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```					Geophysical Field School 2000

Geoelectric DC-Resistivity Soundings and Mappings
1   Why ? (Objective)

Geoelectric (DC-) Resistivity soundings and mappings are geophysical methods to provide an image of
the underground‟s resistivity by non-destructive means. Since the electric resistivity of the underground
depends on properties such as water content, electric resistivity methods help to get information about
groundwater level, moisture distribution near the surface and in artificial buildings like dams etc..

2   When ? (Applications)

Since the (DC-) geoelectric methods provide an image of the electric resistivity these methods are
applied at problems which are closely linked to the electric properties of the underground. The electric
resistivity of a rock sample depends on:

-   its mineral content

-   its porosity (amount and structure)

-   the contained fluid (amount and its resistivity)

Since the electric resistivity depends on more than one parameter the use of known information about the
sample e.g. type of rock usually helps to improve the accuracy of the interpretation.

3   How ? (Getting Data)

For field applications 4-point configurations are used to measure the resistivity of the ground. Between
two electrodes, usually called A and B, a known voltage is set. Due to this voltage (U AB) a current (I)
and a potential field is induced into the ground. With two additional electrodes (M, N) the Voltage (UMN)
between two points can be measured. From the measured current (I), voltage (UMN) and the distribution
of the electrodes the (apparent) resistivity of the underground can be calculated
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3.1   Resistivity Soundings – Schlumberger Layout

As a rule of thumb the measured current and voltage values are increasingly influenced by deeper
parts of the earth when the spacing of the input electrodes (A, B) increases. Since the so called
„Depth of Investigation‟ is mainly influenced by the spacing of electrodes A and B it is easy to
obtain a vertical resistivity profile (“sounding”) by employing different spacings A-B and a
constant spacing between M and N. This is the principle of the Schlumberger-Layout (

Figure 3-1). The POTENTIAL ELECTRODES (M, N) are placed with a fixed spacing (a). The
CURRENT ELECTRODES (A, B) are placed at distances from the potential electrodes which may be
chosen as integer multiples of the inner spacing (a). To cover a large interval of different spacings with a
reasonable number of single measurements usually a (roughly) logarithmic increment is used between
successive spacings, e.g., n = 1, 1.4, 2, 3, 4.5, 6.5, 10, 15, 20, 30, 45, 65, 100. These values are only a
general suggestion. If necessary the number of single measurements can be increased.

For the effective operation in the field at least 3 persons are required (1 operator, 2 electrode slaves). To
have an immediate control of the data quality it is helpful calculate directly the apparent resistivity value
and draw it against AB/2 on a double logarithmic graph. Bad values, e.g., due to bad electrode coupling,
are easily recognised as large deviations from the smooth curve.

Uinp, IAB
UMN
A                                M                     N                                            B
na                     a                               na
2L
2l
Figure 3-1: Principle of a Schlumberger-Layout most suitable for resistivity sounding

  L2  l 2  U MN
   Schlumberger
a              
general: L=AB/2;         l=MN/2                                               2l        I AB

 aSchlumberger    n  n  1  a 
U MN
special case: MN=a; AM=NB=na                                                               I AB
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3.2   Resistivity Mappings – Wenner Layout
If, for some reason, the resistivity in a certain depth is of special interest the electrode configuration
(especially the distance between A and B) remains constant but the whole configuration is moved along
a profile. For such “resistivity mappings” usually the Wenner-Layout (Figure 3-2) is used because of its
easy-to-handle constant spacing (a) between all four electrodes. For smaller spacings 3 persons are
required to take efficient readings (1 operator, 2 electrode slaves). For larger spacings 5 persons may be
necessary (1 operator, 4 electrode slaves)

For a Wenner-Layout the „Depth of Investigation‟ is either (a) or even more conservative (a/2). Keep in
mind that this „Depth of Investigation‟ is only a rough estimate of the physical principles. The definition
of this depth varies and it should only be used to give a (rough) estimate of resistivity distribution or to
facilitate the visualisation of the computed apparent resistivities.

As a quality control for resistivity mappings with a spacing (a) the apparent resistivities (a) are drawn
against the midpoints of the electrode configuration (semi-logarithmic paper). Larger deviations from a
smooth curve should be double-checked since such values might be distorted by bad electrode coupling.

