Resistivity & IP methods

Andrew BinleyLancaster University

International PhD Course in

HYDROGEOPHYSICS

Overview

We have demonstrated links between hydrological and geophysical properties and show the potential value of measuring resistivity and induced polarisation (IP) as a means of determining information about hydrological structures and/or states.

Here we present approaches for measurement of resistivity and IP.

We cover: the basic background to the measurement principle; measurement approaches and limitations; example applications.

Resistivity basic measurement principles

Measurements are usually done at low frequency (DC resistivity).

Four electrodes are used: C+ source current, C- sink currentP+ potential measurement (positive) P- potential measurement (negative)

C+ C-P+ P-

Current is injected between C+ and C-The voltage difference between P+ and P- is measured

The voltage difference is a function of the currentinjected and the resistivity beneath the electrode array

-5 -4 -3 -2 -1 0 1 2 3 4 5

-4

-3

-2

-1

0

)()( rδσ IV −=∇⋅∇

C+

Distance (m)

Elev

atio

n (m

)

The voltage V due to current injection I in the subsurface with electrical conductivity σ (=1/ρ)

satisfies:

If σ is uniform then current injection at the surface leads to: r

IV 12

⋅=π

ρ

C+ C-P+ P-

For a pair of current electrodes (dipole) we can then determine the apparent resistivity given the measured potential difference (dipole) for a given injected current.

For the arrangement below: aRa πρ 2=

Where R is the transfer resistance V(P+)-V(P-)I

a a a

C+ C-P+ P-

The apparent resistivity is the equivalent resistivity if the ground is uniform and is a useful way of expressing measured data (and also for plotting surface data as we will see later)

a a a

aRa πρ 2=

-5 -4 -3 -2 -1 0 1 2 3 4 5-5

-4

-3

-2

-1

0

)()( rδσ IV −=∇⋅∇

Distance (m)

Elev

atio

n (m

)

We can also compute the analytical solution to

C+

Even if the electrode is not on the surface. To do this we make use of an imaginary source above the ground and use superposition (just as in pumping test analysis)

0 0.5 1 1.5 2

Time (s)

+I

-I

0

Time (s)0 0.5 1 1.5 2

0

Volta

ge +V

-V

Curr

ent

Vsp

Vp +Vsp

Current is normally injected as a switched square wave

Why is this ?

C+ C-

P+ P-

A M N B

AB

MNAM NB

GeneralC+ C-

P+ P-aa a

WennerA M N B

C+ C-

P+ P-naa

a

Dipole-dipoleA M NB

C+ C-

P+ P-ana na

SchlumbergerA M N B

Various surface measurement configurations

We can profile the subsurface by moving our array

C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-C+ C-P+ P-

The depth we are sensitive to will depend on the array configuration and the subsurface properties. For the array above we may assume that the apparent resistivity is at about half the electrode spacing.

Resistivity profiling

The Pulled Array Continuous Electrical Profiling (PACEP) method was developed by the Aarhus hydrogeophysics group as a cost effective method for spatially dense measurement over large areas.

An electrode array is towed across the field behind a small vehicle and measurements with three sets of electrodes with different separations are performed continuously and simultaneously while actively towing the electrode array.

Soundings - often called vertical electrical soundings (VES) – allow us to build up a 1D profile of the subsurface.

The array spread is progressively increased and the depth of sensitivity increases.

1 10 100 1000AB/2

10

100

1000

ρ a (O

hm-m

)

Resistivity soundings

Inverse methods (see later) are then used to determine a 1D resistivity structure that best matches the data.

10 100 1000

ρ (Ohm-m)

50

40

30

20

10

0

Dep

th (m

)1 10 100 1000

AB/2

10

100

1000

ρ a (O

hm-m

)

VES has been widely used in hydrological investigations to determine lithological boundaries.

10 100 1000

ρ (Ohm-m)

50

40

30

20

10

0

Dep

th (m

)

The method has also been used to monitor dynamic processes, e.g. responses to recharges and travel times of pollutants.

The approach is clearly limited if the 1D assumption is not valid.

Distance (m)

Electrode

Surv

eyle

vel

1357

C+P+

P- C- P+ P- C-C+

5 10 15 20 25 30 35 40 45

Developments in multi-electrode instruments has led to widespread use of resistivity imaging.

Here profiles are measured at different electrode separations (i.e. different survey depths).

