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 accuracy Photo 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 mGal Photo by Dr. Louie From Telford et al., 1990

Density determination Nettleton (1942) method 1.Gravity survey over hill 2.Determine gravity anomalies for different densities 3.Plot these versus distance 4.Plot elevation vs. distance 5.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’d 2.5 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-line canal 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 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 s Nettleton, L.L., 1942, Determination of density for reduction of gravimeter observations: Geophysics, v. 4, p Telford, W.M., Geldart, L.P, Sheriff, R.E, 1990, Applied Geophysics, Second Edition, Cambridge University Press: Cambridge.