GEOPHYSICS (MAGNETIC SKEMANG) Flashcards
a passive method which measures variation in the existing magnetic field strength oldest and most widely used geophysical tool in mineral exploration
Magnetic
deposits that are applicable to be sujected to Magnetic
Iron Ore Sulfides
Ni-Cu-PGEs
This provides regional coverage of large areas at a comparatively low cost specialy in delineating target areas during reconnaissance
Airborne Potential field sirveys
used as a magnetic sensor fixe to a string in the tail of an aircraft
Bird
Used to tow a sensor behind a ship to remove te magnetic effects of the vessel
Fish
Where is marine potenrial survey used?
Ocean Floor Polymetallic Nodules
The curved orientations of lines in a magnetic field
Lines of Force
Flux of density due to magnetic pole of strength m at dustance R and expressed as the force exerted on a unit positive pole at point P
Magnetic Field B
Units of Magnetics intensity?
Gamma (y) = 10^-9 T or= 1nT
Magnetic intensity at the poles
60,000 y or 60,000 nT
Magnetic intensity at the equator
30,000 y or 30,000 nT
How many degrees is the declination of the geomagnetic poles tp geopgraphical poles?
11.5 deg
The Angle between the Magnetic and Geograhic North
Declination
The Angle made by the point with respect to the horizontal
Inclination
What causes Magnetic Anomaly?
Superimposed presence of magnetic minerals and rocks on the normal geomagnetic field
Rocks’ magnetic suscpetibiloty is depedent on?
Proportion of rock forming ienrals with are either nonmagnetic or very feebly magnetic
Highly Magnetic Minerals
Magnetite, Ilmenite, Pyrrhotite
Hmeatite with a crystal form of Magnetite
Maghemite
Which type of igenous rocks are usually more magnetic due to higher magntite content
Mafic-Ultramafic
Magnetic propoerty of metamorphic rocks
Varying
Magnetic of property of sed rocks
Non magnetic unless enriched with magnetite bearing sulfide
the response signaled by the causative bodyover regional trends of country rocks
Magnetic Anomaly
Temperature above which magetic minerals lose their magnetic property (Curie Temo)
600 deg cel
?No electrons are unpaired, lahat may ka pair
?Magnetic susceptibility is negative (Non Magnetic)
?When magnetic field is applied, magnetization is induced but reuces to zero when the field is removed
?Will be repelled when placed near a magnet
Diamagnetic Subastance
?Magnetic field exists due to the spin of unpaired electrons
?Magnetic Susceptibility is Positive and Small (Weakly Magnetic)
?Tend to line up in the presence of magnetic field
?Magnetization depende linearly and reduces to zero on removal of the field
?Will be weakly attracted to Magnet
?Can be observed only at relatively low temp
Paramagnetic
?Dipoles are PARALLEL to the magnetic domains
?Strong Spontaneous Magnetization w/c can exist even w.o external magnetic field
?Magnetic Susceptibility is very High (Strongly Magnetic)
Ferromagnetic
?Dipoles are ANTI-PARALLEL w. equal number of dipoles in each direction
?There is noexternal magnetic field effects due to self cancellation
?Defects in crystal lattice may caused small magnetization caled Parasitic Antiferromagnetism (Non magnetic low temps but can be magnetic at higher temps until Neel Temp)
Antiferromagnetic
?Dipoles are ANTI-PARALLEL w. unequal strength of dipoles thus no self cancellation
?Weakly attracte to magnet
?Can be transformed into Paramagnetism upon heating
Ferrimagnetism
The temperature at which ferromagnetic or ferrimagnetic materials becomes paramagnetic on heating and lose their magnetic property
Curie temp (570 deg cel)
Inherited Magnetization remaining after the removal of applie field
Remanent or Permanent Magnetization
Type of remanent magentization whrein igenous rock solidifes and cools thtough the curie temp of its magnetic minerals
Thermoremanent Magnetization (TRM)
The Magnetic particles of a sediment align within the Earth;s Field during sedimentation
Detrital Remanent Magnetiztaion (DRM)
Magnetic Minerals recrystallize or grow during diaganesis or metamorphism
Chemical Remanent Magnetization (CRM)
Develop slowly in an ambient magnetic field (Cool) as the domain magnetization relax ito the direction of the field
Viscous Remanent Magnetization (VRM)
Path of Magnetiztaion
Hysterysis Loop
Common causes of Magnetic Anomaly
1) Dykes (vertical)
2) Faulted, Folded or truncated sills and lava flows (Horizontal0)
3) Massive Basic Intrusion
4) Met Basement Rocks
5) Magnetite Ore Bodies
Theory which states that Earths Magnetic Field is created continously within the outer core through an intense current loop due to friction caused by earth’s rotation
Dynamo Theory
Magnetic survey instrument used to measure geomagnetic elements based on priniples of a suspended bar magnet in Earth’s Field
Magnetic Variometer
Standard instrument in magnetic surverys which provides reading every 0.1 of a second
High Sensitivity Cesium Vapor Magnetometer
Differential Magnetormeters
Magnetic Gradiometers
Long term variatons in yhe main magnetic field caused by fluid motion in the Earth;s outer core but will not complicatie data reduction efforts
Secular Variations
How can the longterm regional variations in magnetic field be corrected?
Used the IGRF Formula
Cyclical Daily variations in magnetic field strength ranging from 20-60 nT
Diurnal Variations
Maxmimum and Minimum of such daily variations
10 am max
4pm min
How can the effect of diurnal variation be correcte?
Periodic calibration of insturemnts at a fixed base station
Streams of high energy particles associated with sunsport activties striking the upper atmosphere and enormously increasing ionisphere currents
Magnetic Storms
How can this be corrected?
Delay the Survey
Recommended Flight heights for Airborne Magnetic
100-200 m Low Flight
Maps produced by magnetic surveys which can give qualitative existense of the ore bodies
Isocontour Maps
Mining and Econ Geol
Metallic Mineral
Iron Ore with higher magnetite and hematite
Massive Sulfide deposits (w/ EM)
Structural
Extent of Sed Basins for Petroleum Explo
Faulting in Bed rocks
Used to define structural features
