Lecture 3 - Ocean bottom gravity surveys Flashcards
What is measured in gravity surveys?
Relative Gravity
What graph is produced?
Near-surface density gravity anomaly
What are Gals in ms^-2
10^-2 ms^-2
What are gus?
10^-6 ms^-2
What is the precision of a gravimeter?
0.1 gu
What must you record when doing a gravity survey?
- Relative Gravity
- Time
- Elevation
- Latitude
What corrections must be done?
- Drift
- Latitude
- Free air
- Bouger
- Terrain
What is the drift correction equal to?
Δg = gs - gb
measured g at survey point - g at base station at same time
What is occurring at the Sleipner site?
Extracting natural gas from Ty Reservoir
Injecting CO2 into Utira Sands at 1mn tonnes/year
Why is high precision needed of gravity and ocean bottom gravity surveys?
As changes in gravity anomaly is relatively small
Why are 3 gravimeters placed on the ocean bottom at a time?
- Improve precision
2. Tell if 1 is not working
What must you account for when calculating Δg between two gravimeters at different elevations?
- Effect of more sediment below 1 (leads to increase in g)
- Effect of less water above 1 (leads to increase in g)
- Effect of greater elevation of 1 (leads to decrease in g)
What do we need to invert parameter dg/dz ?
Because density of sea-bottom sediments not known so don’t know precisely how g changes with elevation
Does g increase or decrease with CO2 injection?
Decrease as displaces brine of higher density
How do you account for wave/swell affects when doing a sea-bottom gravity survey?
Apply low pass filter
