Geol 454 Environmental and Exploration Geophysics I Gravity Problem Set 3

Will have opportunities to discuss Thursday and next Tuesday Due date – Nov. 17th)

3. What is the radius of the smallest equidimensional void (e.g. chamber in a

cave) that can be detected by a gravity survey for which the Bouguer gravity values have an accuracy of 0.05 mGals? Assume the voids are formed in limestone (density 2.7 gm/cm3) and that void centers are never closer to the surface than 100m. (Problem 6.5 from Burger et al.)

4. The curve in the following diagram represents a traverse across the center of a roughly equidimensional (i.e. roughly spherically shaped) 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 its depth and approximate radius), given that the object has a relative density contrast of 0.75 g/cm3 with surrounding strata. Problem 6.8, Burger et al.).

Horizontal Position (km)0.0 0.5 1.0 1.5 2.0

Bou

guer

Ano

mal

y (m

Gal

)

-1.50

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

Horizontal Position Bouguer Anomaly (mGal)

0.1 -1.45 0.2 -1.36 0.3 -1.27 0.4 -1.17 0.5 -1.07 0.6 -0.96 0.7 -0.85 0.8 -0.74 0.9 -0.64 1 -0.56

1.1 -0.49 1.2 -0.44 1.3 -0.4 1.4 -0.36 1.5 -0.32 1.6 -0.27 1.7 -0.22 1.8 -0.16 1.9 -0.1 2 -0.03

5. Determine the depth to the center of each of the three equidimensional bodies (~spheres) producing the anomalies (A, B, and C) shown in the figure below. (Similar to problem 6.9 from Burger et al.).

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

-1500 -1000 -500 0 500 1000 1500

Distance from peak (m)

Bou

guer

Ano

mal

y (m

Gal

s)

B.

C.

A.