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.