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Exploration Technique: Electromagnetic Techniques

Exploration Technique Information
Exploration Group: Geophysical Techniques
Exploration Sub Group: Electrical Techniques
Parent Exploration Technique: Electrical 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
Electromagnetic Techniques:
The objective of electromagnetic (EM) techniques is to image the electrical resistivity structure of the subsurface through the measurement of naturally- or artificially-generated electromagnetic fields.
Other definitions:Wikipedia Reegle


 
Introduction
Electromagnetic techniques are utilized for electrical prospecting. The imaging capabilities of EM methods range from near-surface applications to the delineation of the crust-mantle boundary.[1] The depth of penetration of an EM wave largely relies on the electrical resistivity of the subsurface.[2]

EM measurements can be taken at the surface, from an airborne platform, aboard a marine vessel, or in the borehole environment. These measurements can be recorded in the time-domain or the frequency-domain (see Field Procedures below), and in an electromagnetic sounding or an electromagnetic profiling configuration. Additionally, it is possible to take advantage of natural, passive EM fields as well as artificially-generated, active sources. EM techniques may also be divided into near-field sources (magnetic dipole) and far-field sources (plane wave approximation). [3]

There are a wide variety of EM techniques which can be applied to subsurface investigations and those relevant to geothermal exploration will be discussed. Many possible classifications exist; the convention and heirarchy chosen for EM Methods in OpenEI is shown below in Related Techniques.
 
Use in Geothermal Exploration

 
Field Procedures
Electromagnetic measurements can be recorded in either the frequency-domain or the time-domain.

Frequency-domain electromagnetic techniques obtain measurements by transmitting a sinusoidally varying current at a fixed frequency; the transmitted frequency depends on the anticipated depth of investigation for the survey target.[4]

Time-domain electromagnetic techniques (TDEM) transmit a symmetric square wave current pulse and measure the subsurface response as a function of time when the current is shut off; measurements at later times correspond to the electrical resistivity response at successively greater depths.[4]





 
Physical Properties
See Electrical Techniques

Electromagnetic techniques utilize EM induction processes to measure one or more electric or magnetic field components resulting from transient or artificially-generated alternating current sources. [3] The term ‘electromagnetic’ applies toward techniques which use low-frequency induction. [5] Low frequency EM fields are primarily sensitive to the electrical resistivity of the earth.[6]

The fundamental laws dictating the behavior of electrical and electromagnetic fields are Ohm’s Law, Ampere’s Law, and Faraday’s Law. [1]

Generalized schematic of active, artificially-generated EM induction processes and the behavior of the primary and secondary fields in the vicinity of a conductor[2]


 
Potential Pitfalls









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