Geophysical Services & Advance Technologies Co.Ltd.

Resistivity

Resistivity

HOW GEOPHYSICAL METHODS CAN HELP THE ARCHAEOLOGIST

lateral and vertical mapping of contaminant plumes
location of water-filled bedrock fractures
location of gravel aquifers
vertical electrical soundings for stratigraphic information
mineral exploration
The electrical resistivity method is used to characterize vertical and lateral changes in subsurface electrical properties. Vertical changes are measured using the vertical electrical sounding (VES) technique. Lateral changes are measured using the resistivity profiling technique.

Resistivity profiling is used to map spatial changes in subsurface electrical properties. Applications of a resistivity survey are similar to those of electromagnetic (terrain conductivity) surveys. Resistivity profiling is most commonly used to map contaminated groundwater plumes.

Vertical electrical soundings (VES) are used to laterally trace clay layers, and in conjunction with borehole data, to characterize electrically distinct layers.

Constraints: VES: Soundings are affected by changes in surface relief and lateral changes in resistivity. The electrode array length is about 10 times the depth of investigation. Profiling: Resistivity profiling is slower and more expensive than EM surveying.

Method: An electrical current is introduced directly (as opposed to inductively as with electromagnetic surveys) into the ground through a pair of electrodes. The resulting voltage difference is measured between another pair of electrodes. The subsurface apparent resistivity is then calculated. Resistivity is the reciprocal of conductivity. Thus, measuring resistivity provides information on ground conductivity.

The vertical electrical sounding (VES) method measures vertical changes in the resistivity of the geological strata. In the field, a series of resistivity measurements are made at various electrode spacings centered on a common point. Sampling depth is increased by increasing electrode spacing.

Data is interpreted using an appropriate computer program. The client is provided with a full report which includes modelled depths, thicknesses and resistivities of subsurface layers.

In the resistivity profiling method, four electrodes are positioned at a fixed distance from each other. A current is introduced between two of the electrodes and a voltage potential is measured between the other two electrodes. The electrode pairs are moved along a surveyed line and the electrical measurements result in a horizontal profile of apparent resistivity. Different electrode spacings can be used to yield a cross-section of resistivity changes with depth.

The final report includes subsurface apparent resistivity values from profile stations. Results may be plotted as profile lines or contour maps (isopleth resistivity map), or in other presentations according to the clients' needs.

RESISTIVITY METHOD
The resistivity method of subsurface exploration is powerful but often tedious to employ unless an automated instrument is available. The method is simple: Current is introduced into the ground through one pair of electrodes. Current flow between these electrodes fans out through the ground in a pattern and intensity that depends on the conductivity of the ground and any stratification or obstacles that lie in the vicinity of the electrodes. A second pair of electrodes is then used to quantitatively measure the voltage pattern on the surface resulting from the current flow pattern of the first set of electrodes. A number of different electrode configurations are used in practice, but in simplest form the operator takes measurements along a straight line ("traverse"), moving his electrodes in pairs. He then repeats the measurements along a parallel line until the area of interest has been covered with a rectangular grid of electrode positions. If multiple electrodes are used and the results recorded automatically at the push of a button, the area to be examined can be searched more efficiently, and also probed at various depths at the same time. (As a rule of thumb, the depth of maximum sensitivity for resistivity sounding is about 1.5 times the electrode spacing in typical arrays). A crew of two can easily study an area of perhaps 1000 square meters in a day. Typical electrode spacings might be 0. 3 to 1.0 meters for shallow targets.

Once the resistivity data has been collected, a simple computer program quickly generates a three-dimensional map of ground electrical resistivity or conductivity. Targets most easily seen on resistivity surveys are cavities or voids, but buried walls and filled trenches can often be mapped. The target depth divided by the diameter of the target should be less than 3 or 4 for best sensitivity, though some experts claim to be able to detect targets with a depth to diameter ratio of 9 or more. Boulders, geological stratifications and water-table depth can also be successfully located by the use of resistivity by selecting appropriate electrode spacing to allow the probing current to enter the ground to the appropriate depth. Resistivity meters employed in oil prospecting are often powered by large generators using very high voltages and electrodes spaced perhaps hundreds of meters or kilometers apart, but instruments suitable for archaeological use are battery powered, easy to use, and usually priced under $1500. Resistivity instruments no different than those used by professional geophysicists, but with fancy labels attached, are often found advertised for five times the price of standard instruments. Let the buyer beware!