AUSTRALIAN SCIENCE AND MATHEMATICS SCHOOL
The Black Box
Earth and Cosmos
Melanie Bennett
October 2012
Tutor Group 01
Andrew Stone
‘It is rare to use one exploration technique alone to prospect and determine the location of anomalies of likely
mineral ore bodies of economic significance. For this task you will plan and carry out geophysics exploration
within the ‘black box’, a model of a 1 km deep section of the Earth’s crust. The aim is to carry out geophysical
exploration program and identify and describe the location of potential areas of interest.’
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PART 1: Planning an investigation – Exploration Plan
Method
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Collect the ‘Black-Box’ model, magnetometer and gravity meter.
Collect the Geophysical Surveys, one for each of the data sets.
Specify North on survey’s with an arrow like the arrow on the ‘black box’.
Mark the halfway points between each vertex on the ‘black box’.
Note on the Survey the scale of the halfway points between each vertex (100m).
Firstly, test the gravity. Turn the Gravity Meter on by rolling the power wheel on the meter upwards.
Then, roll back the wheel until the meter reads 0.
Place the gravity meter so the arrows are in line with marked points on the model.
Take the readings from all vertexes and halfway points. Record results on the Survey with units ‘milligal’s
(x10)’.
Next, turn on the Magnetometer by flicking the power switch to ‘ON’.
Turn the Sensitivity Knob on meter to full and turn back until reading says ‘0’.
Place the magnetometer so the knob is in line with the marked points on the box.
Take the readings from all vertexes and halfway points and record results on the Survey with units ‘NanoTeslas (x 10,000)’.
When all readings are recorded, turn off the magnetometer and gravity meter.
North
Scale of 200m
Figure 1: The Black Box's Surface Area.
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Instruments Used
Gravity Meter
A gravity meter measures the differences in the gravitational field of the Earth. A change in the gravitational
field of the earth is dependent on the change in density of the rocks under the earth’s surface. This instrument
is useful in finding ore bodies because there will be a difference in density between the ore body and the
surrounding soil. For example, platinum, is much dense than dirt and the gravity meter would pick up this
difference in density by the difference in the gravitational field of the Earth. Below is an example of what a
simple gravity meter may look like. This gravity meter measures the gravitational field of the Earth in units
‘milligal’s X10’ which is a measure of gravitational acceleration.
Figure 2: A Gravity Meter.
Magnetometer
A magnetometer measures the strength of a magnetic field. The three magnetic minerals which would change
the strength of a magnetic field are iron, nickel and cobalt. This instrument is useful in determining whether or
not there are magnetic minerals under the earth’s surface. It can also be useful to use this instrument after
finding an ore body using the gravity meter to determine the type of mineral under the earth’s surface. For
example, if geologists found need to mine iron and they have found an ore body, a magnetometer can tell if
the minerals under the surface are magnetic like iron. Below is an example of what a simple magnetometer
may look like. This magnetometer measures the strength of the magnetic field in units ‘Nano-Teslas X10000’.
Figure 3: A Magnetometer.
Safety Hazards
Overall Hazard Assessment LOW
Possible Hazard
Falling over, sun burn, and
other hazards found outside.
Wooden box injuring body if
tripped over.
Wooden box injuring body by
walking into it or hitting it.
Extinction of forest due to
using paper for the
geophysical surveys.
MEDIUM
HIGH
Suggested Safe Operating Procedure
To decrease the risk these hazards, the investigation was simulated with a
foam box taking the role of the earth’s surface inside. This decreased hazards
found outside.
To decrease the risk of tripping over the box, the box will be stationed on a
table. To decrease the impact of tripping over the box, the investigation will
take place in a carpet area.
To decrease the impact of injuring the body with the wooden box, the box will
be replaced with a foam box instead. To decrease the risk of walking or hitting
into the box, the box will be in the centre of the table with no corners hanging
over the edge.
The likelihood of this occurring is very low but to decrease the impact on
forests, only 2 pages were used and the report will not be printed.
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PART 3: Analysis and Conclusion – interpretation of data
Results – Magnet Anomalies
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Results – Gravity Anomalies
Geophysical Survey Sheet
0m
Figure 5: Geophysical Survey using a gravity meter.
100m
150m 200m
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Table of Data
Gravitational acceleration (X10 milligals)
0
0
0
0
0
0
0
0
0
0
0
0
0
Gravitational
acceleration
(X10 milligals)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
5.1
4
1
0
0
0
0
0
0
0
0
0
2
5
6
5
0
0
0
0
0
0
0
0
0
2
5
6
5.5
2.5
0
0
0
0
0
0
0
0
0
2.5
2
1.5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Figure 6: Table of Data from the Geographical Surveys.
Surface Graph
Gravity Anomalies for a described Exploration Region
6
5
5-6
4-5
3
3-4
2
13
10
1
7
0
1
2
3
4
4
5
6
7
8
9
Distance
X100 m
Figure 7: Gravity Anomalies for a described Exploration Region.
10
11
1
12
13
2-3
Distance
X100 m
Gravity
X10 Gal
4
1-2
0-1
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Analysis – Gravity Anomalies
calculate
Cross-Sectional Reading
Cross-Sectional Readings for peak of Gravity Anomalies
7
6 (g max)
6
Gravity
X10 Gal
5
4
3 (1/2 g)
3
2
1
1.5 (x)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Distance
X100 m
Figure 8: Line Graph - Cross-Sectional Readings for peak of Gravity Anomalies.
Tiberg’s Rule
To calculate the depth of the rock/ore body causing the anomaly in the results, Tiberg’s Rule must be used:
𝑧 (𝑑𝑒𝑝𝑡ℎ) = 1.3 × 𝑥
= 1.3 × 1.5
= 1.95𝑚
Analysis – Written Summary
Magnetic Anomalies
the
Part 3: Analysis and Conclusion – interpretation of data
 The third step is to analyse your data (i.e. graphs and calculations etc) and describe your
conclusions about the shape (size and depth of the ore body). You should submit:
1. Your raw data survey sheets
2. Excel spreadsheet with your table of data, transposed from your raw data
3. Appropriately labelled graphical representations of your data using the surface
graph feature in Excel
4. A line graph of the transect with the highest anomaly for calculating depth of
the ore body
5. Calculations that show the theoretical depth to the centre of the ore body using
Tiberg’s rule (NB: this is likely to be very different to the actual scale so you
must show your working).
6. A detailed written analysis and summary of the graphical data (i.e. summarise
the graphs in words, what anomalies are present (magnetic/gravity)? Are the
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anomalies positive or negative? Where? How many? What are the likely physical
properties of the ore and what is it likely to be?)
7. An explanation of what improvements could be made to your geophysical
survey
8. Include in-text referencing and reference list

Future planning
o If your results indicate an ore body may be present (even if you found a very
weak or no anomalies), what steps would you take to further explore the area
and narrow the target area?
http://galitzin.mines.edu/INTROGP/notes_template.jsp?url=GRAV%2FNOTES%2Freldens.html&page
=Gravity%3A%20Notes%3A%20Density%20Contrast
The peak is the anomalies a very high anomalies in the north west corner.