Time-Domain Electromagnetics

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Exploration Technique: Time-Domain Electromagnetics

Exploration Technique Information
Exploration Group: Geophysical Techniques
Exploration Sub Group: Electrical Techniques
Parent Exploration Technique: Electromagnetic Sounding Techniques
Information Provided by Technique
Lithology: Detection of rock units or geological features with contrasting apparent resistivity.
Stratigraphic/Structural: Structural information may be inferred from TDEM data.
Hydrological: Hydrological information such as depth to groundwater table may be determined.
Thermal: Extent of hydrothermal alteration mineralogy may be inferred.
Cost Information
Low-End Estimate (USD): 62.356,235 centUSD
0.0624 kUSD
6.235e-5 MUSD
6.235e-8 TUSD
/ mile
Median Estimate (USD): 8,609.42860,942 centUSD
8.609 kUSD
0.00861 MUSD
8.60942e-6 TUSD
/ mile
High-End Estimate (USD): 25,000.002,500,000 centUSD
25 kUSD
0.025 MUSD
2.5e-5 TUSD
/ mile
Time Required
Low-End Estimate: 0.26 days7.118412e-4 years
6.24 hours
0.0371 weeks
0.00854 months
/ 10 mile
Median Estimate: 8.48 days0.0232 years
203.52 hours
1.211 weeks
0.279 months
/ 10 mile
High-End Estimate: 27.77 days0.076 years
666.48 hours
3.967 weeks
0.912 months
/ 10 mile
Additional Info
Cost/Time Dependency: Location, Size, Resolution, Terrain, Weather
Time-Domain Electromagnetics:
Time-domain electromagnetic (TDEM) surveys are active-source soundings which provide information about the electrical structure of the shallow subsurface.
Other definitions:Wikipedia Reegle

Use in Geothermal Exploration
TDEM is used in geothermal exploration predominantly as a complement to other electromagnetic methods, specifically Magnetotellurics. TDEM may assist in resolving the static shift issue in magnetotelluric data. A static shift arises as a distortion of the electrical field due to local, shallow geological heterogeneities. [1][2][3] TDEM can be used as a static shift correction for magnetotelluric data because the technique does not rely on direct electrical measurements to obtain the apparent resistivity profile with depth.

Field Procedures
The equipment involved in a TDEM survey is: a transmitter unit; a receiver unit; transmitting insulated wire; a receiver coil or antenna; and 2 12V batteries (depending on the TDEM instrument). TDEM systems are transportable by backpack with 2-3 people (depending on the wire length and gauge, transmitter type, battery weight, etc.).

The insulated wire is placed on the ground in a loop configuration. The area of the loop depends on the desired depth of investigation for the survey. An electrical current is applied to the transmitter loop, then the current is shut off and the decay response of the secondary magnetic field (see Data Access and Acquisition, below) is measured at the receiver coil. The measurement itself takes a few minutes to acquire, and then the wire can be picked up to move to the next station. TDEM does not require direct contact with the soil for data acquisition.
Environmental Mitigation Measures
TDEM is a non-invasive geophysical technique with negligible impact because direct contact with the ground surface is not necessary. The extent of vegetation may be problematic for the geophysical crews and line cutting may be required in thickly vegetated areas. Access to the stations via truck, backpacking, helicopter, etc., is also a potential environmental impact.
Physical Properties
Data Access and Acquisition
Time-domain electromagnetic surveys are based on the following principles. A steady current is applied to the transmitter loop for a sufficient time period to enable the turn-on transients in the subsurface to dissipate, establishing a static primary magnetic field. The current is then shut off over a given ramp time and, according to Faraday’s Law, the rate of change of the primary magnetic field induces an electromotive force. These secondary, or eddy, currents flow and decay as a circular eddy current ring at successively greater depths; the decay is analogous to the dissipation of a smoke ring and depends on the electrical structure in the vicinity of the measurement.[4] The rate of change of the electrical field due to the decay of eddy currents generates a secondary magnetic field and the magnetic flux with time is measured by the receiver coil. [5]

The decay response of the earth may vary by orders of magnitude depending on the ground’s electrical structure, the target size and depth of the target; because of this, TDEM systems must have a large dynamic range. Eddy current behavior within a conductor is characterized by small initial amplitude and relatively slow decay, whereas for a resistor there will be a high initial amplitude and rapid decay. Since the decay time is highly variant, measurements at a large number of time intervals are employed. Sampling “gates” or measurement windows must be very small, especially at early times directly after shutting off the current in the transmitter loop, to avoid distortion from changes in signal amplitude. [5]

Transient current flow in the subsurface after the transmitted current has been shut off. Note the smoke ring-like dissipation of the transient current flow.[6]

Potential Pitfalls
Transient systems are broadband and as such subject to external cultural noise and interference. Effective exploration depth is directly related to the external noise, loop size and transmitted current magnitude.[5]

Also, TDEM is not as effective in regions with thick, resistive overburden. [3]

Page Area Activity Start Date Activity End Date Reference Material
Time-Domain Electromagnetics At Dixie Hot Springs Area (Combs 2006) Dixie Hot Springs Area

Time-Domain Electromagnetics At Glass Mountain Area (Cumming And Mackie, 2007) Glass Mountain Geothermal Area

Time-Domain Electromagnetics At Haleakala Volcano Area (Thomas, 1986) Haleakala Volcano Area

Time-Domain Electromagnetics At Hualalai Northwest Rift Area (Thomas, 1986) Hualalai Northwest Rift Area

Time-Domain Electromagnetics At Kilauea East Rift Geothermal Area (FURUMOTO, 1976) Kilauea East Rift Geothermal Area 1973 1975

Time-Domain Electromagnetics At Kilauea East Rift Geothermal Area (Skokan, 1974) Kilauea East Rift Geothermal Area 1974 1974

Time-Domain Electromagnetics At Kilauea East Rift Geothermal Area (Thomas, 1986) Kilauea East Rift Geothermal Area 1978 1987

Time-Domain Electromagnetics At Kilauea Southwest Rift And South Flank Area (Thomas, 1986) Kilauea Southwest Rift And South Flank Area

Time-Domain Electromagnetics At Long Valley Caldera Geothermal Area (Nordquist, 1987) Long Valley Caldera Geothermal Area 1986

Time-Domain Electromagnetics At Mauna Loa Northeast Rift Area (Thomas, 1986) Mauna Loa Northeast Rift Area

Time-Domain Electromagnetics At Neal Hot Springs Geothermal Area (Colorado School of Mines and Imperial College London, 2011) Neal Hot Springs Geothermal Area 2011 2011

Time-Domain Electromagnetics At Soda Lake Area (Combs 2006) Soda Lake Area

Time-Domain Electromagnetics At Truckhaven Area (Warpinski, Et Al., 2004) Truckhaven Area

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