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– 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

• 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

• 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

Zone B4 sectors

6.56’-54.6’

+/- z dgr

0-1.1 0.00000

1-1.9 0.00133

1.9-2.5 0.00267

2.5-2.9 0.0040

2.9-3.4 0.0053

3.4-3.7 0.0067

3.7-7 0.0133

7-9 0.0267

9-12 0.40

12-14 0.053

14-16 0.067

16-19 0.080

19-21 0.0935

21-24 0.107

24-27 0.120

27-30 0.133

Zone C6 sectors

54.6’-175’

+/- z dgr

0.0-4.3 0.00000

4.3-7.5 0.00133

7.5-9.7 0.00267

9.7-11.5 0.0040

11.5-13.1 0.0053

13.1-14.5 0.0067

14.5-24 0.0133

24-32 0.0267

32-39 0.040

39-45 0.053

45-51 0.067

51-57 0.080

57-63 0.0935

63-68 0.107

68-74 0.120

74-80 0.133

80-86 0.147

86-91 0.160

91-97 0.174

97-104 0.187

104-110 0.200

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• Gravimeter points were spaced 100 meters apart• Points S1-S19 were surveyed (plus Base 1 & Base 2)

Fernley Line 1

2548.6

2548.8

2549

2549.2

2549.4

2549.6

2549.8

2550

2550.2

2550.4

2550.6

4381000 4381500 4382000 4382500 4383000 4383500

Latitude (UTM)

Bo

ug

ue

r A

no

ma

ly (

mG

al)

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

Fernley Line 2

2549

2549.5

2550

2550.5

2551

2551.5

2552

2552.5

4379500 4380000 4380500 4381000 4381500 4382000 4382500 4383000 4383500

Latitude (UTM)

Bo

ug

ue

r A

no

ma

ly (

mG

al)

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

Hazen Line

2555

2555.5

2556

2556.5

2557

2557.5

2558

2558.5

2559

2559.5

2560

4375500 4376000 4376500 4377000 4377500 4378000 4378500 4379000 4379500

Latitude (UTM)

Bo

ug

ue

r A

no

ma

ly (

mG

al)

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)

0419.

gh

Fernley 1 Fernley 1 Anomaly modeling tol=0.5 mGal

-3

-2.5

-2

-1.5

-1

-0.5

0

4381 4381.5 4382 4382.5 4383

UTM Y, km

Gra

vity

, m

Gal

Ganom

Gcalc

Blue lines indicate regional stresses. Red lines indicate shearing within the flower structure

Fernley Line 1

Fernley 1 Depth and Bouguer Slab Depth

0

0.2

0.4

0.6

4381 4381.2 4381.4 4381.6 4381.8 4382 4382.2 4382.4 4382.6 4382.8 4383

UTM Y, km

Mo

de

led

De

pth

, km

Depth

Mag Anomaly

Bouguer Slab Depth

Fernley 2

Fernley 2 anomaly modeling tol= .5 mGal

-4

-3.5

-3

-2.5-2

-1.5

-1

-0.5

0

4379.5 4380 4380.5 4381 4381.5 4382 4382.5 4383 4383.5

Ganom

Gcalc

Fernley 2

Fernley 2 Modeled Depth vs. Bouguer Depth

00.10.20.30.40.50.60.70.8

4379 4380 4381 4382 4383 4384

UTM Y, km

dept

h, k

m Depth

Bouguer Depth

Mag Anomaly

Hazen

Hazen Modeled Anomaly tol=.75 mGal

-6

-5

-4

-3

-2

-1

0

4375 4376 4377 4378 4379

UTM Y, km

Mod

eled

Gra

vity

, mG

al

Gcalc

Ganom

Hazen

Hazen Modeled Depth and Bouguer Slab Depth

0

0.2

0.4

0.6

0.8

1

4375 4376 4377 4378 4379

UTM Y, km

Mo

de

led

De

pth

, km

Depth

Bouguer Depth

The End . . . Questions?