Introduction

Resistivity methods were among the first geophysical techniques developed. The basic concept originated 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 resistivity 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 associated 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 arrangement 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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