The document discusses gravity modeling and the factors that influence gravity anomalies. It explains that there are two main contributions to observed gravity acceleration: regional gravity anomalies caused by large-scale geologic structures, and local anomalies caused by smaller-scale structures. The size, depth, and density contrast of buried objects determine the magnitude and shape of their gravity signature. Deeper sources produce broader, lower amplitude anomalies while shallow sources create sharper, higher amplitude anomalies.

1. Ali Oncel [email_address] Department of Earth Sciences KFUPM Gravity Modeling 2 Introduction to Geophysics Introduction to Geophysics-KFUPM Highest peaks on the planet

3. Introduction to Geophysics-KFUPM Homework Status Red Sea Arabian Gulf DSF Arabian Shield Gulf of Aden Zagros Suture GO AS MS OM 33 ° E 60 ° E 10 ° N 37 ° N

4. DEM http://www.sgs.org.sa/gis/images/mnt.gif Introduction to Geophysics-KFUPM

5. http://www.sgs.org.sa/gis/images/peninsul.gif after Collenette and Grainger, 1994

6. Gravity Modeling Introduction to Geophysics-KFUPM In the given example, the ore body is spherical in shape and is buried in sedimentary rocks having a uniform density. In addition to the ore body, the sedimentary rocks in which the ore body resides are underlain by a denser Granitic basement that dips to the right. This geologic model and the gravity profile that would be observed over it are shown in the figure below. Source: Notes of Thomas M. Boyd- Colorado School of Mine hump

7. Source: Notes of Thomas M. Boyd- Colorado School of Mine Regional Gravity due to dipping Layer Introduction to Geophysics-KFUPM Decrease

8. Introduction to Geophysics-KFUPM A B Local Anomaly= A - B

10. Notice that the Regional Gravity Anomaly is a slowly varying function of position along the profile line. This feature is a characteristic of all large-scale sources. That is, sources of gravity anomalies large in spatial extent (by large we mean large with respect to the profile length) always produce gravity anomalies that change slowly with position along the gravity profile. Local Gravity Anomalies are defined as those that change value rapidly along the profile line. The sources for these anomalies must be small in spatial extent (like large, small is defined with respect to the length of the gravity profile) and close to the surface. Sources of the Local and Regional Gravity Anomalies Introduction to Geophysics-KFUPM

12. Symmetry Introduction to Geophysics-KFUPM Local Gravity

13. The anomaly is attenuated ( smaller l Δ g z l ) as the sphere is buried more deeply within the Earth The width of gravity anomaly increases as the sphere is buried more deeply. Recall : Gravity anomaly profile ( Δ g z ): Buried Sphere Model Introduction to Geophysics-KFUPM (see more page 248 of Lillie’s book).

14. Depth Dependence of Source Wavelength of Anomaly & Magnitude ) Introduction to Geophysics-KFUPM Why a negative anomaly is observed? Example 1 As an example of the effects of burial depth on the recorded gravity anomaly, consider three cylinders all having the same source dimensions and density contrast with varying depths of burial. For this example, the cylinders are assumed to be less dense than the surrounding rocks.

16. Example 2 • anomaly centered over centre of sphere • anomaly becomes broader (longer wavelength) with increasing depth from source =200 m  c =  1 -  2 = 0.4 g/cm 3 =500 m Introduction to Geophysics-KFUPM

18. Gravity Effect of Sphere Most simple object to model is a sphere of uniform r • assume that mass is essentially a ‘point source’ - as if mass were concentrated in the centre • x is distance from centre of sphere • R is radius between centre of sphere and measurement point • z depth to centre of sphere Introduction to Geophysics-KFUPM

19. As you remember from the previous lecture, we can obtain vertical component of gravitational attraction of excess mass in mGal as: Recall : Gravity Effect of Sphere Introduction to Geophysics-KFUPM See pp. 247 of Lillie

20. Half Width Depth Estimates • The depth to the top of a gravity source can be determined approximately from half-width x ½ of anomaly x ½ is half distance from the centre of anomaly at which amplitude has decreased to half its peak value Z is depth for spherical object z = 1.305x½ z = 1.305x ½ Introduction to Geophysics-KFUPM