Magnetic survey

 

The shape of Earth’s magnetic field (EMF) is similar to the field´s shape of a bar magnet (dummy placed in the middle of the earth) with the axis oriented roughly in the direction of the axis of rotation of the Earth (with deflections of about 11 °). EMF intensity at a given point on the Earth’s surface is a total vector T with a certain amplitude and orientation. In the rectangular coordinate system oriented so that the XY plane is tangential to the earth’s surface and the Z-axis is perpendicular to it, can be a total EMF vector disjoined into the components. I – angle (inclination) and D (declination) describe the local orientation of total vector.

EMF in space is influenced by both cosmic magnetic fields and especially by the processes carried on the sun.
EMF intensity in Slovakia is about 47000 nT, the largest amount is in the field of magnetic poles of about 62000 nT, the smallest is near the equator, about 23000 nT.

Daily variations cause changes in intensity to tens of nT, magnetic storms in hundreds of nT, anomalies caused by changing the content of magnetic minerals in geological bodies lead to changes from tens to thousands of nT, and changes greater than ten thousand nT may result in artificial sources, for example various power electrical equipment, metal constructions, etc.

 

Anomalies of the local geomagnetic field, produced by variations in the intensity of magnetization of rock units, are detected in geomagnetic measurements in applied geophysics. Magnetization of rocks is a vector quantity and is characterized by the ability to generate a secondary magnetic field in the external magnetic field, similar to the magnet. Then the total magnetic field is reflected on the surface. Magnetization of rocks is partly due to the induction of its own EMF and partially by the remanent magnetization. Induced magnetization depends primarily on the magnetic susceptibility as well as inducing field (EMF). Remanent magnetization depends on the geological history of rocks. At higher temperatures (such as the Curie temperature – about 600 °) related to the depth of occurrence, rocks lose their magnetic properties.

 

The units used in magnetometry


Magnetic susceptibility, which is dimensionless in SI system, is used to assess the magnetic properties of rocks. The ability of the rocks to magnetize, as well as magnetic induction on the surface of the earth element are expressed in Tesla, respectively its smaller micro and nano multiples.

 

Magnetic properties of rocks

According to the general classification, the rocks (similar to the minerals) can be incorporated into 3 categories according to the magnetic properties:
diamagnetic, paramagnetic and ferromagnetic.

Magnetic susceptibility is negative in diamagnetic substances, so that the intensity of the induced field in it operates in the opposite direction to the intensity of the primary field. It is not interesting substance in terms of localization possibilities by magnetic measurements.

 

Paramagnetic substances have a positive magnetic susceptibility and intensity of the induced field acts in the same direction as the intensity of the primary field. These substances exhibit a magnetic moment also in the absence of the primary field and their knowledge is important for the evaluation of magnetic measurements.

 

Ferromagnetic substances have a positive and high magnetic susceptibility, the intensity of the induced field acts in the same direction as the intensity of the primary field and amplify it. These substances often show significant magnetic moment in the absence of the primary field. Their magnetization takes place according to the famous course called hysteresis curve. They are the most significant in terms of the magnetic survey and the most important minerals include, for example magnetite, ilmenite, pyrrhotite, maghemite, cubanite, hematite, ulvospinel, geothite, lepidocrocite, limonite; garnets and some other minerals such as pyrolusite, polianite, rutile, planterit, cassiterite, greigite may also be included here.

 

Use of magnetometric survey

Geologické mapovanie

Geological mapping

Vyhľadávanie ložísk nerastných surovín
Search for mineral deposits
Vyhľadávanie zakrytých kovových predmetov

Search for the hidden metal objects
Modelovanie tvaru a hĺbky telies

Modelling the shape and depth of bodies
Vyhľadávanie zlomov a iných vertikálnych rozhraní

Searching faults and other vertical linies
Archeologický prieskum

Archeologický prieskum

Archaeological survey

 

Derived maps

Residualmagneticmap - Reziduálne magnetické pole je rozdiel medzi vektorom totálnej intenzity magnetického poľa a IGRF (InternationalGeomagneticReferenceField)

Residual magnetic map – Residual magnetic field is the difference between the vector of total magnetic field intensity and IGRF (International Geomagnetic Reference Field)

Tiltderivative - Tiltova derivácia sa používa na mapovanie plytkých štruktúr pri prieskume nerastných surovín.

Tilt derivative – Tilt derivative is used for mapping shallow structures in minerals exploration.

Analyticsignal - Analytický signál je vhodná metóda pre lokalizáciu hraníc magnetických telies, obzvlášť v prípade veľkej remanentnej magnetizácie

Analytic signal – Analytical signal is a suitable method for locating the border of magnetic bodies, especially for a large remanent magnetization.

 1 Verticalderivative - 1 Vertikálna derivácia je mapa rýchlosti vertikálnej zmeny magnetického poľa s výškou. Poskytuje lepšie predstavu o kontaktoch hornín, zostruje anomálie.

1 Vertical derivative – 1 Vertical derivative is a map of the vertical velocity changes of the magnetic field with height. It provides a better understanding of contacts of rocks, sharpens the anomaly.

2 VerticalDerivative - 2 Vertikálna derivácie je mapa zmien rýchlosti vertikálnej zmeny magnetického poľa s výškou.

2 Vertical derivative – 2 Vertical derivative is a map of the vertical velocity changes of the magnetic field with height.

Reduced toGeomagnetic Pole - Magnetické anomálie nad symetrickým telesom sa v našich zemepisných šírkach prejavujú nesymetrickými krivkami nad telesom. V rôznych zemepisných šírkach sú tieto krivky rôzne. Symetrický prejav anomálie nad symetrickým telesom je možný len na póle, alebo rovníku. Redukcia na geomagnetický pól je matematický postup, ktorý prepočítava merané hodnoty tak, akoby boli merané na geomagnetickom póle ( zabezpečí symetrický prejav anomálie nad symetrickým telesom).

Reduced to Geomagnetic Pole – Magnetic anomalies over symmetric body in our latitudes reflect as non-symmetrical curves over the body. These curves are different in different latitudes. Symmetrical effect of anomalies over symmetric body is possible only at the pole or the equator. Reduction in the geomagnetic pole is a mathematical procedure that calculates the measured values ​​as if they were measured at the geomagnetic field (it ensures symmetric effect of anomalies over the symmetric body).

Horizontal gradient - Predstavuje rýchlosť zmeny geomagnetického poľa v horizontálnom smere.

Horizontal gradient – It represents the rate of change of the geomagnetic field in the horizontal direction.

 

 

Terrain magnetometric survey

Magnetometer measurements (measurement frequency is typically 1 s) are supplemented by measurements of magnetic variations. Each measurement includes not only a measuring of total magnetic field, but also the parameter of measurements quality and coordinates x, y, z for each measured station.