Ground Magnetics

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Exploration Technique: Ground Magnetics

Exploration Technique Information
Exploration Group: Geophysical Techniques
Exploration Sub Group: Magnetic Techniques
Parent Exploration Technique: Magnetic 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.
Cost Information
Low-End Estimate (USD): 160.9016,090 centUSD
0.161 kUSD
1.609e-4 MUSD
1.609e-7 TUSD
/ mile
Median Estimate (USD): 2,835.68283,568 centUSD
2.836 kUSD
0.00284 MUSD
2.83568e-6 TUSD
/ mile
High-End Estimate (USD): 18,000.001,800,000 centUSD
18 kUSD
0.018 MUSD
1.8e-5 TUSD
/ mile
Time Required
Low-End Estimate: 0.60 days0.00164 years
14.4 hours
0.0857 weeks
0.0197 months
/ 10 mile
Median Estimate: 3.09 days0.00846 years
74.16 hours
0.441 weeks
0.102 months
/ 10 mile
High-End Estimate: 8.63 days0.0236 years
207.12 hours
1.233 weeks
0.284 months
/ 10 mile
Additional Info
Cost/Time Dependency: Location, Size, Resolution, Terrain, Weather, Permitting Restrictions
Dictionary.png
Ground Magnetics:
The surface magnetic method is the study of the distribution of magnetic minerals in the upper 20-30km of the earth's crust, recorded at an observation point on the earth's surface.
Other definitions:Wikipedia Reegle



 
Use in Geothermal Exploration





 
Field Procedures
A surface magnetic survey is a low-impact, non-invasive geophysical technique. An instrument called a magnetometer is used for the measurement. There a few varieties of magnetometers used in geophysical exploration (See Data Acquisition below) and all are transportable by one person in the field using a backpack. The measurement can be taken at one point or in a continuous profile and causes no surface disturbance.

Geomatrix G-858 Magnetometer[2]

 
Environmental Mitigation Measures
The largest impact would be due to accessing the stations, by foot or vehicle or whatever means of transport is used. The measurement itself is non-invasive and there is no disturbance to the topsoil.
 
Physical Properties
 
Data Access and Acquisition
The survey design for a ground magnetics survey depends on the anticipated target size and depth. Stations are generally laid out in a grid or profiles perpendicular to the strike. The station interval and line spacing are the two primary parameters for a magnetic survey.

The data must be corrected for: diurnal effects relating to temporal variations in the geomagnetic field; the normal-field correction to account for variations in geomagnetic intensity with latitude and longitude; and elevation and terrain corrections.[3] A base station magnetometer may be utilized to continuously monitor diurnal variation.

There are a few varieties of magnetometers used for geophysical exploration, outlined below.

• Fluxgate magnetometer- The fluxgate magnetometer consists of two identical ferromagnetic cores with a primary winding about each core. The primary windings are connected in series but in opposite orientations, and an AC current is applied to the windings which inversely saturates the cores. A secondary coil is wound around both cores; this coil will not experience an induced current in the absence of an external magnetic field. However when an external magnetic field is encountered, a net current is induced in the secondary coil which depends on the rate of change of the magnetic flux along the axis of the cores. The voltage output of the secondary coil is proportional to the amplitude of the earth’s external magnetic field. A fluxgate magnetometer typically contains three pairs of cores in order to measure the three components of the earth’s magnetic field <x,y,z>. The orientation or alignment of the instrument is crucial to the accurate measurement of the field.[1][3]

• Proton-precession magnetometer- A proton-precession magnetometer utilizes a sensor element containing a liquid with a large amount of hydrogen nuclei (water, decane, benzene). This sensor element is surrounded by a coil through which a direct current is applied. The applied DC current induces a polarizing magnetic field and results in an excitation within the fluid. This causes a transition of the molecules into a higher energy state, resulting in displacement of the protons in the fluid out of alignment with the earth’s magnetic field. Once the DC current is switched off, an electromagnetic field is generated and the protons will precess about the direction of the earth’s ambient magnetic field before returning to the original alignment. The resonance, or precession, frequency (~2kHz) is directly proportional to the intensity of the earth’s magnetic field. When the measured precession frequency is divided by a constant, the gyromagnetic ratio of the proton, the strength of the earth’s total magnetic field is determined.[1][3]