Resistivity surface imaging

10090807060504030

0 5 10 15 20 25 30 35 40 45

0 5 10 15 20 25 30 35 40 45

10090807060

0 5 10 15 20 25 30 35 40 45-8-6-4-20

Elev

atio

n (m

)

Distance (m)

Electrode

1357

135

Surv

eyle

vel

Surv

eyle

vel

Distance (m)

Distance (m)

100 Ohm-m10 Ohm-m

Apparent resistivity (Ohm-m)

Apparent resistivity (Ohm-m)

10 Ohm-mSynthetic model

Wenner arraypseudosection

Dipole-dipole arraypseudosection

A pseudosection is built up using measured apparent resistivities

0 5 10 15 20 25 30 35 40 45-8-6-4-20

Elev

atio

n(m

)

Distance (m)

Electrode

100 Ohm-m10 Ohm-m10 Ohm-m

Synthetic model

Wenner arraymodel

Dipole-dipole arraymodel

These data may be inverted (see later) to determine a resistivity image that is consistent with the data

0 5 10 15 20 25 30 35 40 45-8-6-4-20

0 5 10 15 20 25 30 35 40 45-8-6-4-20

1009080706050403020

Resistivity (Ohm-m)

Distance (m)

Distance (m)

Elev

atio

n(m

)El

evat

ion

(m)

0 5 10 15 20 25 30 35 40 45

0 5 10 15 20 25 30 35 40 45

10090807060504030

1009080706050

0 5 10 15 20 25 30 35 40 45-8-6-4-20

Elev

atio

n (m

)

Distance (m)

Electrode

1357

135

Surv

eyle

vel

Surv

eyle

vel

Distance (m)

Distance (m)

100 Ohm-m

10 Ohm-m

Apparent resistivity (Ohm-m)

Apparent resistivity (Ohm-m)

Synthetic model

Wenner arraypseudosection

Dipole-dipole arraypseudosection

Note that the pseudosection doesn’t always show a structure that resembles the subsurface.

0 5 10 15 20 25 30 35 40 45-8-6-4-20

Elev

atio

n (m

)

Distance (m)

Electrode

100 Ohm-m

10 Ohm-mSynthetic model

Wenner arraymodel

Dipole-dipole arraymodel

Note that the pseudosection doesn’t always show a structure that resembles the subsurface.

10 10020 40 60 80

0 5 10 15 20 25 30 35 40 45-8-6-4-20

0 5 10 15 20 25 30 35 40 45-8-6-4-20

Resistivity (Ohm-m)

Elev

atio

n (m

)El

evat

ion

(m)

Distance (m)

Distance (m)

Surface resistivity imaging has been used widely to look at subsurface structure and dynamic processes

After Slater and Sandberg (2000)

Surface resistivity imaging has been used widely to look at subsurface structure and dynamic processes

Before pumping

20 minutes after pumping

60 minutes after pumping

100 minutes after pumping

140 minutes after pumping

180 minutes after pumping

240 minutes after pumping

20 minutes of recovery

19 hours of recovery

% change in resistivity-15 15After Barker & Moore (1998)

BEDROCK

GPS

P1 P2A B P3 P4P5P6 P7P8 P9

WATER

SEDIMENTS

Surface resistivity imaging based on continuous surveys have been developed for land and marine investigations

1.7 1.8 1.9 2.0 2.1 2.2 2.3

log10 resistivity in Ω m

0 m

16 m

0 m

155 m

log10 resistivity in Ohm-m

Multiple 2D or true 3D data can be used to show subsurface structure …

East Shefford Farm site

5.0-6.0m

0.2-0.5m

1.2-1.6m

2.1-2.6m

0

-70 Distance (m)

NE

Relative increase in resistivity (%)

SW

-100

0

5.0-6.0m

0.2-0.5m

1.2-1.6m

2.1-2.6m

0

-70

Chalk

Alluvial gravel

Soil

Peat

Chalk

Alluvial gravel

Chalk

Distance (m)

NE

Resistivity (Ωm)

SW

10 30 50 70 90 110 130 150 170 190 210

-100

Variation in Apr ‘05 survey as compared to Nov ‘04

56 60 64 68 72 76 80 84 88 92 96 100 104 108 112 116

0

Nov ‘04 survey

-40 -30 -20 -10 0 10

… and changes over time in 3D

Data collection speed will depend on:

Number of channels (detectors),

Source frequency,

Stacking requirements

Basic (single channel) systems may only be capable of around 400 to 500 measurements per hour. Some multi-channel systems can work at around 2000 measurements per hour or faster.

Resistivity data acquisition issues

To gain better resolution at depth we may use electrodes in boreholes.

