Gravity surveys

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Gravity surveys

• Annie, Sue, Betsy

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Regional location
Bango Road canal bank
Reno Highway
Carson Highway
V-line canal bank
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Theodolite

• Measures horizontal vertical angle relative to
  known point (north)
• Measures horizontal vertical offset between
  survey point and instrument location
• Linked to Earth using one GPS point
• Data used to obtain latitude, longitude, elevation

Photo by Dr. Louie
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Gravimeter

• Lacoste-Romberg G-509 gravimeter
• Must be level to get accurate reading
• UNR base station measured in AM PM
• Field base station measured every 3 hrs
• 0.05 mGal instrument accuracy

Photo by Dr. Louie
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Terrain correction

• Zone B 4 sectors
• radius 6.56 54.6 ft
• Zone C 6 sectors
• radius 54.6 175 ft
• Elevation difference estimated for each sector
• Positive number added to SBA

Photo by Dr. Louie
8 ft 0.0267 mGal
From Telford et al., 1990
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Density determination
Must determine appropriate earth density

• Nettleton (1942) method
• Gravity survey over hill
• Determine gravity anomalies for different
  densities
• Plot these versus distance
• Plot elevation vs. distance
• Choose curve with minimum correlation with
  topography

Average crustal density 2.67 g/cc (LaFehr,
1991) However, we can be more accurate!
Example gravity profile, density 2.4 g/cc
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Density determination, contd
2.5 g/cc is the most appropriate density to use
2.5 g/cc
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Calculating Bouguer Anomalies
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V-line gravity survey
March 17, 2009

• 32 stations
• 4 m spacing
• Along canal bank
• Down hill at ends

V-line canal
stations surveyed using theodolite
Photo by Dr. Louie
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Preliminary plot
Note correlation with elevation
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V-line canal bank

• Small anomalies due to user error
• Gradation from high to low along canal bank
• Gravity high in valley

Plot generated by Laura Huebner using Geosoft
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Discussion of V-line results

• Localized anomalies
• we had many people use the gravimeter
• SW --gt NE
• Decreased compaction
• Dryer sediments
• Higher gravity in valley due to compacted,
  natural sediment

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V-line conclusions

• Despite local anomalies, high confidence in
  general decreasing SBA trend
• Trees near higher SBA values
• Increased water raises SBA
• Increased water accounts for vegetation
• Compromised canal bank?
• Overall, low confidence in results
• small range in SBA (0.368 mGal)
• enormous error
• Terrain corrections (0.1 mGal)
• Multiple user error (0.03 mGal)
• Instrument error (0.05 mGal)

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Bango Road gravity survey
March 18, 2009
stations surveyed using theodolite
terrain corrections for zones A, B, C

• 13 stations
• 15 m spacing
• Along canal bank

Note slight offset in angle due to theodolite
error
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Preliminary plot
Note correlation with elevation
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Bango Road canal bank

• Gravity low in sediment fill at lower elevations
• Gravity high at top of hill

Plot generated by Laura Huebner using Geosoft
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Discussion of Bango Road results

• SBA follows geology
• Gravel comprises lower elevations
• Competent basalt comprises higher elevations

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Bango Road conclusions

• Relatively lower error, though still high
• Instrument error (0.05 mGal)
• Multiple user error (0.02 mGal)
• Terrain correction error (0.2 mGal)
• SBA range of 0.824 mGal
• High confidence in relative profile
• Gravity high over basalt
• Gravity low over sediments
• No apparent seepage through canal bank

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References

• LaFehr, T.R., 1991, Standardization in gravity
  reduction Geophysics, v. 57, p. 1170-1178.s
• Nettleton, L.L., 1942, Determination of density
  for reduction of gravimeter observations
  Geophysics, v. 4, p. 176-183.
• Telford, W.M., Geldart, L.P, Sheriff, R.E, 1990,
  Applied Geophysics, Second Edition, Cambridge
  University Press Cambridge.