Subsidence - Investigations in Environmental Geology - page 150

7. Refer to Figure 9.7 to determine the surface area of the
large northeastern subsidence feature in 1970, when sulfur
extraction ended, and record answer here.

QUESTIONS (9, PART C)
1. What is the diameter and surface area of Orchard Dome?

2. a. At what depth is:

8. By 1979, the NE subsidence feature had extended to the
SE beyond the area seen in Figure 9.6. It reached across a
road almost to one of the small open circles (marked "A" in
Figure 9.6) at the end of an unimproved road.
a. According to data in Figure 9.7, by how many acres had
the flooded subsidence area increased between 1970 and
1979?

the top of the salt?

the top of the cap rock?

b. According to spot elevations and contours in Figure 9.6,
what is the approximate elevation of the land surface
above the dome (i.e., above sea level)?

b. What do the small open circles on the map (Figure 9.6)
represent?

3. a. In what unit or part of Orchard Dome is the sulfur that
was extracted?

c. Explain, with the aid of your salt dome cross section in
Question 4, why these circles occur outside the salt dome.
(Hint: Related to the mechanism of salt dome formation.)

b. Describe how the sulfur was mined.

9. What was the average rate of increase in subsidence area
between 1941 and 1979 for this major subsidence trough?

4. From the description of the Orchard Salt Dome and infor-
mation in Figure 9.6, make a simple concept sketch of the salt
dome in cross section showing the shape of the dome, the
position of the cap rock above it, and the overlying rock and
sediment. Label the rock units. Also mark and label the
approximate elevation of the landscape, sea level, top of cap
rock, and top of salt. Structural contours are shown and
explained in Figure 9.6. (Your instructor may provide part of
this concept sketch.)

10. A reconnaissance flight over the dome in 1985 docu-
mented an increase in the number of sinkholes from 16 to 22
in the central dome area since 1979. Investigators were not
able to accurately determine if there were any increases in
the NE subsidence trough in 1985.
a. What is the latitude and longitude marked near the NE
edge of the dome in Figure 9.6?

b. Describe how you might go about determining the cur-
rent number and distribution of sinkholes and troughs in
Orchard Dome today? (Hint: The answer to Question 10.a
should help.)

PART D. SUBSIDENCE ABOVE COAL MINES,
ILLINOIS AND COLORADO

5. How far below sea level is the lowest structural contour
for the dome?

Coal-fired power plants provide the largest share of
electric energy in the United States. Of major concern
is their impact on air quality (sulfur and nitrogen
oxides, mercury, and C0 2 , a greenhouse gas). Carbon
capture with sequestration is proposed to address the
C0 2 problem and this technique will benefit from imple-
mentation of new, clean coal technologies for electric
energy generation. Successful sequestration depends on

6. On Figure 9.6, identify and label the two types of subsi-
dence features that are described in the introduction. The
two types are: