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Resistivity and IP: Introduction

Goals

The goal of this tutorial is to outline the fundementals of galvanic geophysics. Applicable methods include The emphasis will be on profiling. For details on sounding, we recommend the instructional web site called Introduction to Exploration Geophysics, hosted by the Colorado School of Mines. That site also has very good basic material on resistivity in general.

Measurement basics

See our resistivity and IP basics page for a refresher on relations between current, voltage and resistivity. To summarize:

Resistivity is defined as the voltage measured across a unit cube's length (Volts per metre or V/m) divided by the current flowing through the unit cube's cross sectional area (Amps per metre2 or A/m2). The resulting units are Ohm-m2/m or Ohm-m.  This is different from resistance which has units of Ohms.

Conductivity is the reciprocal of resistivity and it's units are Seimens per metre or S/m, (occasionaly referred to as Mhos/m). Milliseimens per metre are sometimes used, where 1000 mS/m = 1 S/m.

Resistivity of a uniform earth can be measured directly by injecting current with a pair of electrodes at one location, and measuring the resulting potentials (voltages) at another location. The resistivity is obtained by recording the current injected, the resulting voltage, and the geometry of the survey array.

Surveying

Mineral exploration and engineering surveys are usually done somewhat differently owing to the differences in the scale of the problems. However, the essentials are the same for both situations. Mineral exploration surveys:

Placing one of the current source electrodes. An operator can carry up to a roughly a kilometer of heavy gauge wire to feed the current electrode.

Sorting wire and equipment to begin a resistivity / ip survey. This is definately not a portable operation!
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Engineering and environmental surveys: Remember that if you have not encountered terms such as apparent resistivity, Ohms law in three dimensions, and the geometric factor (which accounts for the surveying array geometry when resistivity is calculated from current and voltage), then you should spend 10 minutes with our resistivity and IP basics page.

Our next topic concerns how the flow of current is affected by conductive and resistive regions in the earth. We also introduce the important concept of how charge distributions arise at boundaries of materials with differing resistivity.


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