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Gravity in the Fernley & Hazen Flat Areas. Geol 492/692 Spring 2005. Melissa Edwards, Harmony Farnsworth, and Rich Redd. Locations—Fernley. Locations—Hazen. Methods—Gravimeter. Used a LaCoste & Romberg Model G Gravimeter to measure gravity at each location

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gravity in the fernley hazen flat areas

Gravity in the Fernley & Hazen Flat Areas

Geol 492/692 Spring 2005

Melissa Edwards, Harmony Farnsworth, and Rich Redd

methods gravimeter
Methods—Gravimeter
  • 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)*1.02258)+3170.62
    • For 3200 reading: ((Gravimeter reading-3200)*1.0226)+3272.88
methods trimble
Methods—Trimble
  • 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
methods terrain corrections
Methods—Terrain 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
methods bouguer correction
Methods—Bouguer 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 = 0.0419*density (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
methods results fernley line 1
Methods & Results—Fernley Line 1
  • Gravimeter points were spaced 100 meters apart
  • Points S1-S19 were surveyed (plus Base 1 & Base 2)
methods results fernley line 2
Methods & Results—Fernley 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
methods results hazen
Methods & Results—Hazen
  • 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
modeling bouguer slab depths
Modeling—Bouguer Slab Depths
  • Where Δg= The computed Ganom value from grav2d
  • Δρ= -.35
  • The 0.0419 value is a constant

Bouguer Slab Equation (Telford et. Al.1990)

fernley 1
Fernley 1

Blue lines indicate regional stresses. Red lines indicate shearing within the flower structure