Uinp, IAB
UMN
A                        M                                   N                            B
a                         a                              a
Figure 3-2: Principle of a Wenner-Layout most suitable for resistivity mappings

U MN
a
Wenner
 2   a 
I AB
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3.3   The Machine (GGA 30 – Gleichstrom-Geoelektrik-Apperatur 30)

3.3.1 A little bit of theory
The principle of measuring the electric resistivity of the underground is fairly simple. A known voltage
(UAB) is set between two points. This voltage induces a current (IAB) and a potential field into the
ground. The shape of the potential field depends on the underground‟s resistivity. With two other
electrodes (M, N) the potential difference (Voltage UMN) is measured.

The GGA 30 consists of two portable units, the 30L and 30M. The 30L unit controls the two current
electrodes (A and B) and the 30M unit controls the voltage electrodes (M and N).

When you operate the GGA-30 you will hear 4 clicking sounds for each single measurement. With the
first click the voltage between electrodes A and B is switched on (DON‟T TOUCH THE
ELECTRODES ! Otherwise now would be the time when you recognise it ). The second and third
(double-click) reverses the voltage between A and B (s. Figure 3-3). This is necessary because of the so-
called “self potential” of the underground. Some underground materials show a voltage between the
electrodes M and N even if there is no current induced through A and B. Additionally another voltage-
offset is introduced by an insufficient compensation at the GGA-30 (s. 3.3.2). These voltage offsets
disturb the measurement. Therefore the input voltage (UAB) is reversed and only the absolute difference
(divided by two) between the two readings is used for the calculation of the electric resistivity. The
fourth click switches the voltage off

Input Voltage
(UAB)
Voltage

Output Voltage
Sampling Area                  (UMN)

t1        t2                                                  Time

Figure 3-3: Measuring principle of a single reading with the GGA-30
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3.3.2 Resistivity Unit Setup for Schlumberger Array (The Practice - from Mike)

CAUTION – This Equipment Generates High Voltages and Strong Currents !

DO NOT TOUCH Electrodes or Connecting Wires While Measurements Are Being Taken. !

SET 30L TO GROUND INSPECTION MODE BEFORE RE-PINNING !

3.3.3 Placing Electrodes
1. Layout a tape line, pick the mid point and place one electrode in the ground at this point – this will
be the ground electrode and your reference point – if you have two tapes, zero both at this point

2. The two outer electrodes – current electrodes – are TYPICALLY set at 5 times the distance of the
two inner electrodes – potential electrodes –, so…

3. Initially, place the two inner electrodes – potential electrodes – at 20 cm on either side of the

4. Place the two outer electrodes – current electrodes – at 1 m on either side of the midpoint of the tape
line

5. NOTE: the inner electrodes remain FIXED and the outer electrodes are REPINNED.

3.3.4 Connecting the Transmitter and Receiver
1. Place the 30L and 30M units side by side near the midpoint of the tape line

2. Connect the wire from the 30L unit to the 12 pin socket on the 30M unit

3. Connect the ground wire on the 30L unit to the ground electrode (see point 1)

4. Connect the A and B terminals (upper right on 30L) to the current electrodes, with A going to the left
electrode and B to the right
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5. Connect the M and N terminals (upper right on 30M) to the potential electrodes (N to right one, M to
left one)

3.3.5 Testing and Operation
1. FOR THE 30L - turn the battery selection switch to INTERN – you should hear a click and a very
high pitch continuous background noise – if not, jiggle the switch around until you do.

2. Press the TEST button to test the battery – if it doesn‟t go to the green range, you must recharge the
system or switch to an external battery.

3. Set DC CURRENT to 1 A – Set DC VOLTAGE to 120V and 60V via the toggle switches.

4. Turn the Mode Indicator to GROUND INSPECTION – Press the A BUTTON and adjust the gain to
a level in the middle of the scale.

5. Press the B BUTTON and adjust the gain to a level in the middle of the scale.

6. Turn the Mode Indicator to OPERATION.

7. FOR THE 30M – Turn the POWER to ON – Press the TEST button to test the battery – if it doesn‟t
go to the green range, you must recharge the system or switch to an external battery.

