Using the Magnetic Technique to Investigate the

Subsurface in the Limpopo Region of South Africa

Mitchell L. Johnson

Jackson State University

AfricaArray 2009

Abstract

We employed the magnetic technique to take readings of the magnetic field in the

Bushveld Complex. The Bushveld Complex is located in Limpopo, South Africa. The

Bushveld Complex is known for its richness in natural resources. We are here to image a

previously known dyke. This dyke has already been imaged by aeromagnetic data. Our

ground magnetic survey will give one a better image of the dyke. Our survey consisted of

30 east-west lines that were 1000 meters long, oriented perpendicularly to a previously

known dyke, as well as 3 north-south lines that ran parallel to this geologic feature. The

results do show the previously imaged dyke along with 3 smaller dykes. This data could

be used for future mining exploration in the surveyed area. By knowing what is beneath

the subsurface miners will be able to mine in a safe and resourceful way.

Introduction

Magnetics is a branch of geophysics that studies how the properties or effects of

magnetic fields change in different places on the Earth. (McCarthy and Rubdige 2005).

The Earth’s magnetic field can be thought of as a point on a surface, though it is

generated by the movement of the fluid core, which consists of molten rock called

magma. The magnetic field on the Earth will change because of differences in the

geology of the surface. At a given location, the Earth’s magnetic field is described by a

vector and is measured using units of nanoteslas.

In this study, we conducted geophysical investigations in the Bushveld Complex,

located in the Limpopo Province in the northernmost part of South Africa (Fig. 1).

Fig. 1 Map of Bushveld Complex

The Bushveld Complex has an abundance of minerals that are mined. This area is known

worldwide for these richest. (McCarthy and Rubidge 2005). By imaging the subsurface,

we can assist mining companies to be more efficient. We can provide these companies

with locations and images of what lies beneath the subsurface, preventing wasted time

and money on trial and error exploration.

While in the Bushveld Complex we conducted a magnetic survey. This survey

was used to map and locate a previously known dyke. Our results not only confirm the

location of this large dyke, but also identify several smaller dykes in the vicinity. This is

very pertinent for the mining industry.

Geologic background

The Bushveld Complex was formed around 2 billion years ago. There are 3 parts

of the Bushveld Complex: an eastern, western, and northern branch. All of these are very

alike and were formed at about the same time. Magma from the Earth’s mantle that came

to the surface made the Bushveld Igneous Complex. This continued to happen over a

period of time. As the molten rock cooled over time, crystallization of different minerals

at different temperatures lead to the formation layered structures were formed which are

called reefs.

One important layer is the Upper Group 2 (UG2) reef The UG2 contains has

chromite, platinum, and the Merensky Reef. The Merensky Reef has with a width of 30

to 90 cm and the UG2 contains have almost 90% of the worlds known PGE reserves.

Methods

Several geophysical methods were employed to investigate the subsurface

structure of the Bushveld Complex however, for this study, I will focus on the magnetic

method. The magnetic method depends on Coulomb’s Law, which describes the force

generated by a magnetic field.

F=m1m2/mr^2

In this equation, F is the force, m1 and m2 are the strengths of two magnetic

poles, m is the magnetic permeability, and r is the distance between the two poles.

(Burger et al., 2006)

To collect the magnetic data, an instrument called a magnetometer is used. This

instrument measures the strength of the magnetic field at a particular location. The

magnetometer does not measure position, but handheld GPS units can be used to

determine the measurement location.

The magnetometer will not give a true reading if a reading is taken in the vicinity

of metallic objects; therefore, readings should not be taken near railway lines, cars,

houses and electric lines. These places and objects will change the magnetic field in the

area. Also, the magnetometer operator should not carry metallic objects because these

objects will have an influence on the magnetic field.

At our fieldsite, data was collected along a grid of GPS coordinates, consisting of

30 lines that were 1000 meters long, striking roughly perpendicular to the dykes imaged

by Anglo Platinum aeromagnetic data (Fig. 2).

