1、中文 2546 字 ,1458 单词， 7100 英文字符 附录 英文原文 Using the Magnetic Technique to Investigate the Subsurface in the Limpopo Region of South Africa Mitchell L. Johnson JacksonState University AfricaArray 2009 Abstract We employed the magnetic technique to take readings of the magnetic field in the Bushveld Compl
2、ex. 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
3、. 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
4、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 Ea
5、rth. (McCarthy and Rubdige 2005). The Earths 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 surfac
6、e. At a given location, the Earths 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 LimpopoProvince in the northernmost part of South Africa (Fig. 1). Fig. 1 Map of Bushve
7、ld 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 l
8、ies 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 s
9、everal 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 ab
10、out the same time. Magma from the Earths 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 we
11、re 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 m
12、ethods 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 Coulombs Law, which describes the force generated by a magnetic field. 221 mrmmF In this equation, F is the force, m1 and
13、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 locati
14、on. 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
15、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).