8. Turn the mV KNOB to 1 and adjust the COARSE, MEDIUM and FINE COMPENSATION knobs to
zero the display.

9. Turn the mV KNOB to the next position, zero and repeat through to 10^-4 – You may have to repeat

10. Turn the mV KNOB back to 1 and press START and needle on the measuring scale.

11. Adjust the mV KNOB so that the scale is maximized without going offscale.

12. Press the DISPLAY BUTTONS and record the values for A ( x50) and mV ( x0.5), as well as mV
RANGE SELECTOR, DC CURRENT and DC Voltage.
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13. To select a Voltage of 120 or 300, the DC Voltage toggle switch must be changed from 60V to High
Voltage (lightning bolt).

14. CAUTION - RE-PINNING – ensure unit is set to GROUND INSPECTION before re-pinning.

15. Use the RESTART button for Stacking Measurements – already stored values (s Idt and s Udt) are
not deleted – therefore the measurement accuracy is increased – see 4.4.3 example 2 manual – You
must keep track of the times stacked - divide by this number to get the true measurement.

NOTE ON THE DISPLAY:

If the readout sequence begins with a 9, the connections of either AB or MN must be changed

If three dots appear on the Right Side of the display, reduce the gain via the mV SELECTOR - (Sect
4.4.4)

If three dots appear on the Left Side of the display, reduce the sensitivity of the current measuring
range (Sect 4.4.4) – change DC CURRENT A (1, 0.1, 0.01)

***SEE OPERATION MANUAL IF IN DOUBT***
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4     What’s next ? (Inversion)

So far only the “apparent” resistivity (a) was calculated from the measured Voltage (UMN), the Current
(IAB) and the spacing between the electrodes. This apparent resistivity can be roughly assigned to a
specific depth. This “Depth of Investigation” depends on the employed electrode configuration but
always depends on the distance between the current electrodes (A, B).

For a Schlumberger configuration half of the distance between the current electrodes (AB/2) is used. For
a Wenner configuration a conservative estimate of the “Depth of Investigation” is a/2=AB/6 or more
progressive a=AB/3. Several different definitions of the “Depth of Investigation” exist (see e. g.
Oldenburg, Li, 1999)

4.1    Equivalence principle – Suppression principle
Aside from errors which are related to the field work, ambiguity in sounding interpretation may arise
owing to two factors. The first, known as the principle of equivalence, may be stated as follows: it is
impossible to distinguish between two highly resistive beds of different thickness (z) and resistivity ()
values if the product z is the same, or between two highly conductive beds if the ratio z/ is the same.

The suppression principle states that if a bed is very thin compared to those above and below, its effect
on the sounding curve is insignificant unless its resistivity is extremely high or low.

4.2    Manual Inversion Techniques
Although commercial software is available to compute resistivity models from field measurements
manual inversion techniques can easily be used for the inversion of sounding data on simple resistivity
distributions. Only if the dataset is 2-dimensional or 3-dimensional which accounts for lateral resistivity
variations a computer aided inversion becomes inevitable.

A good description of common manual inversion techniques including all necessary master and auxiliary
curves can be found in Telford, Geldert, Sheriff (1992). A short summary of the methods described there
you will find in the appendices to this manual.
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5     So What ? (Interpretation)

Since we now have employed either our own inversion software or a commercial program we have
produced some nicely coloured pictures of the underground resistivity. Although up to now we have
talked a lot about resistivities it might be hard to believe that usually we are not interested in the electric
resistivities itself. We are more interested in correlating electric resistivity with properties like the
mineral content (type ) of rock, its water content and/or porosity.

5.1    Resistivities from different rock materials
For the interpretation of the inverted resistivities in terms of rock types you usually can rely on the work
of other colleagues. Tables with the electric resistivities of certain rock types can be found in most of the
standard geophysical books. A recent and good reference is Schön, 1996 with an extensive chapter on
the relations of electrical resistivities to other petrophysical parameters (e.g. water content, porosity,
crack density, clay content,....)

