Resistivity and IP: Introduction
Goals
The goal of this tutorial is to outline the fundementals of galvanic geophysics.
Applicable methods include
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Resistivity profiling (sometimes referred to as Electrical Imaging or EI,
especially when performed with semi-automated acquisition equipment, and
processed with inversion techniques).
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Resistivity sounding.
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Induced polarization.
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.
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The
current injected into the ground is usually a square wave signal that has
positive and negative excursions. This helps account for naturally occuring
signals in the ground that are called sponteneous or self potentials.
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For resistivity, potentials are measured while current is flowing.
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For IP measurements, residual time varying potentails are measured while
the source current is off, between the alternating positive and negative
on times.
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Alternatively, IP measurements can be made using sinusoidal source current
signals at two or more frequencies.
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See the page on IP for more details of measuring
chargeability.
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Both resistivity and IP profiles were traditionally interpreted directly
from the raw data plots called pseudosections. This form of data presentation
will be outlined later in the tutorial. Formal inversion of 2D profiles
is currently the most useful way of extracting as much information from
data as possible, and 3D inversion methods will soon make it attractive
to perform fully three dimensional surveys.
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More specifics on pseudosections are provided on the pseudosections
page.
Mineral exploration surveys:
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Electrode spacings are typically 20 m - 50 m, although smaller or larger
scale work is sometimes carried out.
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Transmitters capable of sourcing constant currents of several amps at several
hunderds of volts are required, so light weight portable systems are rarely
adequate.
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Potentials are usually measured at six or eight electrode pairs simultaneously
using a portable, automated receiving system that can synchronize with
transmitted waveforms, and that can save data in digital form.
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Induced polarization measurements are almost always carried out simultaneously
with resistivity measurements. Time domain and frequency domain are equally
popular, and the choice of which type to employ depends on several factors.
It is beyond the scope to cover this material here.
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Resistivity and IP surveys are considered rather expensive. Logistics of
placing electrodes can be time consuming, and making measurements at the
necessary large number of array configurations requires several operators.
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There have been recent advances in mineral exploration methods which emulate
to some extent the methods developed for small scale work, but most are
proprietry systems not available for general application.
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!
|
.
Engineering and environmental surveys:
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Electrode spacings are more typically 1 m - 20 m.
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Portable battery operated transmitters sourcing less than an amp at a few
hundred volts are usually adequate.
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There are several automated, multiple electrode, receiving systems available
today, vastly improving the efficiency of performing surveys. The limitation
is that the number of electrodes and their spacing is constrained by the
special purpose cables required.
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In addition to saving setup and surveying time, automated systems allow
a wider range of source - receiver geometries, thus permitting more complete
data sets than usually obtainable in mineral exploration contexts.
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Induced polarization is not often recorded for these purposes, although
there is a growing recognition of situations where chargeability is likely
to be a useful.
Example of an engineering system laid out from the manufacturer's
web page.
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Typical raw pseudosection for a small scale engineering survey.
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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.