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)* ) –For 3200 reading: ((Gravimeter reading-3200)*1.0226)

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 B 4 sectors 6.56’-54.6’ +/- zd gr Zone C 6 sectors 54.6’-175’ +/- zd gr

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 = * elevation –Bouguer Correction = *density (used 2.67) * elevation –Theoretical Gravity = * (1 + ( * sin 2 (latitude)) – ( * sin 2 (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)

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

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

Modeling—Bouguer Slab Depths  Where Δg= The computed Ganom value from grav2d  Δρ= -.35  The value is a constant Bouguer Slab Equation (Telford et. Al.1990)

Fernley 1 Blue lines indicate regional stresses. Red lines indicate shearing within the flower structure

Fernley Line 1

Fernley 2

Hazen

The End... Questions?