5.2    Qualitative Interpretation
In many surveys the interpretation is limited to getting a model of the distribution of rock types (clays,
sandstones, carbonates,....) in the underground or to determine the depth of a specific target horizon (e.g.
groundwater level). A very important but often neglected factor in this kind of interpretations is the
accuracy of the measured values and especially the accuracy of the calculated resistivity model.
Although modern programs usually provide nicely coloured images of the results it remains the
interpreters (Your) task to assess the reliability of these models and to produce a plausible geological
model from the results. In case of a 2-dimensional model it is possible to check the „sensitivities‟ of the
different parts of the model. The sensitivity determines how large the influence of the resistivity in a
given part of the model is on the measurements. A change of resistivity in areas which are far away from
the electrodes will have less influence on the measurements than changes in parts of the model which are
very close to the electrodes. As a consequence the accuracy of a resistivity model decreases with
increasing depth (distance from the electrodes). Since the exact accuracy depends on the exact
circumstances it is not possible to give a universal scheme which is valid for all surveys. If you want to
do a quantitative interpretation more problems crop up.
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6     That’s it ? (Generalised geometric factor, Geoelectric Tomography)

Of course there would be much more to say about the practical application of DC resistivity methods in
the field but the information so far should enable you not only to survive the resistivity experiment at
“Field School 2000” but to understand why you press the buttons on these orange boxes. For those who
are still awake there is another paragraph which might be of some interest.

6.1    Geoelectric Tomography
Since the resistivity distribution in the underground is not usually one- or even two-dimensional as
assumed in the interpretation of single soundings a more sophisticated technique, known as tomography,
became popular in recent years. The simplest tomographic dataset along a profile is a Wenner-mapping
with many different layout parameters (a). Unfortunately such tomographic measurements require many
single readings. A profile with only 20 electrode positions (1 m spacing) requires 57 single
measurements (Table 6-1) to get a complete pseudosection [A pseudosection is the distribution of the
apparent resistivity obtained along a given electrode profile with all possible configurations of a given
array geometry (Wenner, Dipole-Dipole, Pole-Pole,...)]. It is possible to conduct these measurements
with manual placing of the electrodes but large arrays with up to several hundred electrodes or repeated
measurements can only be handled by automatic multi-electrode recording systems. With such a system
a single measurement only last 10 to 15 seconds but all electrodes have to be placed and wired with the
recording unit. For small one-time surveys it is usually not worth the effort to install a complete
electrode array. For monitoring purposes where the surveys are repeated in different time intervals it is a
time saving method.

Table 6-1: Number of single measurements with different Wenner-Layout s along a profile with
20 electrode positions

Wenner-Layout Parameter (a) No. of single masurements
1                            17
2                            14
3                            11
4                            8
5                            5
6                            2
Total: 57
Geophysical Field School 2000

7.1    Books and Papers
Oldenburg, D.W., Li, Y., 1999: Estimating Depth of Investigation in DC Resistivity and IP Surveys,
Geophysics, Vol.64, No. 2, p403-416.

Parkhomenko, E. I., 1967: Electrical Properties of Rocks, Plenum Press [Geophysics Library]

Schön, J. H., 1996: Physical Properties of Rocks, Pergamon Press/ Elsevier [Geophysics Library]

Telford, W.M., Geldart, L.P., Sheriff, R.E., Keys, D.A., 1990: Applied Geophysics, Cambridge
University Press, 2nd edition [Geophysics Library]

userpage.fu-berlin.de/~vrath/mt-n.html:     References for Electromagnetics and Inverse Theory

www.agiusa.com:       inversion software (free Demos), (free) forward modelling software, tutorials,...

http://talus.Mines.EDU/fs_home/tboyd/GP311/MODULES/RES/main.html DC Resistivity Tutorial from
Geophysical Field School 2000
Geoelectric Data Acquisition – Schlumberger Sounding
Location:                                                                M-N Distance [m]:
Date:                                                                    Data-Collecting Slaves:

n        Input           TA4        I-Range         TA5            V-          IAB        UMN           K          AB/2       a [m]          Remarks
Voltage [V] (min. 3-dig.)                                Range                                                                            (e.g.
(min. 3-dig.)                   [mA]       [mV]          [m]         [m]      a  K 
U MN
I AB    stacks)

I[mA]  TA4  I _ Range/stacks  0.05 ; U[mV]  TA5  V _ Range/stacks  0.5    AM  BN  n  MN  K Schlumberger    n  n  1  a
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Geoelectric Data Acquisition – Wenner Mapping
Location:                                                                Layout Parameter (M-N) [m]:
Date:                                                                    Data-Collecting Slaves:

