Introduction to IP

Overview

Resistivity and Induced Polarization (IP)

surveys are commonly-used, surface-

based geophysical methods which

provide information about the electrical

properties of the subsurface. These

properties provide information about

faults, fractures, geologic structures,

mineralization and groundwater porosity.

Zonge geophysical equipment used for

Resistivity-IP surveys acquires both data

sets simultaneously.

Resistivity and IP measurements are

made by introducing an electrical current

into the ground between two electrodes

and making measurements of the

induced voltages between two receiver

dipoles. Electrical properties of the

ground can be calculated by comparing

the transmitted signal to the received

signal. A change in ground resistivity

(the ability of the ground to conduct

electrical current) affects the strength of

the received signal. A change in IP (the

ability of ground materials to polarize at

interfaces) affects the shape or timing of

the received waveform.

Variations in moisture content, porosity,

permeability, and soil or rock type affect

resistivity. Cultural features (man-made

objects) such as fences, power lines,

and pipelines can affect ground

resistivity as well.

In contrast to resistivity, there are

relatively few subsurface conditions that

create an IP response. Metallic

mineralization, particularly disseminated

sulphides, causes increased IP values.

Certain dissolved solids in groundwater

and, in some environments, certain clay

types can increase the IP response, if

the abundance of the clay is within type-

specific ranges. As with resistivity, IP

measurements can be influenced by

cultural features.

IP surveys are

commonly used in

minerals exploration

and environmental

studies. Resistivity

surveys are used

extensively in

exploration for

minerals, ground-

water, geothermal,

and hydrocarbons.

Zonge typically

acquires both data

sets in the same

survey using the

same equipment

and array.

WWW.ZONGE.COMWWW.ZONGE.COMWWW.ZONGE.COMWWW.ZONGE.COM

Field Logistics

IP field crews typically include three to

five people with one pick-up truck. No

trenching, drilling or blasting is involved

and the majority of the survey can be

done on foot using backpack-carried

equipment.

The transmitter dipole consists of thin,

14-gauge insulated wire laid on the

ground. At each end, the wire is

connected to the ground with metal

stakes, each about one-half inch in

diameter, that are pounded into the

ground from one to two feet. These

stakes are doused with saltwater to

provide good electrical contact with the

ground. Each grouping is called an

“electrode.” (See figure below.)

Numerous electrodes are set up in a line

and connected by wires to the

transmitter equipment at a central

location. The wire is laid out by walking

along the ground. Vehicle access along

the length of the wire is not necessary

and driving is kept to a minimum. The

transmitter equipment is usually kept in a

pick-up truck with a generator mounted

in the back or on a small trailer behind

the truck. This equipment transmits a

very carefully controlled signal at specific

frequencies into the ground.

An operator makes measurements with

a portable Zonge receiver system by

connecting to different dipoles. The

dipoles for the receiver are also wires

laid on the ground, but for this purpose,

they are grounded using small porous,

ceramic “pots” about six inches tall and

two inches in diameter, buried an inch or

so into the soil.

The receiver equipment is usually

carried by backpack along the survey

line while the measurements are taken

at different locations. By making

measurements at numerous stations

along a line, a cross section of the

earth’s electrical resistivity properties

can be produced, providing information

about subsurface faults, fractures,

geologic structures, mineralization, and

groundwater.

No trenching,

drilling, road-

making, or

blasting is

involved. The

majority of the

survey can be

done on foot

using backpack-

able equipment.

Typical transmitter electrode set-up for IP-Resistivity surveys

WWW.ZONGE.COMWWW.ZONGE.COMWWW.ZONGE.COMWWW.ZONGE.COM

The geometry between the transmitter

and receiver electrodes varies with

different types of electrode arrays.

Zonge receivers support dipole-dipole,

pole-dipole, gradient, Schlumberger, and

Wenner arrays.

A frequently-used configuration in North

America is the dipole-dipole array in

which current is introduced into the

ground at two adjacent (current)

electrodes and the resulting electric field

is measured at two adjacent (potential)

electrodes. The distance between the

current electrodes is generally the same

as the distance between the potential

electrodes.

As the spacing between the two sets of

electrodes is increased, the relative

contribution of responses from rocks at

depth is increased thus providing

information on the variation of resistivity

and IP effects with depth. The depth of

investigation for any given array

configuration is a function of the relative

resistivity contrasts and the electrode

spacing.

Inversion Models

Resistivity-IP surveys measure apparent

resistivity and apparent polarization

(chargeability in the time-domain). The

measured values are a function of the

variation in resistivity and polarization

within a volume of earth and do not

reflect actual values at specific plot

points. The distribution of intrinsic values

for the resistivity and IP effect is then

interpreted from the measurements

using theoretical curves and computer

modeling.

Software programs are used to produce

results more similar to an image of the

subsurface electrical structure. Smooth-

model inversion is a robust method for

converting measured resistivity and IP

data to smoothly-varying model cross-

sections.

“Inversion” refers to mathematically

back-calculating from the measured data

to determine a likely location, size and

depth of the source(s) of IP and

resistivity changes. Resistivity and IP

values in the two-dimensional model

section are iteratively modified until the

calculated data values match observed

data as closely as possible, subject to

some constraints.

Modeling

programs are

used to convert

the measured

results to profiles

of resistivity or IP

versus depth.

Dipole-dipole array commonly used in North America

www.zonge.comwww.zonge.comwww.zonge.comwww.zonge.com

wwwwwwwwwwww....zzzzoooonnnnggggeeee....ccccoooommmm

Final Product

The results of data processing and

modeling can be presented in several

forms: modeled cross sections of

resistivity and IP data, plan views, and

3D fence diagrams.

When data are collected from stations

along several lines in the same area,

data can be displayed in plan-view

plots at a constant elevation or depth.

Plan views help highlight trends

between collection lines.

Reference

Coggon, J. H., “A comparison of IP electrode arrays.” Geophysics, 38, pp. 737-761, 1973.

Roy, A. and Apparao, A., “Depth of investigation in direct current methods.” Geophysics,

36, pp. 943-959, 1971.

Sumner, J. S., “Principles of induced polarization for geophysical prospecting.” Elsevier, 1976.

Tripp, A. C., Hohmann, G. W., and Swift, C. M., “Two-dimensional resistivity inversion.”

Geophysics, 49, pp. 1708-1717, 1984.

Zonge, K. L., Wynn, J., and Urquhart, S., “Resistivity, Induced Polarization, and Complex

Resistivity,” edited by Butler, D.K. Investigation in Geophysics, Vol. 13, pp. 265-300.

Society of Exploration Geophysicists, 2005.

Parts of this document have been extracted from Practical Geophysics II, Northwest

Mining Association, 1992.

For more information, see www.zonge.com/geophysical-methods/

2015-01

Modeled IP results at 700 ft. depth

Fence diagrams show 2D cross sections of IP and resistivity inversion results in

a spatial 3D context.

Zonge International is

an employee-owned

company providing

ground geophysics field

services, consulting and

customized equipment

to geoscientists and

engineers worldwide.

The company is known

for its expertise in the

development and

application of broad-

band electrical and EM

methods.

Tucson, Arizona, USA

1 520-327-5501

Reno, Nevada

1 775-355-7707

zonge@zonge.com