Objective:  Calculate “residual” gravity values for the data provided and then plot a gravity profile. 

Procedure:

A Banded Iron Formation (BIF) at depth is the target of a an exploratory vertical drill hole.  The BIF lies below volcanic rocks and a nonconformity.  A gravity survey was conducted along a north-south line with 10 kilometer spacings to predict the subsurface geometry of the BIF.  The objective is to plot a gravity profile which indicates large scale folds in the BIF.  The BIF is closest to the surface at the crest of an anticlinal fold.  To minimize the drill depth, and hence the cost of drilling, the drill hole should be positioned directly above the crest of the anticline.  Find the crest of the buried anticline and indicate the best drill site on the profile.  The horizontal scale is shown in kilometers.

Calculate the “residual” gravity values for the stations along the gravity survey line.  The residual gravity value for station 0 N is calculated for you.  Then plot the gravity values on the profile using the scale on the left. 

1. Fill in the values for the Latitude Corrections on Answer Sheet, assuming a linear change going northward along the survey line.

2. Fill in the values for the Residual Gravity Values by adding or subtracting the corrections listed for the measurements at each station.

3. Plot a dot on the Gravity Profile representing each Residual Gravity Value.  Plot each dot directly above the appropriate survey station and using the scale on the right.

4. Draw a smooth curve connecting the dots to show the gravity profile along the survey line.

5. The gravity profile reflects large scale folds in the BIF beneath the nonconformity. In the subsurface below the nonconformity, sketch a large anticline with the following characteristics:

Station	Observed Gravity Value (Mgals)	Latitude Correction (Mgals)	Free-Air and Bouguer Correction (Mgals)	Terrain Correction (Mgals)	Residual Gravity Value (Mgals)
0 N	5.7	(-)   0.1	(+)   0.15	(+)    0.25	6.0
10 N	5.49	(-)   0.15	(+)   0.16	(+)    0.5
20 N	5.05	(-)   0.2	(+)   0.45	(+)    0.7
30 N	5.2	(-)   0.25	(+)   0.2	(+)    0.6
40 N	5.92		(+)   0.18	(+)    0.2
50 N	6.1		0.0	0.0
60 N	5.9		0.0	0.0
70 N	5.47		(-)    0.12	(-)    0.15
80 N	6.64		(-)    0.14	(-)     0.5
90 N	7.73		(-)    0.13	(-)     0.55
100 N	7.95		(-)    0.15	(-)    0.2
110 N	6.9		0.0	0.0
120 N	6.32		(-)    0.12	0.0

Download a PDF version of the profile plot.