Absolute Observations Training

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Absolute Observations Training

• Bill Worthington
• June 10 12, 2008

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Absolute measurements are not that hard to do,
but it takes work to do them well.
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Outline

• Introduction, Background and Principles
• Absolute measurements, what are they and why do
  we need them?
• Introduction to DI-Flux
• Measurement Procedures
• Computation of Results

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Description of Earths Magnetic Field

• D Declination Angle
• I Inclination Angle
• F Total Field
• H Horizontal
• Z Vertical

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Geomagnetic Observations

• Measure the total field, F, which has both
  magnitude and direction and is time dependent.
• Horizontal reference plane, for Inclination
• True North meridian plane, for Declination
• F is measured with a nuclear resonance technique
  using the proton gyromagnetic ratio ?p2.6751525
  40 108 T-1 s-1

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Declination Measurements
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Inclination Measurement
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Observatory Instrumentation

• 3-Axis fluxgate magnetometer
• Scalar magnetometer, proton or overhauser
• DI-Flux magnetometer
• Data collection computer

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Absolute vs. Variation Instruments

• Absolute Instruments directly measure the
  strength of or the orientation of the magnetic
  vector.
• Variation or relative instruments, measures the
  deviation from an undetermined field. To
  determine the value of the undetermined field, it
  is necessary to make an absolute measurement.

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Magnetometer Baselines

• Absolute measurements of I and F are used to
  compute absolute values of H and Z.
• The difference between the absolute values of H,
  D, Z and the H, D, Z output from the variations
  instrument is known as the baseline for each
  component.
• The baseline values are compiled and used to
  adopt a continuous baseline to correct the
  variations data to absolute values.

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Example Baselines, CMO
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Magnetometer Drift

• Variations instruments such as a fluxgate
  magnetometer can drift due to
• Temperature changes in the electronics and/or
  sensor
• Aging of components
• Pier drift magnetometer piers can slowly move
  with time introducing change in baselines
• Hence, baselines are not just used to correct the
  variation data but also to remove magnetometer
  drift.

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DI-Flux Instrument

• Is used to measure the D and I angles
• Non-magnetic Theodolite
• Fluxgate Sensor
• Fluxgate Electronics

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Non-magnetic Theodolite, Zeiss 020

• Telescope microscope
• Circles
• Sensor
• Circle clamps
• Slow motion controls
• Feet
• Mirror
• Pendulum

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Reading the Microscope 020

• Vertical Scale
• Yellow background
• 256 52.2
• Horizontal Scale
• Blue background
• 235 05.0

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Reading the Microscope (010)

• This instrument reads to 1 second and has a much
  different scale.
• The knob with the clamps allows one to switch
  between the horizontal and vertical circles.

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DI-Flux Fluxgate

• Single axis sensor
• The fluxgate sensor orientation is very sensitive
  with respect to magnetic field direction. The
  highest sensitivity is when sensor magnetic axis
  and magnetic field are almost perpendicular
• Null reading when output is 0.0 Volts the
  magnetic vector is perpendicular to the sensor.
• The Sensor axis and the optical axis of telescope
  are aligned so they are parallel.

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DI-Flux Electronics

• Digital display of output in Volts or nT. Few
  units use an analog meter anymore.
• The output in nT may need to be calibrated to use
  with the offset method.
• Usually battery operated
• May be slightly magnetic.

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Measurement Procedure

• Preparation
• Leveling the Instrument
• Reading the Azimuth mark
• Leveling the telescope
• Four Declination readings
• Computation of the meridian
• Alignment to the meridian
• Four Inclination readings

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Observation form

• Hand out

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Absolutes Preparation

• Check magnetic field for disturbance level, at
  high latitudes this can be important.
• Remove all magnetic items from your person. This
  includes keys, coins, watches, knives, pens, and
  cell phones.
• Turn on the clock or set it as necessary.
• Check battery voltage of the DI-Flux electronics.
• Make sure instrument is properly placed and/or
  clamped on the pier.

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Leveling the Theodolite

• 3 adjustable feet
• 2 axes of alignment
• Left hand rule
• Precision of level
• Need for clamps

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Terminology and Cautions

• Descriptions of instrument positions refer to the
  direction the telescope is pointing (N, S, E, W)
  and whether the magnetometer sensor is up or
  down.
• Never handle the instrument by grabbing the
  magnetometer sensor.
• Leave your hands off the instrument when you are
  moving to a new position.
• Do not touch the optics.

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Reading the Azimuth Mark

• The bearing to the Azimuth mark from the
  absolutes pier is a known bearing with respect to
  true north.
• The Azimuth measurements provide the reference
  for the angular measurements of Declination.
• The Azimuth mark is read in the Up and Down
  positions, before and after each set of
  Declination measurements.
• Make sure the telescope is properly focused on
  the mark or target.
• The alignment on the mark should be as precise as
  possible. Double check the alignment.

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Reading the azimuth mark
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Leveling the telescope

• Leveling the telescope to 90 or 270 is
  critically important to obtaining precise
  Declination measurements. If your instrument
  reads in grads, this would be 100 and 300.
• The alignment of the scales in the microscope
  should be done Exactly. Poor leveling is the
  most common problem when the D baselines are not
  repeatable.

