3 Measurement of the magnetic field - Test magnet isattached to a horizontalfiber so that it can rotatein a vertical planeIf the magnetometer isinclined both vertical andhorizontal components ofthe earth’s field act torotate itIn horizontal position of theshaft only the vertical fieldintensity component canact to rotate it- Measures relative changesin the vertical component ofmagnetic field intensityTorsion magnetometer (Askania) – the most convenient to operate, precision ~ 1 gamma (1 nT)Robinson&Coruh, 1988
4 Measurement of the magnetic field - The design is based on theprinciple of magnetic inductionIt can be mounted for continuousoperation on an aircraft, ship,or any moving vehicle- It is not affected by motion orchanges in elevationTo avoid the distortion of thereading by the metal parts of theaircraft it has to be mounted in astreamline container called “bird”which trailed on a long cablebehind the aircraftMeasures the vertical componentof the magnetic field intensityFlux-gate magnetometer, precision ~ 1 gamma (1 nT)Robinson&Coruh, 1988
5 Measurement of the magnetic field Designed for operation in aircraft,ships, other moving vehiclesHand-carried instruments forground surveyingSensitive only to the totalfield intensityProton precession magnetometers, precision ~ 1/2 gammaRobinson&Coruh, 1988
6 Measurement of the magnetic field Proton precession magnetometers - principleSpinMagnetometer principlePrecessionw = g Fppg- gyromagnetic ratio of the protonRobinson&Coruh, 1988F – magnetic field, w - angular velocity
7 Geomagnetic Survey - applications Locating:Pipes, cables, magnetic objectsBuried military ordnance (shells, bombs)Buried metal drums of contaminated or toxic wasteConcealed mine shaftsMapping:Archeological remainsConcealed basic igneous dykesMetalliferous mineral lodes
8 Geomagnetic Surveys - types On the surface – used mostly to map anomalies that aretoo narrow for adequate detection by airborne operations.Use flux-gate, proton-precession, or torsion typemagnetometers2. By airplane (aeromagnetic surveys) –measure the magnitude but not the directionof the magnetic field; they can cover big areasUse flux-gate and proton-precession type magnetometersMagnetometer surveys must include measurement of thediurnal and secular variations of the magnetic field a time base must be designated.Most portable magnetometers measure only vertical relativeintensity a base station must be designated in this caseSharma, 1976; Robinson&Coruh,1988
9 Interpretation of magnetic data End product of magnetometer survey – contoured anomalymap in isogams (or nT)2. Interpretation of magnetic data – similar to gravity datainterpretation but more complicated because of:a) dipolar nature of the magnetic fieldb) the additional unknown parameter introduced bythe direction of the magnetization in rocks.3. Three types of interpretation:a) qualitativeb) quantitative – forward modeling and inversionc) semi-quantitativeSharma, 1976
10 Qualitative interpretation of magnetic data Visual inspection of the shape and trend of themagnetic anomalies2. Delineation of the structural trends3. Close examination of the characteristic features of eachindividual anomaly:- the relative locations and amplitudes of the positive andnegative parts of the anomaly- the elongation and areal extend of the contours- the sharpness of the anomaly as seen by thespacing of contoursIn many cases meaningful geological information can beobtained directly by looking at the map, without makingany calculations.Sharma, 1976
12 Depth to magnetic basement Crystalline basement rocks, which are commonly more mafic than overlyingsedimentary deposits, are the main source of magnetic anomalies in the region.(High ferromagnetic mineral content results in high level of magnetization)
13 Curie DepthMinerals that exhibit strong (ferromagnetic) behaviour at low temperatures haveweaker (paramagnetic) properties when hotter than the Curie temperature (~600).Areas with high geothermal gradient have a shallow bottom to magnetic basement,compared to colder areas. The form of magnetic anomalies can thus be usedto map the approximate Curie depth.
14 Quantitative interpretation of magnetic data From the relative spreads of the maxima and minimaof the anomaly the approximate location and horizontalextend of the causative body can be obtained.2. The form of the anomaly, its shape and depth maybe determined.3. The geometrical parameters must be translated intostructural terms in the light of known geology.4. From the amplitude of the anomaly, the magnetizationcontrast can be determined.Sharma, 1976
17 Approximation by a simple dipole magnet vhApproximation bya dipole magnetModeling of a dipole magnet:Vertical (v), horizontal (h) and total (t) intensity
18 Comparison of total intensity anomalies over a simple dipole magnet (different cases) Different depth ofthe dipole magnetDifferent angle ofthe dipole magnetDifferent oriented dipole magnetsDifferent strength ofthe dipole magnet
19 Total intensity anomalies over plates of infinite horizontal extend (different cases) Plates inclined atdifferent angles,magnetized indifferent direction (i),and oriented indifferent directions (s)relative to themagnetic north.
20 Magnetic anomaly interpretation- simple graphical method Reynolds, 1997
21 Igneous intrusion / Salt dome Igneous intrusion – have large amounts of magnetite and high magnetization.Salt – exhibits diamagnetic behaviour and a weak field opposite in direction to the ambient field.
22 Salt dome Gravity anomaly over the Grand Saline Salt Dome,Texas, USA (contours in gravity units)Magnetic anomalies over the Grand Saline Salt Dome,Texas, USA(contours in nT)The stippled area represents thesubcrop of the dome.low densitynegative magneticsusceptibility