Electrical Techniques

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Exploration Technique: Electrical Techniques

Exploration Technique Information
Exploration Group: Geophysical Techniques
Exploration Sub Group: Electrical Techniques
Parent Exploration Technique: Geophysical 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
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Electrical Techniques:
Electrical techniques aim to image the electrical resistivity of the subsurface through the measurement of potentials, currents and electromagnetic fields at the earth's surface.
Other definitions:Wikipedia Reegle


 
Introduction
The early development of electrical prospecting techniques, the concept of “seeing” through the ground using an electrical field, was initially theorized as early as 1830 by Robert W. Fox and was commercially applied by Conrad Schlumberger in 1912. Since that time, developments in theoretical principles, field hardware and numerical techniques have enabled substantial improvements in modeling capabilities such as inverse solutions in two and three dimensions.[1][2]
 
Use in Geothermal Exploration
Geothermal systems are typically characterized by a low electrical resistivity signature relative to the surroundings. [3] Low resistivity in a geothermal reservoir can result from high ionic concentrations in geothermal brines, increased temperature of the fluid and the rock, and most importantly the hydrothermal alteration resulting from exposure to high temperature fluid circulation.

Hydrothermal alteration yields clay mineral formation (such as illite, smectite, and chlorite); the clay type developed depends upon the temperature of the reservoir. Conductive clay minerals such as smectite and mixed layered clays form at reservoir temperatures of 50-230 degrees Celsius. However, at higher temperatures, more resistive alteration minerals are formed and this applies to clay minerals such as chlorite and epidote.[4] Hence, an increase in resistivity beneath a conductive layer representative of a hydrothermally altered zone is a typical characteristic of a geothermal field. [5] This signature of a geothermal system is very well-suited to detection through the application of electrical methods.

Additionally, downhole resistivity measurements may enable fracture characterization in a geothermal reservoir based on the resistivity distribution in the geothermal field due to the contrast between rock and fluid. [3]

 
Field Procedures
Electrical and electromagnetic measurements can be obtained through sounding and profiling techniques. A sounding examines the contrast in apparent resistivity with depth, while profiling (or mapping) is concerned with lateral changes in apparent resistivity.

Field equipment set up for a 2D electrical resistivity survey. [6]

 
Physical Properties
Ohm's Law is the physical principle which relates electrical current, voltage and resistance in the following mathematical relationship:

V = I x R, where:

Current = I = rate of flow of electric charge through a cross-sectional area (measured in amps [A])
Voltage(a.k.a. Potential)= negative of the work per unit charge performed by the electric field over a given distance (measured in volts [V])
R = Resistance = ability of a material to obstruct electrical current flow (measured in ohms [Ω])

Ohm's Law states that the electrical current flowing through a conductor is proportional to the potential (voltage) difference and inversely proportional to the resistance of the material.

Electrical resistivity is an intrinsic property of a material and the unit of measurement is the ohm meter (Ωm).[7] Resistivity is a measure of the ability of current to flow through a volume of earth material, which is the product of the resistance multiplied by a geometric factor to account for the subsurface volume. [8] The apparent resisitivity is the average resistivity over an equivalent uniform half-space. There exist a wide range of typical electrical resistivities of earth materials, shown in the table below.

Electrical properties of rocks are affected by rock composition; clay and mineral content; porosity; fluid type, salinity and saturation; temperature; and grain size distribution, among others. [9]

Army Corps of Engineers, 1995[9]

 
Data Access and Acquisition
Electrical techniques consist of a few subcategories including self-potential, electrical resistivity, and electromagnetic methods. The self-potential technique is based on naturally occuring voltage distribution in the subsurface and is a passive technique. Electrical resistivity methods directly inject electrical current into the ground as an active source for the survey, while electromagnetic methods utilize either passive or active electromagnetic induction processes. [9]

 
Best Practices
Raw electrical field data always have to be properly interpreted in order to derive models of the electrical structure of the subsurface. Most modern interpretation methods involve either or both forward modeling or inversion, In forward modeling, as proposed resistivity model of the subsurface is constructed and the response of this model to the particular electrical method being used is computed for comparison with the observed data. The model is adjusted until there is a sufficient match to the observed data. In the inversion method, the process of comparison is done within the computer program. Electrical interpretation software is very sophisticated and must be used by experienced experts for reliable results. As noted below, the models yielded are almost always non-unique, but independent geological or geophysical data can be used to reduce non-uniqueness.
 
Potential Pitfalls
Developing a model of subsurface electrical properties from surface electrical measurements is what is termed an ill-posed problem. The problem is ill-posed when there are a multitude of potential solutions resulting in non-uniqueness. The issue of ambiguity and non-uniqueness may be reduced through the introduction of constraints to the model and a geological understanding of the system. An integrated approach with multiple geophysical data sets reduces this ambiguity as well. [1]








 
References
  1. 1.0 1.1 Michael S. Zhdanov. 2010. Electromagnetic geophysics: Notes from the past and the road ahead. Geophysics. 18.
  2. S. Ward. 1980. Electrical, electromagnetic, and magnetotelluric methods. Geophysics. 45(11):1659–1666.
  3. 3.0 3.1 Lilja Magnusdottir,Roland N. Horne. 2011. Characterization of Fractures in Geothermal Reservoirs Using Resistivity. In: Thirty-Sixth Workshop on Geothermal Reservoir Engineering; 2011/02/01; Stanford, California. Stanford, California: N/A; p.
  4. Benedikt Steingrímsson. 2011. Geothermal Well Logging: Geological Wireline Logs and Fracture Imaging. In: United Nations University Geothermal Training Programme. Short Course on Geothermal Drilling, Resource Development and Power Plants; 2011/01/16; Santa Tecla, El Salvador. Reykjavik, Iceland: Iceland GeoSurvey; p.
  5. H.M. Bibby,G.F. Risk,T.G. Caldwell,S.L. Bennie. 2005. Misinterpretation of Electrical Resistivity Data in Geothermal Prospecting: a Case Study from the Taupo Volcanic Zone. In: Geological and Nuclear Sciences. World Geothermal Congress 2005; 2005/04/24; Antalya, Turkey. New Zealand: ?; p. 1-8
  6. USGS. Integrated Surface Geophysical Methods for Characterization of the Naval Air Warfare Center, New Jersey [Internet]. 2013. [updated 2013/01/03;cited 2013/11/22]. Available from: http://water.usgs.gov/ogw/bgas/toxics/NAWC-surface.html
  7. J.D. McNeill (Geonics Limited). 1980. Electrical Conductivity of Soils and Rocks. TN-5 Edition.  ?: Geonics Limited. Report No.: TN-5.
  8. Rhett Herman. 2001. An introduction to electrical resistivity in geophysics. American Journal of Physics. 69(9):943-952.
  9. 9.0 9.1 9.2 N/A. 1995. Chapter 4: Electrical and Electromagnetic Methods. N/A: N/A. 4-1-4-57p.


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