Gravity in the fernley hazen flat areas
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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

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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


Locations fernley

Locations—Fernley


Locations hazen

Locations—Hazen


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


Fernley line 1

Fernley Line 1


Fernley 2

Fernley 2


Fernley 21

Fernley 2


Hazen

Hazen


Hazen1

Hazen


The end questions

The End . . . Questions?


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