Gravity Method in Geology

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Questions and Answers

What is the mean value of gravity at the Earth's surface?

9.6 m s-2

10.0 m s-2

9.8 m s-2 (correct)

9.81 m s-2

What is the unit of gravity referred to as the gravity unit (gu)?

1 gu = 1 mgal

1 gu = 1 μms-2

1 gu = 100 μms-2

1 gu = 10 μms-2 (correct)

Which of the following accurately describes the accuracy obtainable during gravity surveys on land?

±1 gu

±100 gu

±0.1 gu (correct)

±0.01 gu

How does latitude affect the value of gravity on Earth?

Gravity changes according to latitude position Signup and view all the answers

What is the mathematical expression for Normal Gravity (gn) in relation to latitude (Ф)?

gn(Ф) = 978.031846 (1 + 0.005278895 sin^2(Ф) + 0.000023462 sin^4(Ф)) Signup and view all the answers

What is the difference between the equatorial radius and the polar radius of the Earth?

21.385 km Signup and view all the answers

Which factor contributes to the variation of gravity across the Earth's surface?

Composition of the Earth's crust Signup and view all the answers

What is the main principle behind gravity prospecting?

It uses variations in the Earth's gravitational field due to density differences. Signup and view all the answers

Which of the following is NOT an application of the gravity method?

Astrobiology research Signup and view all the answers

What characteristic is associated with micro-gravity surveys?

They often detect anomalies such as buried cavities. Signup and view all the answers

According to Newton's Law of Gravitation, what happens to the gravitational force between two masses if the distance between them is doubled?

The gravitational force becomes four times weaker. Signup and view all the answers

What is a gravity anomaly?

A localized perturbation in the gravitational field due to subsurface mass differences. Signup and view all the answers

Which factor does NOT influence the gravitational field measured in gravity surveys?

The temperature of the surrounding environment Signup and view all the answers

Which gravitational constant value is used in the formula representing Newton's Law of Gravitation?

$6.673 × 10^{-11}$ (m³.kg-1.sec-2) Signup and view all the answers

Which geological feature is generally associated with small negative gravity anomalies?

Salt domes Signup and view all the answers

Signup and view all the answers

Study Notes

Gravity Method

Gravity prospecting uses Earth's passive potential fields.
Subsurface geology is mapped based on density variations in subsurface rocks.
A causative subsurface mass creates a localized gravity anomaly.
Small-scale anomalies indicate buried landforms like valleys.
Larger-scale anomalies are linked to salt domes, granite plutons, or sedimentary basins.
Micro-gravity surveys pinpoint very small features like cavities.

Gravity Method Applications

Shape of the Earth is determined.
Hydrocarbon exploration is enhanced.
Regional geology is studied.
Isostatic compensation is determined.
Subsurface cavities (microgravity) are detected.
Buried valleys are localized.
Glacier thickness is measured.
Tidal oscillations are studied.
Basin geometry is analyzed.

Basic Theory

Newton's Law of Gravitation is fundamental.
The gravitational force between two masses is inversely proportional to the square of the distance between their centers.
The universal gravitational constant (G) is a key factor.
G's experimentally determined value dictates the strength of the attraction.

Units of Gravity

The average surface gravity is approximately 9.8 m/s².
Variations due to density differences are around 100 μms⁻².
Gravity units (gu) are used in surveys.
High accuracy (±0.1 gu) is achievable on land.
Sea surveys have lower accuracy (±10 gu).
The milligal (mgal) is the c.g.s. unit; 1 mgal = 10⁻³gal = 10⁻³cm/s².

Basic Theory (Additional Factors)

Earth's gravitational field changes across space.
Numerous factors impact regional gravity.
Measured gravity represents all components' vector sum at the observation point.
Earth's non-uniformity stems from shape, rotation, and composition.

Gravity Variation

Earth's rotation and flattening cause global gravity variations.
The equatorial radius is greater than the polar radius.
This results in a difference in these two values.
There are other types of local variations too.

Normal Gravity

Normal gravity (gn) changes based on latitude (°).
It's a mathematical function expressing both of the Earth's flattening and rotation effects.
The function displays a curve when plotted against latitude.

The Earth Gravity Variation

Global and local gravity variations are categorized.
Specific materials have density ranges and approximate average densities.
Space and geographic features are influential.
Time-varying factors, like tidal effects and instruments, impact gravity measurements.

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Description

Explore the intricacies of gravity prospecting and its applications in understanding subsurface geology. This quiz covers concepts such as gravity anomalies, Newton's Law of Gravitation, and practical uses in hydrocarbon exploration and regional geology studies. Test your knowledge on how gravity helps map the Earth's hidden features.