Gravity in the Fernley

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Gravity in the Fernley Hazen Flat Areas
Geol 492/692 Spring 2005
Melissa Edwards, Harmony Farnsworth, and Rich Redd
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LocationsFernley
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LocationsHazen
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MethodsGravimeter

• Used a LaCoste Romberg Model G Gravimeter to
  measure gravity at each location
• Before each trip to the field a base station
  measurement was taken at UNR outside of SEM (Base
  1)
• At both the Fernley and Hazen sites a base
  station was established and a gravity measurement
  was taken the morning, afternoon, and the evening
  of each trip (Base 2 Base 3)
• The base station gravity measurements were all
  within 0.5 mGals of each other, indicating the
  gravimeter was working correctly
• A calibration correction for the reading for the
  gravimeter was made using one of the following
  formulas (depending on what the gravity reading
  was)
• For 3100 reading ((Gravimeter reading-3100)1.022
  58)3170.62
• For 3200 reading ((Gravimeter reading-3200)1.022
  6)3272.88

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MethodsTrimble

• Used a Trimble XT GPS to measure elevation and
  lat/long at every location
• Instrument was placed 10 ft or less away from the
  spot where a gravity reading was taken, but at
  the same elevation level
• Instrument was allowed to collect GPS points for
  10 minutes or 120 points, which ever came first
• Data
• Downloaded using GPS Pathfinder Office 3.0
• Used the Reno airport as the reference for
  corrections to all the GPS data collected from
  the Trimble since it had data for all the days we
  recorded.
• Horizontal error at least 6 cm.
• 68 confident elevation is within 2 meters
• Amounts to a 0.3 to 0.6 mGal error in the Bouguer
  calculations

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MethodsTerrain Corrections

• Estimations were made of the elevation difference
  b/w the place where the gravity reading was taken
  and the surrounding topography
• For zone B (6.56-54.6), 4 directions
• For zone C (54.6-175), 6 directions
• With these values a chart was used to find the
  dgr values for each direction all of these
  values were added to obtain the total terrain
  correction value

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MethodsBouguer Correction

• Bouguer anomaly value calculations were made
  using the gravity readings from the gravimeter,
  the terrain corrections, the elevation/lat/long
  data downloaded from the Trimble, and the
  following formulas
• Free Air Correction 0.3086 elevation
• Bouguer Correction 0.0419density (used 2.67)
  elevation
• Theoretical Gravity 978.03185 (1 (0.0053024
  sin2 (latitude)) (0.0000058 sin2 (2
  latitude))
• Bouguer Anomaly (Observed Gravity Free Air
  Correction Bouguer Correction) Theoretical
  Gravity

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Methods ResultsFernley Line 1

• Gravimeter points were spaced 100 meters apart
• Points S1-S19 were surveyed (plus Base 1 Base 2)

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Methods ResultsFernley Line 2

• Gravimeter points spaced 100 meters apart
• Points S201-S221 were surveyed (plus Base 1
  Base 2)
• Poor satellite coverage for S204 S205 with the
  Trimble

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Methods ResultsHazen

• Gravimeter points were spaced 100 meters apart
• S128 was spaced 150 meters form S127
• S131 was spaced 330 meters from S130
• Points S100-S131 were surveyed
• Poor satellite coverage for S121 S122 with the
  Trimble

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ModelingBouguer Slab Depths

• Where ?g The computed Ganom value from grav2d
• ?? -.35
• The 0.0419 value is a constant

Bouguer Slab Equation (Telford et. Al.1990)
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Fernley 1
Blue lines indicate regional stresses. Red lines
indicate shearing within the flower structure
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Fernley Line 1
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Fernley 2
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Fernley 2
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Hazen
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Hazen
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The End . . . Questions?