Gravity Methods 2

1. Ali Oncel [email_address] Department of Earth Sciences KFUPM Gravity Methods 2 Introduction to Geophysics Introduction to Geophysics-KFUPM

3. Recall: 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 =90)=983,217.72 mGal Introduction to Geophysics-KFUPM

5. Δ g fa = g – g t + FAC g = gravitational acceleration observed at station g t = theoretical gravity Introduction to Geophysics-KFUPM Δ g fa = free gravity anomaly

9. Introduction to Geophysics-KFUPM The extra mass of mountains results in higher gravity relative to the near sea level. To account for the excess mass above a sea level datum , The BC assumes an infinite slab of density ( ρ ), with thickness (h) equal to the station’s elevation. Bouguer Correction BC= 0.419 ρ h

10. To determine the Bouguer correction, the density of the infinite slab ( ρ ) is needed to assumed as a reduction density of 2.67 g/cm , which is equal to typical density of granite. Bouguer Gravity Anomaly on Land Δ g B = Δ g fa -BC The simple Bouguer gravity anomaly results from subtracting the effect of the infinite slab (BC) from the free air gravity anomaly: BC= 0.419 ρ h =0.419 (2.67g/cm 3 ) h =(0.112 mGal/m)*h (m) Δ g B = Δ g fa -(0.112 mGal/m) h Introduction to Geophysics-KFUPM

11. Bouguer Gravity Anomaly on Sea Δ g B = Δ g fa - 0.419 ρ h Station elevations (h) are zero, Then, Δ g B = Δ g fa Thus, if a type of Bouguer correction is applied, then equation (see page 233 of Lillie) is: Δ g B = Δ g fa + (0.0687 mGal/m)h w Introduction to Geophysics-KFUPM

12. Bouguer Gravity Anomaly on Sea Introduction to Geophysics-KFUPM Δ g fa = g – g t + FAC Δ g B = Δ g fa - BC BC = 0.419 ρ h FAC = h x (0.308 mGal/m)