Gravity

Data Analysis
DATA ACQUISITION
We chose the ground floor of Madison Hall as our base station, the gravimeter was read at the base station three (3) times to determine the error in the instrument, which was found to be ±0.1 dialunits
Then the gravimeter was read at increasing elevation see table below for gravimeter values obtained at every elevation s/n Time of Reading
Thermometer Reading ((±1 F)
Gravimeter
(±0.1 dialunits)
Elevation
(±0.1 ft)
1
11:35
69
1077.9
0.0
2
11:40
69
1076.6
1.1
3
11:46
69
1076.6
2.3
4
11:57
69
1075.5
3.5
5
12:01
69
1075.1
4.6
6
12:10
69
1073.6
5.8
7
12:15
69
1073.6
6.9
8
12:22
69
1072.3
8.1
9
12:27
69
1071.1
9.2
10
12:36
69
1070.2
10.4
11
12:40
69
1069.6
11.6
Fig 1:
CORRECTING FOR DIURNAL VARIATION
We returned to take readings at the base station after every 2 elevation readings see table below for details s/n Time of Reading
Thermometer Reading (±1 F)
Gravimeter
(±0.1 dialunits)
Increase (dialunits)
1
11:35
69
1077.9
0.0
2
11:53
69
1078.5
0.6
3
12:05
69
1078.9
1.0
4
12:18
69
1081.0
3.1
5
12:32
69
1080.9
3.0
6
12:44
69
1080.9
3.o
Fig 2
We adopted the time 11:35 as our reference (the time recording began) and then we subtracted the readings obtained at subsequent times from the reading at 11:35, the differences obtained were subtracted from the readings taking within each time interval.
For example, readings taking within the time interval 11:35 and 11:53 i.e. readings ‘2’ and ‘3’ in Table 1 were corrected by subtracting 0.6. Similarly readings within the time interval 11:53 and 12:05 i.e. readings ‘4’ and ‘5’ in Table 1 were corrected for diurnal variation by subtracting 1.0.
See ‘Table 3’ below for values obtained after diurnal variation correction

REDUCTION OF DATA
The gravimeter measures change in gravity, therefore it is appropriate to reduced our data to obtain changes in gravimeter readings, hence, we set the base reading at zero elevation as reference and subtracted readings at subsequent elevations from the base station reading see Table 3 for details s/n Gravimeter
(dialunits)
Corrected
(dialunits)
Reduced reading (dialunits)
Elevation
(ft)
1
1077.9
1077.9
0
0.0
2
1076.6
1076.0
-1.9
1.1
3
1076.6
1076.0
-1.9
2.3
4
1075.5
1074.5
-3.4
3.5
5
1075.1
1074.1
-3.8
4.6
6
1073.6
1070.5
-7.4
5.8
7
1073.6
1070.5
-7.4
6.9
8
1072.3
1069.3
-8.6
8.1
9
1071.1
1068.1
-9.8
9.2
10
1070.2
1067.2
-10.7
10.4
11
1069.6
1066.6
-11.3
11.6
Table 3
CONVERSION FROM dialunits to mGal
To