Lecture 3 - Ocean bottom gravity surveys

Question 1

What is measured in gravity surveys?

Answer 1

Relative Gravity

Question 2

What graph is produced?

Answer 2

Near-surface density gravity anomaly

Question 3

What are Gals in ms^-2

Answer 3

10^-2 ms^-2

Question 4

What are gus?

Answer 4

10^-6 ms^-2

Question 5

What is the precision of a gravimeter?

Answer 5

0.1 gu

Question 6

What must you record when doing a gravity survey?

Answer 6

1. Relative Gravity
2. Time
3. Elevation
4. Latitude

Question 7

What corrections must be done?

Answer 7

1. Drift
2. Latitude
3. Free air
4. Bouger
5. Terrain

Question 8

What is the drift correction equal to?

Answer 8

Δg = gs - gb

measured g at survey point - g at base station at same time

Question 9

What is occurring at the Sleipner site?

Answer 9

Extracting natural gas from Ty Reservoir

Injecting CO2 into Utira Sands at 1mn tonnes/year

Question 10

Why is high precision needed of gravity and ocean bottom gravity surveys?

Answer 10

As changes in gravity anomaly is relatively small

Question 11

Why are 3 gravimeters placed on the ocean bottom at a time?

Answer 11

1. Improve precision

2. Tell if 1 is not working

Question 12

What must you account for when calculating Δg between two gravimeters at different elevations?

Answer 12

1. Effect of more sediment below 1 (leads to increase in g)
2. Effect of less water above 1 (leads to increase in g)
3. Effect of greater elevation of 1 (leads to decrease in g)

Question 13

What do we need to invert parameter dg/dz ?

Answer 13

Because density of sea-bottom sediments not known so don’t know precisely how g changes with elevation

Question 14

Does g increase or decrease with CO2 injection?

Answer 14

Decrease as displaces brine of higher density

Question 15

How do you account for wave/swell affects when doing a sea-bottom gravity survey?

Answer 15

Apply low pass filter

Question 16

How do you establish water depth of gravimeter?

Answer 16

Instruments record water pressure and P = ρgh

Question 17

What is the ‘CO2 injection response’?

Answer 17

The gravity change as a result of CO2 injection for CO2 of different densities

Calculate from ΔM = Vco2(ρw - ρco2)

Question 18

In the Sleipner Case Study, what are the observed gravity changes due to?

Answer 18

1. Elevation Changes
2. Changes in producing reservoir
3. CO2 injections

Question 19

What ‘unknowns’ are present following the gravity survey?

How do you constrain these?

Answer 19

1. Gravity gradient dg/dz
2. Reservoir Response Rgty
3. CO2 injection response gco2
4. Absolute gravity

Constrain by inverting. Minimise the objective function linearly