Input           TA4        I-Range         TA5            V-         IAB        UMN         K      Midpoint     a [m]          Remarks
Voltage [V] (min. 3-dig.)                                 Range                                          [m]                        (e.g. stacks)
(min. 3-dig.)                 [mA]        [mV]       [m]                 a  K 
U MN
I AB

I[mA]  TA4  I _ Range/stacks  0.05 ; U[mV]  TA5  V _ Range/stacks  0.5   AM  MN  NB  a  K Wenner  2    a
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Double-Logarithmic Graph (Schlumberger-Sounding)

100

-1
10
a / 1 [ - ]

10-2

10-3 0
10                            101                         102
AB/2 [ m]
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Semi-Logarithmic Graph (Wenner-Mapping)
104

103
Apparent Resistivity [ m ]

2
10

101

0
10
0        25                50                 75   100
Configuration Midpoint [ m ]
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Partial Curve Matching (Telford et al., 1990)

The procedure for matching successive left-to-right segments of a field sounding is as follows:

1. The left-handportion of the field sounding curve, plotted on a transparency of identical log-log scale,
is fitted to as many points as possible on the master, maintaining the respective axes parallel. This fit
provides the location of the first cross or auxiliary point where the field sheet coincides with 1=L=1.
the origin on the master. Hence we obtain 1, z1, whereas the best-fit segment gives (2/1) or 2.
[This segment may be extended beyond the fitted portion along the (2/1) line with pencil for a
check on the next step.]

2. The sounding curve is transferred to the appropriate auxiliary curve set where the cross is placed at
the origin and the same (2/1) curve of the auxiliary as that in step 1 is drawn in pencil on the
sounding.

3. Replacing the sounding curve on the master and maintaining the (2/1) line from step 2 on the
master origin, a second master segment further to the right is fitted to the sounding curve. The second
cross is marked over the master origin, giving e2 and ze2 where ze2=z1+z2 and e2 is related to the
other parameters by z e 2  e 2  z1  z 2   e 2  z1 1  z 2  2 (Clearly e1=1 and ze1=z1 at the first

cross.) We also obtain (3/e2) and hence 3 from the fitted segment.

4. The sound ing curve is returned to the auxiliary and step 2 is repeated.

5. Repeat step 3 to get e3, ze3, as well as 4 from the third cross.

6. Repeat steps 4 and 5 until the sounding curve is completely fitted.

It should be noted that this procedure gives good results only if the bed thicknesses increase rapidly with
depth; in fact, each successive layer should be thicker than he total thickness above. The technique of
partial curve matching, although rather crude compared to complete analysis of the sounding curve by
computing methods, is quite useful in the field to keep abreast of daily measurements and as a control for
the more sophisticated approach later.
Geophysical Field School 2000

Schlumberger Master-Curve (Ascending Type)

102
2/1=100
2/1=80
2/1=60

2/1=40

2/1=20
a / 1 [ - ]

101                                                        2/1=9
2/1=7

2/1=5

2/1=3

100 0                                                      2/1=1
10                     101                         102
AB/2 [ m ]
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Schlumberger Master-Curve (Descending Type)

100                                                       2/1=1/1

2/1=1/3

2/1=1/5
2/1=1/7
-1                                                       2/1=1/9
10

2/1=1/20
a / 1 [ - ]

2/1=1/40
2/1=1/60
2/1=1/80
10-2                                                          2/1=1/100

10-3 0
10                      101                          102
AB/2 [ m]
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Auxiliary Curves fo Schlumberger Soundings (H-Type)

100                                                         2/1=1/1

2/1=1/3

2/1=1/5
2/1=1/7
-1                                                         2/1=1/9
10
a / 1 [ - ]

2/1=1/20

2/1=1/40

2/1=1/60
2/1=1/80
10-2                                                            2/1=1/100

10-3 0
10                      101                           102
AB/2 [ m ]
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Auxiliary Curve (Type A)

102

2/1=100
2/1=80
2/1=60

2/1=40

2/1=20
a / 1 [ - ]

101
2/1=9
2/1=7

2/1=5

2/1=3

100 0                                      2/1=1
10                101              102
z2/z 1 [ - ]
Geophysical Field School 2000

Schlumberger Sounding-Curve (EXAMPLE)

103

2
10
Resistivity [ m ]

101

100 0
10                  101                        102
AB/2 [ m ]                        y

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