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1. Physical Geodesy

2. What is gravity? Gravitation is the force of attraction between two bodies, such as the Earth and our body. The strength of this attraction depends on the mass of the two bodies and the distance between them. A mass falls to the ground with increasing velocity, and the rate of increase is called gravitational acceleration, g, or gravity. Gravity is not the same everywhere on Earth, but changes with many known and measurable factors, such as tidal forces. Gravity surveys exploit the very small changes in gravity from place to place that are caused by changes in subsurface rock density. Higher gravity values are found over rocks that are more dense, and lower gravity values are found over rocks that are less dense.

3. GRAVITY keeps the moon orbiting Earth . . . and Dactyl orbiting Ida . . . It holds stars together . . . Prevents planets from losing their atmospheres . . . And binds galaxies together for billions of years . . .

4. How do scientists measure gravity? Scientists measure the gravitational acceleration, g, using one of two kinds of gravity meters. An absolute gravimeter measures the actual value of g by measuring the speed of a falling mass using a laser beam. A second type of gravity meter measures relative changes in g between two locations. This instrument uses a mass on the end of a spring that stretches where g is stronger. A relative gravity measurement is also made at the nearest absolute gravity station, one of a network of worldwide gravity base stations.

5. What is a gravity anomaly? Gravity meters measure all effects that make up the Earth’s gravity field. Many of these effects are caused by known sources, such as the Earth’s rotation, distance from the Earth’s center, topographic relief, and tidal variation. Gravity caused by these sources can be calculated using realistic Earth models and removed from the measured data, leaving gravity anomalies caused by unknown sources. To the geologist, the most important unknown source is the effect of the irregular underground distribution of rocks having different densities. A sequence of gravity corrections are applied to the original gravity reading and result in various named gravity anomalies.

6. .. con’t The observed gravity anomaly has been corrected for Earth rotation, latitude, tidal effects, and gravity meter fluctuations. The free air gravity anomaly has been corrected for the gravity effect caused by the elevation difference between the station and sea level (a correction for distance) and is a standard for oceanic gravity interpretation. The Bouguer (pronounced Boo-gay ́) gravity anomaly has been further corrected for the mass that may exist between sea level and the observer (a correction for mass) and is a standard used in geologic interpretation on land. The isostatic (pronounced iso -stat ́- ic ) gravity anomaly is calculated by subtracting the gravitational effect of low density mountain roots below areas of high topography.

7. What is a gravity map? A gravity map is made using numerous gravity measurements across the area of interest. Gravity surveying by aircraft is still a new science, so most gravity measurements are made on the ground at discrete stations. Gravity measurements are often processed to a complete- Bouguer or isostatic gravity anomaly. These data are then gridded, so that the randomly spaced data are converted to a representation of the gravity field at equally spaced locations. The distance chosen between grid points depends on the average distance between gravity stations. Gravity anomaly maps can be shown as color figures with warm colors (reds and oranges) showing areas of higher gravity values and cool colors (blues and greens) showing lower values or as contour line maps, where each contour line follows a constant gravity value.

8. How is gravity measures: Falling objects Pendulum Mass on a spring

9. Falling objects: The distance a body falls is proportional to the time it has fallen squared. The proportionality constant is the gravitational acceleration, g: g = distance / time 2 . To measure changes in the gravitational acceleration down to 1 part in 40 million using an instrument of reasonable size, we need to be able to measure changes in distance down to 1 part in 10 million and changes in time down to 1 part in 10 thousands!! As you can imagine, it is difficult to make measurements with this level of accuracy.

10. Pendulum measurements: The period of oscillation of the pendulum, T, is proportional to one over the square root of the gravitational acceleration, g. The constant of proportionality, k, depends on the pendulum length: Here too, in order to measure the acceleration to 1 part in 50 million requires a very accurate estimate of the instrument constant k, but k cannot be determined accurately enough to do this.

11. But all is not lost: We could measure the period of oscillation of a given pendulum by dividing the time of many oscillations by the total number of oscillations. By repeating this measurement at two different locations, we can estimate the variation in gravitational acceleration without knowing k.

12. Mass on a spring measurements: The most common type of gravimeter used in exploration surveys is based on a simple mass-spring system. According to Hook’s law: X = mg / k , with k being the spring stiffness.

13. Like pendulum the measurements, we can not determine k accurately enough to estimate the absolute value of the gravitational acceleration to 1 part in 40 million. We can, however, estimate variations in the gravitational acceleration from place to place to within this precision. Under optimal conditions, modern gravimeters are capable of measuring changes in the Earth's gravitational acceleration down to 1 part in 1000 million.

14. Reading Assignment Various undesired factors affect the measurements: Temporal (time-dependent) variations : Instrumental drift Tidal effects Spatial variations : Latitude variations Altitude variations Slab effects Topography effect

15. Quiz 5% Define the following: Gravity List and Explain Gravity Meter Gravity anomaly Geophysics

16. Assignment 10% Explain about CHAMP (Challenging Minisatellite Payload ) and GRACE(Gravity Recovery and Climate Experiment ) gravity gradiometer