Gravity Methods 1

1. Ali Oncel [email_address] Department of Earth Sciences KFUPM Gravity Methods 1 Introduction to Geophysics Introduction to Geophysics-KFUPM

2. Field Trip: Refraction Seismology Introduction to Geophysics-KFUPM

3. Field Trip: Refraction Seismology Introduction to Geophysics-KFUPM

4. Field Trip: Refraction Seismology Introduction to Geophysics-KFUPM

5. Field Trip: Refraction Seismology Introduction to Geophysics-KFUPM

6. Field Application by Gravimeter Introduction to Geophysics-KFUPM This Wednesday, Mid Term Next Wednesday, May 10

9. Differences in Earth’s Potential Fields Earth’s gravity field is simply compared to the magnetic field . Lines of the force for the gravity field are directed toward the center of the Earth while magnetic field strength and direction depend on the Earth’s positive and negative poles. Introduction to Geophysics-KFUPM

11. The variation of g across the earth’s surface provides information about the distribution of density contrasts in the subsurface since m =  V (i.e. density x volume). m s spring mass, m E mass of the earth = 5.967 x 1024 kg R E radius of the earth Like apparent conductivity and resistivity , the acceleration of gravity (g) is a basic physical property we measure, and from which, we infer the distribution of subsurface density contrast. Earth’s Gravitational Acceleration For perfectly spherical and non-rotating Earth with uniform mass distribution, g E represents the acceleration of gravity at a particular point on the earth’s surface: Introduction to Geophysics-KFUPM R E m S m E

13. The main factors responsible for the difference in gravitational acceleration at the equator compared to the poles. Gravity increases from 978,000 mGal at the equator, to about 983,000 mGal at the poles. Variation in gravity from equator to pole , according to 1967 Reference Gravity formula. Introduction to Geophysics-KFUPM

14. Theoretical Gravity The average value of gravity for a given latitude is approximated by the International Association of Geodesy in 1967. g n = Normal Gravity: Gravitational acceleration expected for a rotating ellipsoidal earth without any geologic complications and no surface features . Ф =latitude Gravitation increase from the equator ( Ф =0) =978,03185 mGal to the pole ( Ф =0)=983,217.72 mGal Introduction to Geophysics-KFUPM

15. dg/dR≈-0.308 mGal/m dg/dR ≈ average value for the change in gravity with increasing elevation Introduction to Geophysics-KFUPM

16. Free Air Gravity Correction Rising upward from Earth’s surface, gravitational acceleration decreases by about 0.308 mGal for every meter of height. A gravity station at high elevation tends to have a lower gravitational acceleration (g) than a station at lower elevation Introduction to Geophysics-KFUPM

19. Free Air Gravity Anomaly Δ g fa = g – g t + FAC Δ g fa = free gravity anomaly g = gravitational acceleration observed at station g t = theoretical gravity Introduction to Geophysics-KFUPM

22. Bouguer Correction Introduction to Geophysics-KFUPM