These can be in single boreholes, e.g. mise-à-la-masse …

C- P-P+

C+

V

Resistivity single borehole surveys

0.96

0.97

0.98

0.99

1

1.01

1.02

1.03

1.04

WellConductive Anomaly Meters

Contour Interval = 0.005

0 2 4 6 8

Nimmer(2005) used mise-à-la-masse to study tracer migration in fractured basalt. She also used enhanced the survey by using borehole electrodes for potential measurements.

drivecurrentbetweenelectrode

pair

measurevoltagebetweenelectrode

pair

electrode

Electrodes in two (or more) boreholes can also be used to gain maximum resolution – cross-borehole electrical resistivity tomography (ERT)

Resistivity cross-borehole imaging

Stainless steel mesh, copper and lead are common electrode materials.

Low cost systems developed for surface imaging may be used for cross-borehole work but some suffer from:

Single channel (slow),

Poor dynamic range (limited application),

Constant current source (limited control)

Resistivity cross-borehole data acquisition issues

Electrodes need to have contact with the soil/rock and

the medium allowing this contact should resemble

(electrically) the native soil/rock.

Below the water table electrodes may be temporarily installed in open holes or inside slotted plastic cased wells. In such cases inflatable packers may be used to prevent current flow along the borehole conductive fluid

Above the water table electrodes are normallypermanently installed.

Pushed holes in unconsolidated sedimentsminimise electrode effects.

If drilled holes do not collapse backfill is required - typically drill returns or sand but avoid Bentonite.

A

BM

N

A

MB

N

Many different types of measurement schemes are possible

10090807060504030

0 2 4 6 8

-14

-12

-10

-8

-6

-4

-2

0

0 2 4 6 8

-14

-12

-10

-8

-6

-4

-2

0

100 Ohm-m

10 Ohm-m

Elev

atio

n (m

) Electrode

Distance (m)

Resistivity (Ohm-m)

We can’t build up pseudosections like in surface imaging but can invert data in the same way to get a model that is most consistent with the data

We have to be careful about borehole spacing since we loose sensitivity away from the boreholes

10090807060504030

0 2 4 6 8 10 12 14

-14

-12

-10

-8

-6

-4

-2

0

0 2 4 6 8 10 12 14

-14

-12

-10

-8

-6

-4

-2

0

100 Ohm-m

10 Ohm-m

Elev

atio

n (m

) Electrode

Distance (m)

Resistivity (Ohm-m)

Distance (m)

Elev

atio

n (m

)

0 5 10 15 20 25 30 35 40 45 50

Distance (m)

-15

-10

-5

0E

leva

tion

(m)

15 20 25 30 35

Distance (m)

-15

-10

-5

0

Ele

vatio

n (m

)

15 20 25 30 35

Distance (m)

-15

-10

-5

0

0 5 10 15 20 25 30 35 40 45 50

Distance (m)

-10

-5

0

Ele

vatio

n (m

)

1 2

(a)

(b)

(c) (d)

0 5 10 15 20 25 30 35 40 45 50-15

-10

-5

0

(a)

10 10020 40 60 80Resistivity (Ohm-m)

Distance (m) Distance (m)

Distance (m)

Distance (m)

Resistivity (Ohm-m)El

evat

ion

(m)

Elev

atio

n (m

)El

evat

ion

(m)

In some cases a combination of arrays is more suitable

0 5 10 15 20 25 30

-16

-14

-12

-10

-8

-6

-4

-2

0Borehole D Borehole E

0

1

2

3

Log 1

0R

esis

tivi

ty (Ω

m)

Distance (m)

Peat

Weathered chalk

Soil

Flinty Chalk

River channel

Alluvial gravel

Dep

th (

m)

Example application to study subsurface structure beneath a river channel

Dep

th (m

)

Distance from midpoint (m)

reactive iron

sand

clay backfill

silty clay

basal gravel

shale bedrock

0

2

4

6

8

10

0-1-2 1 2

Cross-borehole imaging of permeable reactive barriers

BH2-1 BH4-3

sandclay backfill

water table?

