Geoscience Reference
In-Depth Information
5
Resistivity Methods
Barry J. Allred, Douglas Groom, M. Reza Ehsani,
and Jeffrey J. Daniels
ContentS
5.1 Introduction ........................................................................................................................... 85
5.2 Ohm's Law and Resistivity (or Electrical Conductivity)....................................................... 86
5.3 Factors Influencing Resistivity (or Electrical Conductivity) in Soil Materials ..................... 86
5.4 Theoretical Current Flow in a Homogeneous Earth and Apparent Resistivity .................... 91
5.5 Common Electrode Arrays and Corresponding Apparent Resistivity Equations................. 93
5.6 Conventional Equipment....................................................................................................... 95
5.7 Computer-Controlled MultiElectrode Systems..................................................................... 95
5.8 Continuous Measurement Systems ....................................................................................... 96
5.8.1 Galvanic Contact Resistivity...................................................................................... 96
5.8.2 Capacitively Coupled Resistivity ............................................................................... 98
5.9 Field Data Collection Modes............................................................................................... 100
5.10 Data Analysis Products ....................................................................................................... 101
5.11 Resistivity Method Applications in Agriculture ................................................................. 106
References ...................................................................................................................................... 107
5.1 IntRodUCtIon
Resistivity methods were among the first geophysical techniques developed. The basic concept orig-
inated with Conrad Schlumberger, who conducted the initial resistivity field tests in Normandy,
France, during 1912 (Sharma, 1997). As an historical note, Conrad Schlumberger and his brother,
Marcel Schlumberger, later founded the global oilfield services company, Schlumberger Limited,
which has been a leader in the development of borehole geophysical systems. Resistivity methods
were originally employed in the petroleum and mining industries, and afterward found use in arche-
ological, hydrological, environmental, and geotechnical investigations. Development of continuous
resistivity measurement techniques in the late 1980s and early 1990s has transformed the basic resis-
tivity method into an effective and efficient tool to assess soil conditions in large agricultural fields.
The resistivity method, employed in its earliest and most conventional form, uses an external
power source to supply electrical current between two “current” electrodes inserted at the ground
surface. The propagation of current in the subsurface is three-dimensional, and so, too, is the associ-
ated electric field. Information on the electric field is obtained by measuring the voltage between a
second pair of “potential” electrodes also inserted at the ground surface. The two current and two
potential electrodes together compose a four-electrode array. The magnitude of the current applied
and the measured voltage are then used in conjunction with data on electrode spacing and arrange-
ment to determine a subsurface resistivity value. The resistivity measurement obtained is a bulk
value representing a continuous volume of the subsurface beneath the four-electrode array.
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