth’s magnetic field. When the strip is not saturated, the gate is open
and the lines of flux bunch together and flow through the strip. However, when
the strip is saturated by passing and electric current through a coil wound on
it, the gate closes and the lines of flux pop out and resume their original paths.
One of the basic laws of electricity, Faraday’s law, tells us that when a line of
magnetic flux cuts or passes through an electric conductor a voltage is produced
in that conductor. If an AC current is applied to the drive winding A-A, of Figure
4-228, the flux gate will be opening and closing at twice the frequency of the
AC current and we will have lines of flux from the earth’s field moving in and
out of the alloy at a great rate. If these lines of flux can be made to pass through
Flux Lines
Figure 4-227. Magnetic flux-lines representation in a highly permeable iron
alloy core.