UTAM 2004

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UTAM 2004
Travis Crosby
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UTAM 2004
Travis Crosby
Very Low Frequency EM Surveys for the Purpose
of Augmenting Near-Surface Seismic Studies
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• Introduction
• Instrument Theory
• Geophysical Results
• Other Instrument Applications
• Future Work

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Travis Crosby

• For Single Method Surveys
• Instrument may record excessive noise.
• Ground may not provide sufficient contrast.
• Overlapping anomalies may hinder
• interpretation.

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UTAM 2004
Travis Crosby

• For Single Method Surveys
• Instrument may record excessive noise.
• Ground may not provide sufficient contrast.
• Overlapping anomalies may hinder
• interpretation.

Reconciliation of multiple data sets
often provides a more true interpretational
picture.
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UTAM 2004
Travis Crosby
VLF Theory

• 42 transmitting stations worldwide (15-30 kHz,
  10-20 km ?).

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UTAM 2004
Travis Crosby
VLF Theory

• 42 transmitting stations worldwide (15-30 kHz,
  10-20 km ?).
• At distance, EM field behaves as a plane wave
  with predictable
• magnetic and electrical components.

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UTAM 2004
Travis Crosby
VLF Theory

• 42 transmitting stations worldwide (15-30 kHz,
  10-20 km ?).
• At distance, EM field behaves as a plane wave
  with predictable
• magnetic and electrical components.
• Eddy currents are generated when field passes
  through a buried
• conductor, creating a secondary magnetic field.

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UTAM 2004
Travis Crosby
VLF Theory

• 42 transmitting stations worldwide (15-30 kHz,
  10-20 km ?).
• At distance, EM field behaves as a plane wave
  with predictable
• magnetic and electrical components.
• Eddy currents are generated when field passes
  through a buried
• conductor, creating a secondary magnetic
  field.
• Instrument measures anomalous response to the
  induced current.

Surface Location of Anomaly
Hz/Hx
Secondary EM Field
Vertical Anomaly
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UTAM 2004
Travis Crosby
VLF Theory

• 42 transmitting stations worldwide (15-30 kHz,
  10-20 km ?).
• At distance, EM field behaves as a plane wave
  with predictable
• magnetic and electrical components.
• Eddy currents are generated when field passes
  through a buried
• conductor, creating a secondary magnetic
  field.
• Instrument measures anomalous response to the
  induced current.
• Karous Hjelt filter applied to each data point
  (Dn) to convert
• complicated anomalies into simple peaks.

Surface Location of Anomaly
Hz/Hx
Secondary EM Field
Vertical Anomaly
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UTAM 2004
Travis Crosby
VLF Theory

• 42 transmitting stations worldwide (15-30 kHz,
  10-20 km ?).
• At distance, EM field behaves as a plane wave
  with predictable
• magnetic and electrical components.
• Eddy currents are generated when field passes
  through a buried
• conductor, creating a secondary magnetic
  field.
• Instrument measures anomalous response to the
  induced current.
• Karous Hjelt filter applied to each data point
  (Dn) to convert
• complicated anomalies into simple peaks.

Filtered Data n - 0.102 Dn-3 0.059 Dn-2
0.561 Dn-1 0 Dn 0.561
Dn1 - 0.059 Dn2 0.102 Dn3
Where Dn Hz / Hx, as measured by the instrument
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UTAM 2004
Travis Crosby
VLF Theory

• 42 transmitting stations worldwide (15-30 kHz,
  10-20 km ?).
• At distance, EM field behaves as a plane wave
  with predictable
• magnetic and electrical components.
• Eddy currents are generated when field passes
  through a buried
• conductor, creating a secondary magnetic
  field.
• Instrument measures anomalous response to the
  induced current.
• Karous Hjelt filter applied to each data point
  (Dn) to convert
• complicated anomalies into simple peaks.
• By increasing filter spacing (Dn-2, Dn,
  Dn2,), information
• about deeper apparent depths can be obtained,
  and used to
• produce cross-section plots of current density.

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UTAM 2004
Travis Crosby
VLF Theory

• 42 transmitting stations worldwide (15-30 kHz,
  10-20 km ?).
• At distance, EM field behaves as a plane wave
  with predictable
• magnetic and electrical components.
• Eddy currents are generated when field passes
  through a buried
• conductor, creating a secondary magnetic
  field.
• Instrument measures anomalous response to the
  induced current.
• Karous Hjelt filter applied to each data point
  (Dn) to convert
• complicated anomalies into simple peaks.
• By increasing filter spacing (Dn-2, Dn,
  Dn2,), information
• about deeper apparent depths can be obtained,
  and used to
• produce cross-section plots of current
  density.
• VLF used to look for bodies of low electrical
  resistance, i.e.
• vertical fractures and ore deposits up to
  depths of 300 m.

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Seismic Refraction
VLF

• 595 m profile length
• 120 Geophones 5 m Spacing
• 41 Shot Points 15 m Spacing
• Source used 550 lb EWG
• First arrival P-wave measured

• 595 m profile length
• 3 m station spacing
• Instrument used Abem Wadi
• Frequency used 25.1 kHz
• (Tx located in North Dakota)

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Site Location
N
20 km
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Line Location
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N
Legend
Water Well
Mapped Fault, dotted where inferred
Seismic or VLF Line
VLF Detected Fault
Seismic Detected Fault
600 m
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Seismic Tomogram
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Ray Path Density
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Seismic Tomogram
VLF Data (Karous Hjelt Filtered)
SE
NW
?
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Site Interpretation
UTAM 2004
Travis Crosby
N
Legend
Water Well
Mapped Fault, dotted where inferred
Seismic or VLF Line
VLF Detected Fault
Seismic Detected Fault
600 m
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UTAM 2004
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Test Profile
N
?
?
Legend
Water Well
Mapped Fault, dotted where inferred
Seismic or VLF Line
VLF Detected Fault
600 m
Seismic Detected Fault
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VLF Data (Karous Hjelt Filtered)
UTAM 2004
Travis Crosby
NW
SE
Water Well
Stream
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?
UTAM 2004
Travis Crosby
Site Interpretation
N
?
Legend
Water Well
Mapped Fault, dotted where inferred
Seismic or VLF Line
VLF Detected Fault
500 m
Seismic Detected Fault
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Other VLF Studies Ore Deposits

• Survey area 90 km northeast of Yellowknife, NW
  Territories, Canada.
• Original defined strike limits Ag-Pb-Au banded
  sulfide lens was 160 m.
• VLF survey complementing other data sets
  suggested a greater strike
• length of 400 m.

Data from Fugro Airborne Surveys
VLF
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Future Work
UTAM 2004
Travis Crosby

• To augment seismic refraction tomography studies
  by conducting
• smaller scale, higher resolution VLF to detect
  normal and antithetic
• faults bounding larger colluvial wedge
  structures.

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Future Work
UTAM 2004
Travis Crosby

• To augment seismic refraction tomography studies
  by conducting
• smaller scale, higher resolution VLF to detect
  normal and antithetic
• faults bounding larger colluvial wedge
  structures.
• Multi-electrode, high-resolution, 2-D, DC
  resistivity imaging of
• colluvial wedges for comparison with seismic
  tomograms.

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UTAM 2004
Travis Crosby