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1、英文原文Using the Magnetic Technique to Investigate the Subsurface in the Limpopo Region of South AfricaMitchell L. Johnson Jackson State University AfricaArray 2009 AbstractWe employed the magnetic technique to take readings of the magnetic field in the Bushveld Complex. The Bushveld Complex is located
2、 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-w
3、est 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 th
4、e surveyed area. By knowing what is beneath the subsurface miners will be able to mine in a safe and resourceful way. IntroductionMagnetics 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). Th
5、e 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 surface. At a given location, the Earths m
6、agnetic 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 ComplexThe Bushveld Complex has
7、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, preventi
8、ng 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
9、. 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 Ea
10、rths 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. On
11、e 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
12、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. In this equation, F is the force, m1 and m2 are the strengths of two magnetic poles, m
13、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
14、, 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 w
15、ill 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, st
16、riking roughly perpendicular to the dykes imaged by Anglo Platinum aeromagnetic data (Fig. 2). Fig. 2 GPS tracks overlaid on aeromagnetic data showing a known dykeWe 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 co
17、nducted 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 ou
18、r raw magnetic data. Diurnal variation is the repetitive variation with time in the Earths magnetic field (USGS 2008). Everyday after collecting field data the group would process data. This included correlating the GPS readings with the position of the magnetometer at the time the measurement was t
19、aken, 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
20、 field by placing dykes in the forward model (Fig 3). Figure 3 2D Forward Model ProfileThere 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 sur
21、vey 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 dataDiscussion The major dyke imaged has a northeast-southwest strike and has very high magnetic values,
22、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
23、 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 surv
24、eys 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
25、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 beco
26、me weak and collapse. This is very important because peoples 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 lon
27、g 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 dyk
28、e 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 f
