Magnetotelluric Techniques

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Exploration Technique: Magnetotelluric Techniques

Exploration Technique Information
Exploration Group: Geophysical Techniques
Exploration Sub Group: Electrical Techniques
Parent Exploration Technique: Electromagnetic Sounding Techniques
Information Provided by Technique
Lithology: Rock composition, mineral and clay content
Stratigraphic/Structural: Detection of permeable pathways, fracture zones, faults
Hydrological: Resistivity influenced by porosity, grain size distribution, permeability, fluid saturation, fluid type and phase state of the pore water
Thermal: Resistivity influenced by temperature
Dictionary.png
Magnetotelluric Techniques:
Magnetotellurics is an electromagnetic geophysical method used to image the electrical resistivity structure of the subsurface through the measurement of electrical and magnetic fields at the earth's surface.
Other definitions:Wikipedia Reegle


 
Introduction
‘Magneto’ refers to magnetic fields and ‘telluric’ refers to electric currents in the subsurface, derived from tellus (Latin for ‘earth’). The fundamentals of magnetotellurics (MT) for a 1D structure were established by Tikhonov, Cagniard and Rikitake and the first papers were published on MT theory in the early 1950’s. The term magnetotellurics was coined by Louis Cagniard in 1953.

 
Use in Geothermal Exploration
Magnetotellurics is a classical technique applied toward geothermal exploration due to its sensitivity to signatures of geothermal systems and its superior depth of investigation relative to other geophysical techniques.[1]

The MT method is capable of resolving the low resistivity signature associated with the clay cap developed due to hydrothermal alteration resulting from fluid circulation within a geothermal system. This clay cap has a low permeability and acts as a trap over high temperature geothermal reservoirs. The type of clay mineral formed due to hydrothermal alteration largely depends on the temperature of the fluid (see Electrical Techniques: Use in Geothermal Exploration).

The resistivity profile with depth as given in an MT sounding can assist in detecting the geometry and depth of the clay cap, and also in determining the boundary between the alteration zone and the geothermal reservoir. The clay cap composition typically consists of a conductive clay from the smectite group. At the temperature transition at the base of the smectite clay cap, the more electrically resistive illite and chloride are present, indicating the top of the permeable geothermal reservoir. Thus the MT method is capable of providing information on the depth to the potential reservoir, the thickness of the clay cap and the geometry of the reservoir based on electrical resistivity modeling.[2]

3D MT resistivity model at Glass Mountain Known Geothermal Area [3]

 
Environmental Mitigation Measures
MT surveys have very minimal impacts on the environment. The small holes dug for the electrodes should be refilled when the equipment is removed at the completion of the recording.

Means of access to the survey location (4WD, helicopter, ATV, etc.) are another potential impact from an MT survey.
 
Physical Properties
The depth of penetration of an electromagnetic (EM) wave, known as the skin depth, is dependent on its period (in seconds) and the electrical resistivity of the subsurface. Conceptually, MT measurements represent skin depth as a function of period and allow for the apparent resistivity distribution of the subsurface to be determined. Magnetotelluric techniques encompass a wide range of penetration depths and are capable of imaging as deep as the upper mantle.[4]

The main component of the earth’s magnetic field is due to magnetohydrodynamic interactions within the earth’s outer core. Additional components contributing to the overall magnetic field of the earth result from variations in solar winds, solar flares, and lightning discharges. Magnetotellurics takes advantage of these fluctuations in the geomagnetic field due to solar and meteorological activity as the source of the signal for the method.[4] [5]

The passive MT method is based on the following principles. The time-varying magnetic field described above induces an electric field within the earth in accordance with Faraday’s Law of EM induction. The total electrical current in the subsurface can be determined through the measurement of fluctuations in the magnetic field at the earth’s surface, based on Ampere’s Law. The electrical current is also measured directly at the earth’s surface to obtain electrical resistivity at a specific location.[4]

Magnetotelluric techniques can be categorized based on the period of the source EM wave. EM fields with periods shorter than 1 second (frequency of 1 Hz) occur due to lightning discharge activity typically generated in equatorial regions within the tropics. Periods longer than 1 second result from ionospheric and magnetospheric interactions with solar winds. The greatest fluctuation in geomagnetic field strength is on the order of a few hundred nanoteslas (nT) [1 gamma = 1 nT] and this occurs during magnetic storms.[4] [5]

The daily geomagnetic index is provided by NOAA and allows for the monitoring of solar activity and therefore MT signal strength; the daily K-index can be found at:

http://www.swpc.noaa.gov/ftpdir/indices/DGD.txt

The electric and magnetic field data are recorded versus time and then converted into the frequency domain to determine the apparent resistivity versus period (see figure below). The magnetotelluric response function is a tensor which relates the electric and magnetic fields to the electromagnetic impedance and allows for the determination of electrical resistivity as a function of depth.[4]

Also, see Electrical Techniques: Physical Properties

Apparent resisivity and phase versus period, compared with results from TDEM survey and the static shift concept [3]



 
Best Practices
• Remote referencing techniques should be utilized to reduce the effects of local noise sources on MT data.

• A parallel sensor test should be performed at the beginning of the survey to QC the magnetometers, instrument channels ad verify all equipment appears to be functioning properly.

• Salt water and bentonite may be used during the electrode installation to reduce contact resistance at the electrode-soil interface.

• All wires and electrical connections should be carefully monitored for corrosion and replaced or re-spliced if corrosion exists.

• MT stations should be moved as far from cultural noise sources as possible while maintaining an even grid for the MT stations.

• Wires must be as close to the ground as possible for overnight recordings to reduce or prevent wind noise or animals tripping on the cables.

• Wires should be closely checked when picking up each station for chew marks or breaks in the wire from animals nibbling or chewing on the wires.

• If it is possible to view the time series in the field, it is recommended to run a brief test on the equipment and setup as a QC check prior to initiating the overnight recording.

 
Potential Pitfalls
• Magnetotelluric surveys are sensitive to cultural noise such as powerlines, pipelines, electric fences, and any other source of EM noise.

• Static shifting is an inherent issue due to near-surface inhomogemeities and distorts the magnetotelluric response. Static shifting techniques have been applied utilizing TDEM surveys as shown in the figure above.






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