• Alkali-vapor magnetometer- The alkali-vapor magnetometer is also known as the optical absorption magnetometer. An alkali metal such as cesium, rubidium, or potassium is subjected to optical pumping where the electrons are brought to a higher energy level through the application of the proper frequency of light. The glass cell in which the vapor is contained becomes transparent during this process as no more electrons remain to absorb the applied radiation. A radio-frequency sweep is then applied to the glass cell and at the appropriate output frequency, the electrons return to a lower energy state and the glass cell becomes opaque once again. The output frequency is measured with a high level of accuracy, making the alkali-vapor magnetometer a very sensitive instrument (to the order of 0.01 nT).The precise frequency at which the electrons return to a lower energy level depends on the external field and is used to derive the earth’s magnetic field strength.[1][3]
 
Best Practices
In the survey design planning:
• The line direction should be positioned perpendicular to the dominant geologic strike direction
• Measurement spacing should be designed to include at least five magnetic measurements per anomaly
• Line spacing and station interval need to be spaced finely enough to characterize spatial distribution of anticipated anomalies








 
References


Page Area Activity Start Date Activity End Date Reference Material
Ground Magnetics (Nannini, 1986) Unspecified


Ground Magnetics At Alum Area (DOE GTP) Alum Geothermal Area


Ground Magnetics At Blue Mountain Geothermal Area (U.S. Geological Survey, 2012) Blue Mountain Geothermal Area 2008 2009


Ground Magnetics At Chena Geothermal Area (Kolker, 2008) Chena Geothermal Area 1973 1974


Ground Magnetics At Chocolate Mountains Area (Alm, Et Al., 2010) Chocolate Mountains Area


Ground Magnetics At Coso Geothermal Area (1984) Coso Geothermal Area 1984 1984


Ground Magnetics At Cove Fort Area (Warpinski, Et Al., 2002) Cove Fort Geothermal Area


Ground Magnetics At Cove Fort Area (Warpinski, Et Al., 2004) Cove Fort Geothermal Area


Ground Magnetics At Cove Fort Area - Vapor (Warpinski, Et Al., 2002) Cove Fort Geothermal Area


Ground Magnetics At Cove Fort Area - Vapor (Warpinski, Et Al., 2004) Cove Fort Geothermal Area


Ground Magnetics At Crump's Hot Springs Area (DOE GTP) Crump's Hot Springs Geothermal Area


Ground Magnetics At Dixie Valley Geothermal Area (Iovenitti, Et Al., 2013) Dixie Valley Geothermal Area 2012 2012


Ground Magnetics At Kilauea East Rift Area (Leslie, Et Al., 2004) Kilauea East Rift Area


Ground Magnetics At Kilauea East Rift Geothermal Area (FURUMOTO, 1976) Kilauea East Rift Geothermal Area 1975 1975


Ground Magnetics At Kilauea East Rift Geothermal Area (Leslie, Et Al., 2004) Kilauea East Rift Geothermal Area 1998 1998


Ground Magnetics At Marysville Mt Area (Blackwell) Marysville Mt Area


Ground Magnetics At Neal Hot Springs Geothermal Area (Colwell, Et Al., 2012) Neal Hot Springs Geothermal Area 2011 2011


Ground Magnetics At Neal Hot Springs Geothermal Area (Shaltry, 2012) Neal Hot Springs Geothermal Area 2011 2011


Ground Magnetics At Neal Hot Springs Geothermal Area (U.S. Geothermal Inc., 2007) Neal Hot Springs Geothermal Area 2007 2007


Ground Magnetics At North Brawley Geothermal Area (Edmunds & W., 1977) North Brawley Geothermal Area 1977 1977


Ground Magnetics At Raft River Geothermal Area (1979) Raft River Geothermal Area 1979 1979


Ground Magnetics At Roosevelt Hot Springs Geothermal Area (Ward, Et Al., 1978) Roosevelt Hot Springs Geothermal Area 1978 1978


Ground Magnetics At San Emidio Desert Area (DOE GTP) San Emidio Desert Area


Ground Magnetics At San Francisco Volcanic Field Area (Warpinski, Et Al., 2004) San Francisco Volcanic Field Area


Ground Magnetics At Silver Peak Area (DOE GTP) Silver Peak Area



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