Frequency-Domain Electromagnetic Survey

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Exploration Technique: Frequency-Domain Electromagnetic Survey

Exploration Technique Information
Exploration Group: Geophysical Techniques
Exploration Sub Group: Electrical Techniques
Parent Exploration Technique: Electromagnetic Profiling Techniques
Information Provided by Technique
Lithology: Detection of high-conductivity bodies in the subsurface.
Thermal: Detection of the presence of a thermal anomaly through its resistivity signature.
Cost Information
Low-End Estimate (USD): 2,928.38292,838 centUSD
2.928 kUSD
0.00293 MUSD
2.92838e-6 TUSD
/ mile
Median Estimate (USD): 4,505.20450,520 centUSD
4.505 kUSD
0.00451 MUSD
4.5052e-6 TUSD
/ mile
High-End Estimate (USD): 7,079.60707,960 centUSD
7.08 kUSD
0.00708 MUSD
7.0796e-6 TUSD
/ mile
Time Required
Low-End Estimate: 9.12 days0.025 years
218.88 hours
1.303 weeks
0.3 months
/ 10 mile
Median Estimate: 16.89 days0.0462 years
405.36 hours
2.413 weeks
0.555 months
/ 10 mile
High-End Estimate: 27.35 days0.0749 years
656.4 hours
3.907 weeks
0.899 months
/ 10 mile
Additional Info
Cost/Time Dependency: Location, Size, Resolution, Terrain, Weather
Frequency-Domain Electromagnetic Survey:
Frequency-domain electromagnetic techniques are continuous wave field methods which enable the mapping of the electrical conductivity of the subsurface through electromagnetic induction.
Other definitions:Wikipedia Reegle

Lawrence Berkeley National Laboratory (LBNL) has conducted geothermal exploration projects utilizing frequency-domain electromagnetics. In general, frequency-domain EM tools (such as the Geonics EM-31, EM-38 and EM-34) are suitable for shallow geophysical investigations for mapping soil conductivities and detection of buried conductors. [1] However, LBNL has applied a prototype frequency-domain EM induction system, the EM-60, to geothermal exploration projects in Nevada with a depth of investigation ranging from 2-5km. [2]
Use in Geothermal Exploration
Frequency-domain EM tools such as the LBNL EM-60 are able to resolve the depth to conductors which may be associated with thermal anomalies. [3] Current literature or case studies regarding the application of the EM-60 to geothermal exploration are limited.

Field Procedures
The field equipment required for the EM-60 EM induction survey is the following. A Hercules gasoline engine runs an aircraft alternator; this transmitter is mounted in a one-ton four-wheel drive truck bed. The transmitter loop design considerations such as wire gauge, length and weight are survey-specific. The LBNL experiment utilized four turns of #6 wire at a total length of 1,372m for a 100m diameter loop. A magnetometer records the earth response at various stations in the vicinity of the current transmitting loop. [4]

This technique is less expensive and less time-consuming than a Direct-Current Resistivity Survey or Magnetotellurics. [5]
Environmental Mitigation Measures
The installation of the transmitting loop and access to the receiver station locations will have the greatest surface impact and depend on the ease of access to the field area. Gasoline usage for the generator is also a consideration.
Physical Properties
Frequency-domain EM induction tools obey Faraday’s Law of EM Induction. The measured quantities are the amplitude and phase of the secondary magnetic field.
Data Access and Acquisition
A transmitter loop (of #6 AWG copper welding cable with four turns and a 50 m radius) is laid out on the ground surface. The EM-60 transmitter (60 kW, 400 Hz, 3 phase alternator) applies square wave alternating current pulses to the transmitting loop at frequencies ranging from 10^-3 to 10^3 Hz and up to 400 A. [2] [4]

The current applied to the transmitting loop generates a primary magnetic field; this primary magnetic field induces secondary electrical eddy currents in the subsurface according to Faraday’s Law. The eddy currents, in turn, generate a secondary magnetic field which is measured with a three-component magnetometer (see Ground Magnetics) at pre-defined station spacing, depending on the survey design. The terrain resistivity is inversely proportional to the ratio of the primary and secondary magnetic fields when operating at low induction numbers. [6] The associated maximum depth of investigation for these particular survey parameters is approximately 5 km. [2]

Best Practices
• It is best practice to record multiple receiver measurements relative to the same transmitter loop. This is because the penetration depth of the induction sounding depends on the distance between the transmitter and receiver, as well as the period of the transmitted wave. This practice enables enhanced resolution of both the shallow as well as the deeper information. [5]

• The transmitter-receiver separation is approximately equivalent to the maximum depth of exploration; survey design should take this into account. [2]

• A reference or remote magnetometer can be used in addition to the field measurements as a correction for natural geomagnetic variations. [3]
Potential Pitfalls
• Working in areas with steep changes in topography creates challenges in laying out the transmitter wire, as well as difficulties in processing and interpretation of the collected data due to an inclined loop. [3]

• 2D and 3D computer modeling algorithms not sufficient at the time (1983). [3]

• Insensitivity of the technique to buried resistive bodies. [3]

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