Magnetic Techniques

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Exploration Technique: Magnetic Techniques

Exploration Technique Information
Exploration Group: Geophysical Techniques
Exploration Sub Group: Magnetic Techniques
Parent Exploration Technique: Geophysical Techniques
Information Provided by Technique
Lithology: Presence of magnetic minerals such as magnetite.
Stratigraphic/Structural: Mapping of basement structures, horst blocks, fault systems, fracture zones, dykes and intrusions.
Hydrological: The circulation of hydrothermal fluid may impact the magnetic susceptibility of rocks.
Thermal: Rocks lose their magnetic properties at the Curie temperature (580° C for magnetite) [1] and, upon cooling, remagnetize in the present magnetic field orientation. The Curie point depth in the subsurface may be determined in a magnetic survey to provide information about hydrothermal activity in a region.
Magnetic Techniques:
The magnetic method is the study of the distribution of magnetic minerals in the upper 20-30km of the earth's crust. The magnetic method may also be used to estimate the thickness of the crust or to constrain temperatures in the crust using the Curie isotherm (the temperatures at which minerals lose their strong magnetic properties), whichever is shallower.
Other definitions:Wikipedia Reegle

Magnetic measurements in geophysical exploration record variations in the magnetic field due to lateral variability in the magnetization of the crust. The lateral variation may produce anomalous regions which are indicative of structural or lithological contrasts in the subsurface. These data can be collected at the earth's surface, from the air, the sea or in a borehole environment. [1]
Use in Geothermal Exploration
Magnetic surveys are an effective method to locate a prospective geothermal reservoir. First of all, 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. [2]

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]

Magnetic anomalies from 2006 airborne magnetic survey over the Taupo Volcanic Zone, New Zealand.[4]

Field Procedures
Environmental Mitigation Measures
Magnetic techniques are non-invasive, passive geophysical surveys. The primary impact is due to the means of access to the stations (i.e. by foot, four-wheel drive vehicle, boat, plane, helicopter, etc.)
Physical Properties
The primary component of the earth’s magnetic field originates from convection of liquid iron in the outer core of the earth and the field strength is on the order of ~50,000 nT. Additionally, the earth’s magnetosphere is influenced by diurnal variations and solar winds. The remaining component of the earth’s magnetic field is due to magnetized materials in the upper 20-30km of the earth’s crust.[1]

1Tesla = 10^9 nT = 10^4 gauss = 10^9 gamma

The magnetic susceptibility is the physical property which defines the magnetic characteristics of a rock, i.e. how easily the material can be magnetized. Magnetic susceptibility relies on the volume percent content of ferromagnetic minerals such as magnetite.[5]The image below outlines typical magnetic susceptibilities for various rock types.[6]

Typical range of magnetic susceptibilities for specified rock types.[6]

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