Gravity Surveying

Question 1

What is a gravity anomaly?

Answer 1

A localised perturbation in gravitational field, often caused by bodies of anomalous mass

Question 2

What is the geoid?

Answer 2

The shape the global ocean surface would take under the influence of Earth’s gravity in the absence of other influences

Question 3

What is a gravity unit composed of?

Answer 3

100 micrometers per second squared

Question 4

What are gravimeters and how do they work?

Answer 4

Spring balances carrying a constant mass, which experiences variation in weight based on variations in gravity, changing the length of the spring as a measure of change in gravity

Question 5

How are horizontal and vertical accelerations produced at sea eliminated or controlled?

Answer 5

Horizontal accelerations are eliminated by mounting the meter on a gyrostabilised horizontal platform
Vertical accelerations are addressed by damping suspension and averaging readings over an interval longer than wave time

Question 6

What is cross-coupling?

Answer 6

The effect of circular phase relationships between vertical and horizontal motion components of a ship causing beam displacements that do not average out over time

Question 7

How is drift monitored when surveying far from an IGSN station?

Answer 7

A series of alternate readings are recorded at 2 stations are averaged to find the measure of drift-corrected gravity difference

Question 8

What readings are recorded at every survey station?

Answer 8

Location, time, elevation or water depth, and gravimeter readings

Question 9

What are the drift and latitude corrections?

Answer 9

The drift correction uses repeated readings at a base station throughout the day to account for variations in gravimeter reading, and the latitude correction uses latitude to account for gravity difference between the Equator and the Poles

Question 10

How does gravity differ at the Poles from the Equator?

Answer 10

Gravity is higher at the Poles as points at the Equator are farther from the centre of the mass of the Earth due to Earth’s oblate spheroid shape

Question 11

What are the 3 elevation corrections and how do they work?

Answer 11

The free air correction corrects for decrease in gravity with height in free air due to distance from the Earth’s centre, the Bouguer correction removes the effect of rocks present between observation and datum points, and the terrain correction accounts for topographic relief

Question 12

What is the tidal correction and how does it work?

Answer 12

It accounts for periodic variation in the gravitational effects of the sun and the moon for gravimeters with low drift rates that use few base measurements

Question 13

What is the Eotvos correction and how does it work?

Answer 13

It removes the effect of centripetal acceleration that reinforces or opposes gravity based on direction of travel

Question 14

What are the Free Air and Bouguer Anomalies?

Answer 14

Free Air = observed gravity - latitude correction + free air correction (±eotvos correction)
Free Air Anomaly ± Bouguer correction + terrain correction

Question 15

What controls density of sedimentary, igneous, and metamorphic rocks?

Answer 15

Sedimentary rock density is mainly controlled by porosity and increases with depth (due to compaction) and age (due to cementation)
Igneous and metamorphic rocks are mainly controlled by composition and density increases with acidity

Question 16

What is the inverse problem of potential field interpretation? How is ambiguity decreased?

Answer 16

Although the anomaly of a given body may be calculated uniquely, there are an infinite number of bodies that could give rise to any specified anomaly
All external constraints on the nature and form of an anomalous body are used to decrease ambiguity

Question 17

What information does direct and indirect interpretation provide?

Answer 17

Direct interpretation provides information on the anomalous body that is largely independent of true body shape
Indirect interpretation simulates the causative body to closely match the observed anomaly

Question 18

How do anomalies change with volume, density, and depth

Answer 18

Anomalies increase in magnitude with volume and density and decrease with depth

Question 19

How do the half-width, gradient-amplitude, and second derivative methods of limit depth differ?

Answer 19

The half-width method uses the horizontal distance from the anomaly maximum to the point at which the anomaly has reduced to half of its maximum, the gradient-amplitude method uses maximum anomaly magnitude and maximum horizontal gravity gradient, and the second derivate method uses maximum rate of change in gradient

Question 20

What is excess mass?

Answer 20

The difference in mass between the body and the mass of the country rock that would otherwise fill its space

Question 21

What is inflection point and where is it in structures with outward and inward dipping contacts

Answer 21

The position where the horizontal gradient changes most rapidly
Outward dipping contact: near the base
Inward dipping contact: near the uppermost edge

Question 22

How are 2D, 3D, and irregular anomalies modelled in indirect interpretation?

Answer 22

2D anomalies are computed for horizontal or half cylinders
3D anomalies are computed for spheres, vertical cylinders, or right rectangular prisms
Irregular anomalies are computed for a series of polygonal horizontal slices

Question 23

What are the 4 steps for indirect interpretation?

Answer 23

Construction of a reasonable model, computation of gravity anomaly, comparison between computed and observed anomalies, and alteration of model

Question 24

What are the main applications of gravity surveying?

Answer 24

Surveying large and medium scale geological structures (island arcs, ocean trenches), delineating major surface features, and locating petroleum and potential aquifers

Question 25

What are the steps for gravity anomaly correction and interpretation?

Answer 25

1. Drift reduction
2. Latitude, elevation (free air, bouguer, terrain), tidal, and eotvos corrections
3. Free air and Bouguer anomaly calculation
4. Limiting depth, excess mass, inflection point, and approximate thickness calculation
5. Indirect interpretation

Question 26

What are Airy and Pratt Isostasy?

Answer 26

Airy’s theory is that excess mass is balanced out in the mantle with a less dense root
Pratt’s theory is that excess mass is supported by density changes in the crust