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Unit Three - Geophysical
Methods Objectives - Students
will be able to:
Explanation Magnetic Minerals Magnetism has been studied for a very long time in human history. Early Greek philosophers knew about the attraction of iron to a magnet. The first magnets consisted of a naturally occurring rock called lodestone, a variety of massive magnetite (almost pure iron oxide). Magnetite is the only naturally occurring mineral with distinctly obvious magnetic properties. Only a few other minerals have any detectable magnetism. However, extremely sensitive magnetometers can detect trace magnetism in many different minerals. Iron, because of its atomic structure, has the greatest tendency to become magnetized or aligned. Other elements, such as cobalt and nickel, also have lesser tendency to become magnetic. Any mineral or rock which contains any of these elements is likely be more magnetic. Magnetism occurs when like poles of adjacent individual atoms are in alignment, creating a “dipole” effect. Another word for this alignment effect is called “polarization”. An analogy can be made with the north and south poles of a typical bar magnet. Like poles repel each other, and opposite poles attract each other. At extreme temperatures, there is a loss of alignment, leading to a loss of magnetism. Experiments have shown this loss of magnetism occurs at a temperature of approximately 550o C (the “Curie point”). Magnetic strength of a mineral or rock is a function of two things: 1) the amount of iron, nickel or cobalt, and 2) the amount of alignment which takes place. The measure of magnetic strength of a mineral or rock is called the “magnetic susceptibility” (Table T6). This can be measured with a simple magnet by testing the “pull”, or it can be measured with very sensitive, highly sophisticated instruments. The susceptibility of a completely nonmagnetic substance is equal to 0. The susceptibility of a highly magnetic mineral like magnetite is about 20.
Earth’s Magnetic Field It is uncertain why the earth has a magnetic field. Although it is much more complex, the earth’s magnetic field can be characterized as a giant bar magnet having a north and south pole. The many measurements which have been made indicate this force field completely surrounds the planet, with lines of force plunging into the polar regions. Geologists believe the earth’s core is largely made up of molten iron. Siesmic studies indicate that the inner core is solid and the outer core is liquid. The inner core is above the Curie temperature, so it cannot contribute to the Earth’s magnetic field. Modern theories suggest the Earth’s magnetic field is caused by flow of material in the outer core which generates electricity, which is associated with the flow of electrical current. The flow of these electrical currents effectively create a huge electromagnetic field. As an igneous rock crystallizes, its own internal magnetic field will line up with that of the earth. The locked orientation of the internal field of the rock is called the “remnant magnetic field”. The orientations of these internal fields has been measured for rocks of different ages and at numerous locations. The measurements indicate the earth’s magnetic poles have been slowly moving over geologic time. The slow migration, called “polar wandering”, is unpredictable. The measurements also indicate that there have been relatively sudden reversals of polarity of the earth’s magnetic poles numerous times in the geologic past. Measurements of ancient polarity and field orientation in rocks has allowed geologists to piece together a time scale which has become invaluable for age dating of rocks. The study of the ancient magnetic field of the earth is called paleomagnetism.
Magnetic Instruments The magnetic instrument we are most familiar with is the compass. A piece of lodestone suspended from a string will align itself with the earth’s magnetic field. In the 12th century the Chinese discovered that rubbing a needle against a piece of lodestone would cause the needle to become magnetic. Later the magnetic needle was suspended, and the first compass was created. A magnetometer is a more complex instrument which measures both the orientation and strength of a magnetic field. When the magnetic field of a rock sample is measured, the result is actually a measure of the field as it is being effected by the earth’s magnetic field, as well as any other large bodies of magnetic rock which are near by. Magnetic Surveys The orientation and strength of the earth’s magnetic field has been measured and studied in detail at many locations. As a result, geophysicists have been able to develop a mathematical model for its shape and intensity, which is called the “magnetosphere”. It is not a perfect sphere, but instead is an imperfect sphere with many bumps and irregularities.
In most cases these rock masses contain the mineral magnetite. Rocks which contain lots of magnetite include gabbro, diorite, basalt and other “mafic” igneous rocks. These rocks will show up as a strong magnetic high an a map of the total field strength. Felsic igneous rocks, like granite or rhyolite, and most sedimentary rocks are notably non-magnetic except in rare cases. These rocks may show up as distinct magnetic lows which are also mappable. In fact, in some cases magnetic lows are of keen interest in mineral exploration because they indicate areas of alteration associated with ore deposits. Magnetic surveys are always conducted by collecting measurements at specified locations along pre-established survey lines. The surveys can be conducted either on the ground or from the air. Those conducted on the ground collect detailed data at very close spacings. These surveys can outline the expanse and even the dip angle of the the magnetic body with some degree of accuracy. Surveys conducted from the air, called aeromagnetic surveys, are designed to cover large areas (100’s of square miles). Magnetometer image courtesy USGS
Field Methods | Geochemical Methods | Geophyscial Methods | Drilling Methods | Petroleum Exploration |
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Magnetometer
surveys conducted on a local scale indicate there are many unexpected
variations in the model, called “magnetic anomalies”.
A magnetic high is where the measured field strength is higher than
the value predicted by the global model, and a magnetic low is where
the measured field strength is lower than the value predicted by the
global model. Possible causes for magnetic highs include the presence
of magnetically charged rocks in the subsurface. Sophisticated
techniques have been developed which can predict the depth, shape, and
orientation of a magnetic rock mass below the surface.