Gravity surveys Annie, Sue, Betsy. Regional location Bango Road canal bank V-line canal bank Reno...
-
Upload
gary-jonah-mclaughlin -
Category
Documents
-
view
215 -
download
0
Transcript of Gravity surveys Annie, Sue, Betsy. Regional location Bango Road canal bank V-line canal bank Reno...
Gravity surveys
Annie, Sue, Betsy
Regional location
Bango Road canal bank
V-line canal bank
Reno Highway
Carson Highway
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
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
accuracyPhoto by Dr. Louie
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
8 ft+0.0267 mGal
Photo by Dr. Louie
From Telford et al., 1990
Density determination
Nettleton (1942) method1. Gravity survey over hill2. Determine gravity
anomalies for different densities
3. Plot these versus distance4. Plot elevation vs. distance5. Choose curve with
minimum correlation with topography
Example gravity profile, density = 2.4 g/cc
Must determine appropriate earth density
Average crustal density = 2.67 g/cc (LaFehr, 1991)
However, we can be more accurate!
Density determination, cont’d2.5 g/cc is the most appropriate density to use
2.5 g/cc
Calculating Bouguer Anomalies
instrument reading +
drift correction +
Free Air anomaly +
Bouguer anomaly =
Simple Bouguer Anomaly
terrain correction
instrument reading +
drift correction +
Free Air anomaly +
Bouguer anomaly =
Complete Bouguer Anomaly
+
V-line gravity survey
• 32 stations• 4 m spacing• Along canal bank• Down hill at ends
March 17, 2009
North
stations surveyed using theodolite
V-lin
e can
al
Photo by Dr. Louie
Preliminary plot
Note: correlation with elevation
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
Discussion of V-line results
• Localized anomalies – we had many people use
the gravimeter
• SW --> NE– Decreased compaction – Dryer sediments
• Higher gravity in valley due to compacted, natural sediment
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)
Bango Road gravity survey March 18, 2009
North
• 13 stations• 15 m spacing• Along canal bank
stations surveyed using theodolite
terrain corrections for zones A, B, C
Note: slight offset in angle due to theodolite error
Preliminary plot
Note: correlation with elevation
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
Discussion of Bango Road results
• SBA follows geology
– Gravel comprises lower elevations
– Competent basalt comprises higher elevations
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
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.