Unit 3-Gravimetry gravity how differ.pptx

Gravimetry is about gravity about earth realeated to geodesy .this lecture tells about time how gravity is different on all part of world

1. Unit 3. Gravimetry

2. Introduction - Gravimetry is a part of geodesy that deals with determination of strength of gravitational field, the effects of gravity at different points on the earth’s surface. - Gravimetry in general sense is the measurement and analysis of gravity.

3. Gravity -That force which tends to pull bodies towards the earth i.e. to give bodies weight. -Gravity is an important and useful tool for studying the mass (density) distribution in the Earth’s interior . -Measurement of gravity provides useful information to derive elevation Earth’s gravity acts downward toward the center of the Earth.

4. Gravity -Gravity plays a major role in determining Mean Sea Level . -Knowing how gravity changes in sea level helps geodesists make more accurate measurements. -In general, in areas of the planet where gravitational forces are stronger, the mean sea level will be higher. -In areas where the Earth's gravitational forces are weaker, the mean sea level will be lower.

5. - Newton, in 1665 discovered the existence of a force between two masses. - The force between masses is called gravitational force. - The gravitational force is always attractive. - That is each body (or mass) attracts towards it the other body (or the other mass). This force exists universally among all bodies . Gravitational Force

6. Newton Gravitational law Newton's law of universal gravitation states that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Where  F  is the force,  m 1  and  m 2  are the masses of the objects interacting,  r  is the distance between the centers of the masses and  G is the gravitational constant.

7. Contd … Mathematically, in which G is a Universal Gravitational Constant. It is also Newton constant . It has a value of G is: kg

8. The unit of gr avity is : Meter per Second 2 ( m/sec 2 )

9. Contd … We know the Newton’s second law F = m * a, assume m is 1 kg and mass of earth be M and r the radius, then the acceleration due to gravity also known as gravitational attraction is given by : Fig : Gravity

10. Gravity Measurement ( Gravimetry ) -To measure the Earth's gravity field, geodesists use instruments in space and on land. -In space, satellites gather data on gravitational changes as they pass over points on the Earth's surface . -On land, devices called gravimeters measure the Earth's gravitational pull on a suspended mass. -With this data , geodesists can create detailed maps of gravitational fields and adjust elevations on existing maps. -Gravity principally affects the vertical datum because it changes the elevation of the land surface

11. Surface gravity measurement: Gravimeter An instrument used to measure gravity on Earth surface is known as a gravimeter. Gravimeters can be divided as following on the basis of the principle used : Absolute Gravimeters : measures the acceleration of small masses falling freely in a vacuum. Example A10 gravimeter, FG5 etc Provides the absolute gravity value

12. Relative Gravimeters : spring based gravimeters carrying fixed mass , measuring the difference in gravity value between two locations . Example CG5 Gravimeter. The principle is that the changes in gravity will result in a change in weight of fixed mass with change of location. Provides relative gravity value.

13. A bsolute G ravity Vs Relative Gravity absolute gravity Relative gravity Absolute Gravimeters measures absolute Gravity. Rel ative gravimeter measures relative gravity. Absolute Gr avity value is determined from the fund amental acceleration quantity i.e. Length and Time. relative gravity is measured from either one of the acceleration quantity i.e. Length or time keeping other qu antity const ant. The methods like free f all, rise and fall and pendulum method are used to determine absolute gravity. Pendulum or spring is used to determine rel ative gravity. Formul a is : T= 2 π = T 1 2 / T 2 2 absolute gravity Relative gravity Absolute Gravimeters measures absolute Gravity. Rel ative gravimeter measures relative gravity. Absolute Gr avity value is determined from the fund amental acceleration quantity i.e. Length and Time. relative gravity is measured from either one of the acceleration quantity i.e. Length or time keeping other qu antity const ant. The methods like free f all, rise and fall and pendulum method are used to determine absolute gravity. Pendulum or spring is used to determine rel ative gravity.

14. Airborne Gravity Measurement -Airborne gravity measurement is the measurement of gravity from aerial perspective -Gravimeters are placed in the aircraft and the aircraft is flown over the project area -Problem with vibration/drift etc is controlled by compensators -Very effective for gravity survey of large area

15. Space borne gravity survey

16. Basic Terms in gravimetry Level Surface -Curved surface at which each point is perpendicular to the direction of gravity at the point. -Surface which is parallel to mean spheroidal surface of the earth Plumb Line -Line connecting the point to the centre of the earth Geoid -Geoid is one of the equipotential surface that best approximates the Mean Sea Level Mean Sea Level (MSL ) -Mean Seal level or simply sea level is an average level of the surface of one or more of Earth's bodies of water from which heights such as elevation may be measured. MSL is represented as the vertical datum.

17. Basic Terms in gravimetry

18. Basic Terms in gravimetry

19. Basic Terms in gravimetry

20. Application areas of Gravity -Shape of the Earth (Geodesy) Determination of the Geoid (represents the mean sea level of the oceans and the extrapolation of this surface under the continents) -Tectonics & Fault locations -Influence of atmosphere, cryosphere & hydrosphere on the Earth - Water level (particularly, mean sea level) studies -Structure of the Earth’s interior -Geology & mineral resource exploration -Satellite and deep space orbit determination

21. THANK YOU