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Geomagnetic method
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- 2. INTRODUCTION • The magneticmethod is a very popular and inexpensive approach for near-surface metal/iron detection. HOW DOES IT WORK? • When a ferrous material is placed within the Earth's magnetic field, it develops an induced magnetic field. • The induced field is superimposed on the Earth's field at that location creating a magnetic anomaly. • Detection depends on the amount of magnetic material present and its distance from the sensor. • The anomalies are typically presented on colour contour maps.
- 3. • The magneticfield values are normally expressed in gamma or nanoTesla. • 1 gamma = 1 nanoTesla
- 5. INDUCED MAGNETIZATION • Whena magnetic material, say iron, is placed within a magnetic field, H, the magnetic material will produce its own magnetization. This phenomenon is called induced magnetization. • 3 types of magnetic material: - Diamagnetism - Paramagnetism - Ferromagnetism
- 6. REMANANT MAGNETIZATION • Ifthe magnetic material has relatively large susceptibilities, or if the inducing field is strong, the magnetic material will retain a portion of its induced magnetization even after the induced field disappears. This remaining magnetization is called Remanent Magnetization. • Remanent Magnetization is used to map the motion of continents and ocean basins resulting from plate tectonics. • The only way to measure the remanent magnetic component of a rock is to take a sample of the rock back to the laboratory for analysis. This is time consuming and expensive. So, we typically assume there is no remanent magnetic component in the observed magnetic field.
- 7. VARIATIONS IN EARTH’SMAGNETIC FIELD • The Earth’s magnetic field varies with time, i.e. it is not constant. • As the Earth rotates, the outer layers of the ionosphere interact with the solar wind to cause minor fluctuations in the magnetic field. • Depending upon the frequency, duration and intensity of these fluctuations, they are given different names.
- 8. SECULAR VARIATION • Geomagneticsecular variation refers to changes in the Earth’s magnetic field on time scales of about a year or more. • These changes mostly reflect changes in the Earth's interior, while more rapid changes mostly originate in the ionosphere.
- 9. DIURNAL VARIATION • Fluctuationswith a period lasting of several hours to one day are called diurnal variations. • However, they are not predictive and are usually not a problem when conducting magnetic surveys. • This diurnal drift can cause a variation of the order of 50 nT per hour.
- 10. NORMAL FIELD, TERRAINAND ELEVATION CORRECTION • Normal Field Correction – variation in field with latitude and longitude • Terrain Correction – only comes into play near the base of a steep slope of high k material • Elevation Correction – generally not required for ground surveys, only in airborne when large changes in elevation is experienced
- 11. MICRO-PULSATION • Erratic, short-termblips or spikes in the magnetic field are called micro-pulsations. • These can range in intensity from a few through to tens, or even hundreds, of nanoTeslas in intensity. • These variations can present a problem when we are surveying where it may appear similar to anomalies caused by buried objects.
- 12. MAGNETIC STORMS • Whenthe amplitude and duration of micro-pulsations becomes severe it is then called a magnetic storm. • Typical micro-pulsations last a few hours whereas magnetic storms can last for days. • It is not recommended to conduct a total-field survey during a magnetic storm, as you may not be able to remove all of the rapidly changing variations in the magnetic field, giving rise to perhaps false anomalies.
- 13. FIELD INSTRUMENTS • FluxgateMagnetometer • Proton Precession Magnetometer • Optically Pumped Magnetometer • Overhauser Magnetometer
- 14. APPLICATION OF MAGNETICSURVEY • Mineral Exploration • Geothermal Exploration • Fault study • Mapping buried utilities • Locating buried tanks or drums
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Editor's Notes
- #3 Engineering and environmental site characterization projects often begin with a magnetometer survey as a means of rapidly providing a layer of information on where utilities and other buried concerns are located -relatively easy and cheap and a few corrections need to be applied
- #4 This magnetizing field strength H is defined, following Biot-Savart’s Law, as being the field strength at the centre of a loop of wire of radius r through which a current I is flowing such that H = I/2r
- #6 The nature of material magnetization is in general complex, governed by atomic properties
- #15 -Pipelines -Igneous and metamorphic rocks generally have a higher magnetic susceptibility than sedimentary rocks. An igneous intrusion or pluton is detectable in a magnetic survey due to the contrast in magnetic susceptibility with the surrounding rock. -Alteration mineralogy may be present in zones of circulation of hydrothermal fluids. This alteration is the transition from magnetic minerals (such as magnetite) to hydrous oxide or clay minerals with low magnetic susceptibility. This alteration mineralogy lowers the magnetic susceptibility in the vicinity of hydrothermal activity and indicate the presence of the geothermal reservoir and conduit structures such as faults or dikes. [3] In addition, it is possible using magnetics to map the Curie point at depth. This enables an inference of the temperature gradient and has been applied to geothermal fields around the world. [2]