Understanding Magnetic Disturbance

1
Understanding Magnetic Disturbance unreliable
azimuths

• The magnetic environment
• Measuring field strength dip angle
• Setting the thresholds for MagIC
• Running a survey
• MagIC identifies the disturbance
• Adjust unreliable azimuths
• Calculate and plot the true hole path
• E N C O R E
• Borehole Surveys WHERE DID THAT
  DRILLHOLE GO?

2
Magnetic threshold and Azimuth accuracy
At any specific place on the earth's surface,
these values are known. Variation - angular
difference between True North and Magnetic
North Magnetic Field Strength - the force of the
earth's magnetic field Magnetic Dip Angle - angle
the field force lines make with the horizontal
3
Measure local magnetic data Take several
single-shot measurements, with SensIT on the
ground, clear of any magnetic disturbance.
Enter values into MagIC set-up, with reasonable
tolerances.
4
Run a multi-shot survey of the hole, or section

• 1 If possible, survey before running casing.
• 2 Take shots at close intervals 3m or 6m max.
• 3 Pull out and download SensIT into StoreIT
• 4 Transfer survey to MeasureIT on PC

Its so simple with FLEXIT
5
Understanding Magnetic Disturbance unreliable
azimuths

• This example is from a hole in Chile
• Collared at dip of - 67 degrees and a
• direction of 270 degrees true
• Cased NX to 85 metres
• Cored NQ to 390 metres

6
The MagIC quality check

• SensIT records Magnetic Field Strength Dip
  Angle at each station
• Within the set thresholds (white), the measured
  hole azimuth is reliable MagIC will then apply
  the variation.
• Outside the thresholds, (yellow) the azimuth at
  that station is unreliable.

7
MagIC quality check adjusting the hole azimuth.

• Take an average of the nearest clean azimuth
  above
• and below the unreliable values (yellow).
• Enter that average into the Azimuth box and click
  Change

8
Plotting the hole paths, in section. The
measured hole the MagIC hole.
9
Understanding Magnetic Disturbance unreliable
azimuths

• This example is from Scotland
• Collared at a dip of - 74 degrees and a
  direction of 50 degrees true
• Depth of 120 metres no Casing
• Two separate magnetic disturbances
• between 48 - 60m and 78 - 84 metres

10
Set the threshold for the magnetic environment.
Its so simple with FLEXIT
11
Identifying magnetic disturbance.
Its so simple with FLEXIT
12
Removing the effects of magnetic disturbance.
Its so simple with FLEXIT
13
Plotting the hole paths, in plan.The measured
hole the MagIC hole.
14
Plotting the hole paths, in section. The
measured hole the MagIC hole.
15
Now I know where that drillhole went,in spite of
magnetic disturbance!
E N C O R EBorehole Surveys