MAGNETIC ANOMALY PRESENTED BY : • ASHUTOSH AND TEAM MEMBERS WHAT ACTUALLY IS MAGNETIC ANOMALY? IN GEOPHYSICS, A MAGNETIC ANOMALY IS A LOCAL VARIATION IN THE EARTH’S MAGNETIC FIELD RESULTING FROM VARIATIONS IN THE CHEMISTRY OR MAGNETISM OF THE ROCKS. MAPPING OF VARIATION OVER AN AREA IS VALUABLE IN DETECTING STRUCTURES OBSCURED BY OVERLYING MATERIAL. MEASURING MAGNETIC ANOMALIES Magnetic anomalies are generally a small fraction of the magnetic field. The total field ranges from 25,000 to 65,000 nanoteslas (nT). To measure anomalies, magnetometers need a sensitivity of 10 nT or less. THERE ARE THREE TYPES OF MAGNETOMETERS Proton Precession Magnetometer Fluxgate Magnetometer Optically Pumped Magnetometer PROTON PRECESSION MAGNETOMETER • A Proton Precession Magnetometer is an instrument that measures the scalar intensity of the local magnetic field and relies upon the proton-precision measurement technique. This is a well-established technique and is successfully implemented in classical proton precision magnetometers. PPM derives its name from the fact that it utilizes the Precession of spinning protons or nuclei of the hydrogen atoms in a sample of hydrocarbon fluid, to measure the total magnetic field intensity. FLUXGATE MAGNETOMETER • The fluxgate magnetometer was developed during World War II to detect submarines. It measures the component along a particular axis of the sensor, so it needs to be oriented. On land, it is often oriented vertically, while in aircraft, ships and satellites it is usually oriented so the axis is in the direction of the field. It measures the magnetic field continuously, but drifts over time. One way to correct for drift is to take repeated measurements at the same place during the survey. OPTICALLY PUMPED MAGNETOMETERS Optically pumped magnetometers, which use alkali gases (most commonly rubidium and caesium) have high sample rates and sensitivities of 0.001 nT or less, but are more expensive than the other types of magnetometers. They are used on satellites and in most aeromagnetic surveys ROLE OF MAGNETIC ANOMALY IN VARIOUS DISCOVERIES • We knew less about the deep ocean of planet earth MOST OF THE THINGS EXPLORED IN LAST 60 Years when technology finally able to see through the huge expansion of water. Ocean Sets just on the oceanic crust just as continent on continental crust. IN 1950group of scientist became interested in oceanic crust and in particular its magnetic properties .What they discovered had changed the view of how we understand not just ocean but the entire history of the earth . • In the late 18th century sailors from Iceland noticed local disturbance and magnetic field that causes navigational error. • In the world war 2, When FLUXGATE MAGNETOMETER used to detect submarines in Atlantic ocean .PLANES flew back and forth across the Atlantic with magnetometer creating magnetic map of sea bed .Scientist gets amazed by getting the particular pattern of magnetic anomaly . CONTINUED….. Scientist saw a positive strip next to a negative one next to another positive one and so on What caused this pattern of magnetic strips ??It turns out the earth magnetic field is not entirely static its moves over time which causes the North and South poles to drift slowly in every few million years it even flips around so North and South are completely switched that is called pole reversing . When the rocks in the oceanic crust are formed they preserve the direction of magnetic field at that time Showin positive and negative anomaly . MAGNETIC ANOMALY AND OCEAN FLOOR STRIPES • Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge • Magnetic surveys over the oceans have revealed a characteristic pattern of anomalies around mid-ocean ridges. They involve a series of positive and negative anomalies in the intensity of the magnetic field, forming stripes running parallel to each ridge. They are often symmetric about the axis of the ridge. The stripes are generally tens of kilometers wide, and the anomalies are a few hundred gamma . The source of these anomalies is primarily permanent magnetization carried by magnetite minerals in basalt and gabbros. They are magnetized when ocean crust is formed at the ridge. As magma rises to the surface and cools, the rock acquires a magnetization in the direction of the field. Then the rock is carried away from the ridge by the motions of the tectonic plates. Every few hundred thousand years, the direction of the magnetic field reverses. Thus, the pattern of stripes is a global phenomenon and can be used to calculate the velocity of seafloor spreading. APPLICATIONS OF MAGNETIC ANOMALIES Magnetic anomalies are used principally in two kinds of applications. • One is to find the depth to basement (if it has magnetic minerals in it). This is especially useful in obtaining the thickness of sedimentary basins in reconnaissance surveys. • The second application is in locating buried geological bodies, which give rise to magnetic anomalies. Bodies which contain ores are typical examples of such bodies, but interpretation methods are also applied to buried plutons, etc. DETERMINING THE DEPTH OF THE BASEMENT USING MAGNETIC ANOMALY The fact that regions with high amplitude, short wavelength and steep gradient indicate that the anomalous mass is at shallow depth while deep bodies give rise to long-wavelength anomalies forms the basis for using spectral techniques for determining depth to basement. The magnitude of the gradient is directly related to the depth to the magnetic basement. This is just one method of finding it and there are many other methods to find out like Tilt depth method ….. LOCATING BURIED GEOLOGICAL BODIES • There are several factors which control the shape of the magnetic anomaly of the buried susceptible bodies like: • The depth to the top surface and to the center of the buried body according to ground surface. Shallow bodies give sharp peak anomalies, while deep ones give broadened peak anomalies. The actual shape for the magnetic buried susceptible body. And this could be approximated geometrically to the sphere, horizontal and vertical cylinder or prism shapes in order to easily calculate the depth to its top and center and the volume. Magnetic anomalies and geological mapping Knowledge of the geology of a region is the scientific basis for resource exploration ( petroleum, solid minerals, groundwater) the world over. Among the variety of rock types to be found in the Earth's crust, many exhibit magnetic properties, whether a magnetization induced by the present‐day geomagnetic field, or Paleomagnetism acquired at some time in the geological past, or a combination of both. Mapping the patterns of magnetic anomalies attributable to rock magnetism has proved to be a very effective way of reconnoitering large areas of geology at low cost per unit area. The fact that most sedimentary rocks and surface‐cover formations (including water) are effectively nonmagnetic means that the observed anomalies are attributable to the underlying igneous and metamorphic rocks (the so‐called “magnetic basement”), even where they are concealed from direct observation at the surface. Anomalies arising from the magnetic basement... THANK YOU !