29、or 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.
30、 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 文献翻译应用磁法勘探技
31、术探查南非林波波河地区的地下情况米切尔约翰逊 杰克逊州立大学 AfricaArray 2009摘要 我们采用磁技术在布什维尔德杂岩带的磁场读数。布什维尔德杂岩位于南非林波波河。布什维尔德杂岩以它丰富的天然资源而出名。我们在这里假设有一个先前已知的堤坝。这个堤坝已经用航磁数据成像。我们的地面磁测将给一一个更好的图像的堤坝。我们的调查是由30条东西线，长1000米，垂直于先前已知的堤坝，以及3条南北线跑这一地质特征。结果表明先前成像的堤防3小脉。这些数据可用于在测区未来矿业勘查。清楚地知道地下是什么能够使矿工们以一个安全的和善于随机应变方法在矿井里开采。测区简介 磁学是地球物理学的一个分支学科，主
32、要研究磁场的变化属性或磁域在不同的地方变化对地球的影响。（麦卡锡和rubdige 2005）。地球的磁场可以被认为是某一表面上的一点，但它是通过流体运动产生的核心，由熔化的岩石组成的那部分被称为岩浆。由于地球表面地质的差异，地球上的磁场会发生改变。在一个给定的位置，地球磁场是一个由向量描述和使用单位奈特士拉测量的物理量。在这项研究中，我们对位于南非林波波省的最北端的布什维尔德杂岩进行了地球物理调查（如下图1）。图1. 布什维尔德杂岩的地图 布什维尔德杂岩有丰富的矿物质而且正在被开采。这个地区闻名全球主要是因为有这些丰富的资源。（麦卡锡和鲁比奇2005）。为了更好地了解底下的情况，通过地下采矿公
33、司的成像，对我们能帮助更有效。我们可以提供这些公司地表下面存在物质的具体位置和图像，避免在试验和错误的探索上浪费太多的时间和金钱。 我们在布什维尔德杂岩地区进行了一项磁测量研究调查。这项调查是使用地图并定位一个先前已知的堤坝。我们的研究结果不仅证实了这个大堤坝的位置，而且也确定了附近的几个小堤坝的存在。这对采矿业的研究调查是非常中肯的。测区的地质背景 布什维尔德杂岩大约形成于二十亿年前。布什维尔德杂岩主要有三个部分：一个是东方的，一个是西方的，还有一个是北方的分支。所有这些都是非常相似的，而且大约形成在同一时间。从地球的地幔出来的岩浆来到了地表形成了布什维尔德杂岩中的火成岩。这个变化持续了一段
34、较长的时期。作为熔融岩石的冷却时间，在不同温度下的不同的矿物结晶导致形成层状结构而形成的物质被称为珊瑚礁。其中一个重要的层是上组2（UG2）礁，UG2包含有铬，铂，和Merensky礁。Merensky礁的宽度在30至90厘米之间，UG2包含有几乎90%的世界上已知的PGE储备。勘探方法 多种地球物理勘探方法被用来对布什维尔德杂岩的地下结构进行调查，然而在这项研究调查中，我将会把重点放在磁法勘探上。磁方法依赖于库仑定律，它描述了一个磁场产生的力。 在这个方程中，F是力，m1和m2是两个磁极的长处，m是磁导率，而r是两极之间的距离。（汉堡等人，2006）通过收集到的磁数据显示，它是用了磁力计这种
35、仪器。该仪器测量在一个特定位置的磁场强度。磁力仪测量不能显示位置，但手持GPS单元可以用来确定具体的测量位置。. 如果读数时周围有其他金属物质，地磁仪将不能显示出真实的数据。因此，读数时应该尽量避开铁路线，汽车，房子和电线等。这些地方的对象将会引起在该地区的磁场的变化。同时，磁力计的操作人员也不应携带金属物体，因为这些对象会对磁场变化有影响。 在我们的勘查区域，数据收集在一个GPS坐标网格内，由30条一千米长的线租成，惊人的大致垂直于墙英美铂金航磁数据成像（如下图2）。图2. 一个已知堤坝的GPS轨道重叠航磁数据显示 由于电源线和植被中的荆棘的影响，我们只能够测量800米。这里有3条南北向的线
36、，是在最初的调查时已经完成了的。每两条测线之间的距离为25米。在800米的测长中我们采取了每隔五米读取一组数据。基站磁强计测量也被用来测量测区的每天昼夜的变化规律，并被用来纠正我们的原磁数据。昼夜的变化就是地球磁场随时间的不断重复性变化（USGS 2008）。在每天收集好现场数据后勘测人员要对采集到的数据进行审核。这包括GPS读数的采集和磁强计测量时的拍摄位置相关的字段值，并转换成Mag2D，在Excel中制作电子表格。测量结果 我们的调查发现了一个主要磁脉磁。这个堤坝是位于约15米以下的表面。我能插用二维正演。我试图将其放在先前的模型中与现场采集到的数据联系起来（如下图3）。图3. 二维正演
37、模型简介 这有一条先前被发现的长15米的英美铂金覆盖层。除了这条主脉，我们也发现了其他3个异常，可能是较小的堤坝。 这个已经在一个三维视图内插的调查数据中显示出。接下来，我们做了一个异常的二维剖面。我们认识到还有有一个二级堤防，位于非常接近我们的主要堤防（如下图4）。图4. 三维视图的数据探讨 主要脉成像有西南东北走向，具有很高的磁场值，峰值在4677nT。（图3）。其他三条脉成像较小但也存在于更深的地下。有一点需要注意的是，每天都有不同的人来读取记录磁性的数据。这意味着可能会出现采取的的读数是那些不够正确的人读取的数据。 另一点需要注意的是，我们的磁数据是在地面上收集的，这是不同于由英美铂金
38、的航磁数据的收集的。航磁调查主要用于大面积的映射。这是由一架飞机飞行在调查区域进行的。而地面磁测图面积较小但是是更详细的调查。两种方法都已经用在矿业勘探中。一些矿物的沉积与磁性矿物丰富程度的增加相关。地面磁法测量将使我们能够确定一个具体的磁读，而在航磁调查只能给出该地区的概述。 根据我们给出的布什维尔德杂岩产生的图像，矿工将会知道如何去开采和去哪里开采。堤防是岩石侵入的自然地质结构并成为在地下的薄弱点。如果矿工把矿井建立在靠近堤处，矿井可能会比较软弱和容易崩溃。这一点是非常重要的，因为人的生命将可能随时处在危险之中。结论 磁法勘探已经被用来反应林波波布什维尔德杂岩，南非等地区的地下区的图像。利
39、用磁强计，我们调查的一个地区，这是一个由30条长800米组成的测区内的磁场变化的调查。在磁场中的这些变化是很重要的，因为每种确定的矿物都有一个具体的磁性矿物的价值和功能，比如堤坝，可以被识别。只有了解了一个堤坝的具体位置才能在未来的矿区开采中确保矿工的安全。我们拍摄并观察了一个以前掌握的堤坝，我们也发现3个较小的堤坝。清楚的了解了地下是什么，英美铂金将利用这些数据将进行进一步的安全和有效的处理自然资源勘探。致谢 我要感谢滨州州立大学成为夏季研究机会计划的一部分。安迪博士让我参加AfricaArray程序和田间学校。我要感谢我的导师萨曼莎博士汉森博士和瑞克他们给我的指导和时间，给了我这个夏天。同时，我还要感谢主办方美国金山大学。参考文献汉堡，相对湿度，琼斯，C.H.，希恩，应用地球物理导论。诺顿公司，2006麦卡锡，T.和鲁比奇，B.地球与生命的故事。南非：Struik，2005美国地质调查局，2008年7月13日