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Declination observations

• Declination is measured with four orientations.
  This is to average out any mis-alignment of the
  sensor and telescope.
• USGS uses the following orientations
• West Down
• East Down
• West Up
• East Up

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Nulling and Timing for D

• Once the orientation is set, the magnetometer
  should be nulled or zeroed using the Horizontal
  Slow motion control.
• The null can be within 0.2 nT of zero.
• Once the magnetometer is nulled, the time and
  reading should be recorded.
• Typically the null is performed at the top of the
  minute, 10 seconds.
• It is important that the timing of the null is
  recorded correctly to compute the baselines.

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Computing the magnetic meridian

• This is necessary to align the telescope in the
  H-Z plane for the Inclination measurements.
• The meridian is the average of the four angular
  measurements for D.
• One must be careful because the measurements are
  in degrees, minutes, and sometimes seconds.
• Most scientific calculators have buttons or
  functions to convert from degrees, minutes, and
  seconds to decimal degrees.

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Computing the meridian (cont)

• First convert to Decimal degrees
• Add up Decimal Degrees
• Divide by 4
• Convert back to D, M, S
• Result is 184 50 33 and its reverse bearing 4
  50 33

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Aligning the meridian

• To perform the inclination readings the
  Horizontal Circle must be aligned with the
  computed meridian.
• Using the example in the previous slide, rotate
  the instrument until the Horizontal Circle reads
  184 50 33.

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Inclination observations

• Inclination is measured with four orientations.
  They are often referred to as plungings.
• USGS uses the following orientations
• South Down (SD)
• North Up (NU)
• Realign the horizontal circle to the reverse
  bearing of the meridian.
• South Up (SU)
• North Down (ND)

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Nulling and Timing for I

• Once the orientation is set, the magnetometer
  should be nulled or zeroed using the Vertical
  Slow motion control.
• The null can be within 0.2 nT of zero.
• Once the magnetometer is nulled, the time and
  reading should be recorded.
• Typically the null is performed at the top of the
  minute, 10 seconds.
• It is important that the timing of the null is
  recorded correctly to compute the baselines.

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Repeat the readings

• The measurement of D and I just described should
  be repeated 2-3 more times. The goal its to have
  at least three repeatable readings from one
  absolute session.

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F measurements

• For the Declination and Inclination readings to
  be useful the total field or F needs to be
  measured at the same time as the observations.
• At some observatories, including the USGS, F is
  recorded along with variation recordings of H, D,
  Z. In this case the values of F are easily
  obtained.
• F must be corrected back to the Absolute pier.

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Computation of Results

• Basic Formulas
• Absolute Values
• Baseline Values
• USGS Baseline Program

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Computation of D

• Compute Meridian
• Compute the mean value of the Mark Readings.
• Mean Az (((MU1MD1)/2 (MU2MD2)/2)180)/2
• Subtract the Meridian from the Mark readings to
  get the Magnetic Azimuth
• Mag. Az Mean Az Meridian
• D True Az Mag. Az 180

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Computation of I and H and Z

• I (((SD NU) (SU ND)) 360) / 4
• H F cos(I)
• Z F sin(I)
• F should be the average of F measured when the
  four inclination angles were measured. In
  addition the average F needs to be corrected to
  the Absolutes Pier.

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Computation of Baseline Values

• The values of H, D, and Z, also called ordinates,
  recorded by the variometer are needed.
• The ordinate values used for each component
  should correspond to the same time that the
  measurements of D and I were performed.
• For example, the D ordinates should be for the
  same time D was observed and the H and Z
  ordinates should be at the same time I was
  observed.

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Computation of H and Z Baseline Values

• The baseline value is the absolute value minus
  the ordinate value.
• For example
• Hbl HAbs HOrd .
• Zbl ZAbs ZOrd .

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Computation of D Baseline Values

• The D baseline computation requires more care
  because the Absolute value of D is measured in
  Degrees and the D ordinate is usually recorded in
  nT.
• Need to convert D in nT to D in degrees or
  minutes
• Commonly the D scale value (Dsv) converts nT to
  minutes and is defined as
• (36060) / 2p / HAbs or 3437.747 / HAbs.
• Then Dbl DAbs ( DOrd Dsv)

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Baseline Computations

• While it is possible to compute the absolute and
  baseline values by hand, it is much easier to
  make use of a spreadsheet or write a computer
  program to do the computations.
• During this course we will use a program that we
  developed to work with our data acquisition
  program. One feature of this program is that it
  has the capability to obtain the values of F and
  the H, D, and Z ordinates from the data
  collection computer.

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Baseline Program
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Baseline Program
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Baseline Program

• The program works somewhat like a spreadsheet.
  There are open blocks where the measurements can
  be entered. With this program all that is needed
  to do is to enter the DI-Flux readings and the
  time of each reading. Once entered and double
  checked for accuracy, the Calculate button is
  clicked on. The sequence is repeated for each
  set.

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Baseline Results

• On the far right of the program there is a tab
  labeled DCP Summary that displays the absolute
  values and baseline results for all observed
  sets.
• With this screen it is possible review and
  evaluate your results.

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Baseline Summary