introduction to ip -...
TRANSCRIPT
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.
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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
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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
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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