Fig. 2 GPS tracks overlaid on aeromagnetic data showing a known dyke

We were only able to survey 800 meters due to a power line and thorns in the

vegetation. There were also 3 north-south lines that were conducted after the original

survey had been completed. The survey line spacing was 25 meters between each line.

We took a reading every 5 meters for the entire 800 meters of the survey line. A base

station magnetometer was also operated to measure diurnal variations, which was then

used to correct our raw magnetic data. Diurnal variation is the repetitive variation with

time in the Earth’s magnetic field (USGS 2008). Everyday after collecting field data the

group would process data. This included correlating the GPS readings with the position

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Mag (nT)

of the magnetometer at the time the measurement was taken, converting the field values

into Mag2D, and making a spreadsheet in Excel.

Results

Our magnetic survey found one major magnetic dyke. This dyke is located

around 15 meters below the surface. I was able to interpolate this by using 2D forward

modeling. I tried to match the data collected in the field by placing dykes in the forward

model (Fig 3).

Figure 3 2D Forward Model Profile

There is an overburden of 15 meters that has been previously found by Anglo Platinum.

In addition to the main dyke, we also found 3 other anomalies that may be smaller dykes.

This is shown in a 3-D view of the survey data that has been interpolated. Next, we made

a 2-D profile of the anomalies. We realized that there is a secondary dyke that lies very

close to our major dyke (Fig 4).

Figure 4. 3D view of data

Discussion

The major dyke imaged has a northeast-southwest strike and has very high

magnetic values, peaking at 4677 nT. (Fig. 3). The 3 other dykes that were imaged were

smaller but were also deeper in the subsurface. One thing to take note of is that everyday

someone different took magnetic readings. This means that there may be readings that

were taken that are not as accurate as others.

Another thing to take note of is that our magnetic data was collected on the

ground, which is different than the aeromagnetic data collected by Anglo Platinum. The

aeromagnetic survey is mainly used to map large areas. This is done by the use of an

airplane flying over the survey area. Ground magnetic surveys map smaller areas but in

greater detail. Both have uses in the mining exploration. Some mineral deposits are

associated with an increase in abundance of magnetic minerals. Ground magnetic

surveys will allow us to pinpoint a specific magnetic reading, where as aeromagnetic

surveys will only give an overview of the area.

With the images we have produced of the Bushveld Complex, miners will know

how and where to mine. A dyke is an intrusion of rocks into the natural geological

structure and will be a weak point in the subsurface. If miners were to put a mine close to

a dyke, the mine may become weak and collapse. This is very important because

people’s lives will be at stake.

.

Conclusion

The magnetic method was employed to image the subsurface within the Bushveld

Complex in Limpopo, South Africa. Using a magnetometer, we surveyed an area that

consisted of 30 lines that were 800 meters long in search of changes in the magnetic field.

These changes in the magnetic field are important because certain minerals have a

specific magnetic value and features, such as dykes, can be identified. Knowing the

location of dykes in a future mining area will keep miners safe. We imaged one major

dyke that was previously observed and we also found 3 smaller dykes. By knowing what

is beneath the subsurface, Anglo Platinum will use this data to further their exploration of

natural resources in a safe and efficient process.

Acknowledgements

I would like to thank The Pennsylvania State University for being a part of the

Summer Research Opportunities Program. Dr. Andy Nyblabe for letting me participate

in the AfricaArray program and field school. I would like to give thanks to My Mentors

Dr. Samantha Hansen and Dr. Rick Brazier for their guidance and time that has been

given to me this summer. Also, The University of Witwatersrand for hosting us.

References

Burger, R.H., Jones, C.H., Sheehan, A.F. Introduction to Applied Geophysics. Norton &

Company, 2006.

McCarthy, T. and Rubidge, B. The Story of Earth & Life. South Africa: Struik, 2005.

U.S. Geological Survey. 2008. 13 July 2009 http://geomag.usgs.gov/intro.php