1. Graphical Separation of Residual
Examine the map at right. Note the regional and residual (or local) variations in the gravity field through the area.
The graphical separation method involves drawing lines through the data that follow the regional trend.
The green lines at right extend through the residual feature and reveal what would be the gradual drop in the anomaly across the area if the local feature were not present.

2. The residual anomaly is identified by marking the intersections of the extended regional field with the actual anomaly and labeling them with the value of the actual anomaly relative to the extended regional field. -1
-0.5
-0.5
After labeling all intersections with the relative (or residual ) values, you can contour these values to obtain a map of the residual feature.

3. Zero
Max = 0
Min ~-1.8
negative
Vertical Cylinder or Sphere?
6000’

4. Problem-
Determine which anomaly is produced by a sphere and which is produced by a horizontal cylinder.

5. Which estimate of Z seems to be more reliable? Compute the range.
You could also compare standard deviations.
Which model - sphere or cylinder - yields the smaller range or standard deviation?

6. Second Gravity Problem Set
We will spend more time on simple geometrical objects during the next lecture, but for now let’s spend a few moments and review the problems that were assigned last lecture.
Pb. 4 What is the radius of the smallest equidimensional void (such as a chamber in a cave - think of it more simply as an isolated spherical void) that can be detected by a gravity survey for which the Bouguer gravity values have an accuracy of 0.05 mG? Assume the voids are in limestone and are air-filled (i.e. density contrast = 2.7gm/cm3) and that void centers are never closer to the surface than 100.

7. Begin by recalling the list of formula we developed for the sphere.

8. Pb. 5: The curve in the following diagram represents a traverse across the center of a roughly equidimensional ore body. The anomaly due to the ore body is obscured by a strong regional anomaly. Remove the regional anomaly and then evaluate the anomaly due to the ore body (i.e. estimate it’s deptj and approximate radius) given that the object has a relative density contrast of 0.75g/cm3

9. You could plot the data on a sheet of graph paper
You could plot the data on a sheet of graph paper. Draw a line through the end points (regional trend) and measure the difference between the actual observation and the regional (the residual).
You could use EXCEL or PSIPlot to fit a line to the two end points and compute the difference between the fitted line (regional) and the observations.
residual
Regional

10. In problem 6 your given three anomalies
In problem 6 your given three anomalies. These anomalies are assumed to be associated with three buried spheres. Determine their depths using the diagnostic positions and depth index multipliers we discussed in class today. Carefully consider where the anomaly drops to one-half of its maximum value. Assume a minimum value of 0. A. B. C.

11. Due Dates Nov. 16th Terrain correction Nov. 18th Gravity lab
Problems 4, 5, & 6.
Gravity paper summaries
Extra credit sphere or cylinder exercise
Due Dates

12. ?? Grav Lab