0.2 0.4 0.6 0.8 1.0Conductivity (S/m)

active electroderedundant electrode

After Slater and Binley (2003)

Cross-borehole imaging of permeable reactive barriers

Monitoring leakage into the vadose zone at Hanford

Start of event B End of event B

End of event DStart of event D

End of event FStart of event F

27-Jul 30-Jul 6-Aug 16-Aug 18-Aug

20-Aug 22-Aug 23-Aug 25-Aug 27-Aug

28-Aug 30-Aug 31-Aug 1-Sep 6-Sep

After Daily, Ramirez and Binley (2004)

Monitoring leakage into the vadose zone at Hanford

Drive currentbetween electrode pair

Measure voltagebetween electrode pair

I

V

Resistivity core and block imaging

We can also apply the same methods for imaging core and blocks (and any shape object)

After Ramirez & Daily (2000)

The methods may be used to look at changes within the block or core due to changing environmental conditions or hydraulic loading

IP measurement principles

0 0.5 1 1.5 2

Time (s)

+I

-I

0

Time (s)0 0.5 1 1.5 2

0

Volta

ge +V

-V

Curr

ent

Vsp

Vp +Vsp

So far we have just looked at DC resistivity

Recall the expected response of the voltage measurement:

In practice, there is a charge up and charge down response.

This forms the basis of time domain induced polarisation measurements

0 0.5 1 1.5 2

Time (s)

+V

-V

0

Vol

tage

t2t1

Vs Vp

Time (s)

Volta

ge

Seigel (1959) defined the apparent chargeability as:

0 0.5 1 1.5 2

Time (s)

+V

-V

0

Vol

tage

t2t1

Vs Vp

Time (s)

Volta

ge

p

sa V

Vm =

Vp is the primary voltage and Vs is the secondary voltage

0 0.5 1 1.5 2

Time (s)

+V

-V

0

Vol

tage

t2t1

Vs Vp

Time (s)

Volta

ge

But Vs is difficult to measure accurately and so an integral measure of chargeability is normally used:

∫−=

2

1

)(1)(

1

12

t

tpa dttV

Vttm (units mV/V)

But Vs is difficult to measure accurately and so an integral measure of chargeability is normally used:

∫−=

2

1

)(1)(

1

12

t

tpa dttV

Vttm (units mV/V)

Note that the measurement is dependent on the chosen time window (and so is never an intrinsic measure).

Also note that the charge up and charge down effect (if strong) can influence the DC resistivity readings.

To measure IP non-polarising potential electrodes (e.g. copper-copper sulfate in a porous pot) are normally used.

Injection currents need to be much higher than normal DC resistivity measurements to ensure good voltage signals.

Also electrical coupling across cables can be a problem and so multi-core cables used for DC resistivity may be problematic.

IP can also be measured in the frequency domain by looking at the change in amplitude and phase lag of an injected and measured signal.

Volta

geCu

rren

t

Phase laq φ

Ip

Vp

Time

Time

The measurement is thus a complex resistivity with magnitude |ρ| = Vp /Ip and phase φ

Volta

geCu

rren

t

Phase laq φ

Ip

Vp

Time

Time

The advantage of the complex resistivity measurement is that it is an intrinsic measure.

Frequency domain instruments are typically more expensive than time domain IP instruments. Few multi-electrode systems are available.

(a)

(b)

(a) SIP Fuchs II base unit and fiber optic cable reels (b) Zonge GDP32 receiver

ρ (Ohm m)

130 135 140 145 150 155 160 165 170 175 180 185 190

Distance along line (m)

-2.5-0.5

Dep

th (m

)

130 135 140 145 150 155 160 165 170 175 180 185 190

Distance along line (m)

-2.5-0.5

Dep

th (m

)

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.4

2.7

-5.0

0.0

1.2

2.4

3.6

4.8

6.0

7.2

8.4

M (mV/V)

Low

tide

(07/

24/9

8)

A A

A A

TP1

TP1

Hig

h

tide

mar

k

130 135 140 145 150 155 160 165 170 175 180 185 190

Distance along line (m)

-2.5-0.5

Dep

th (m

)

-0.4

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

MN (mS/m)

A A TP1

w=

0.2

mS/

cm

w=

20.4

mS/

cm

w=

0.2

mS/

cm

w=

20.4

mS/

cm

w=

0.2

mS/

cm

w=

20.4

mS/

cm

0

0.5

1 Dep

th (m

)

fine sand

till

Summary Logfor TP1

110100

Direct measure of IP: not influenced by fluid chemistry

Crescent Beach State Park, ME

Resistivity

Relative measure of IP:

After Slater & Lesmes (2001)

Separating lithological variation from fluid chemistry changes:

Cross-borehole ERT and IP imaging at the Drigg nuclear waste disposal site

0 5 10

40

0

5

10

15

20

25

30

35

200

400

600

200

400

600

Resistivity (ohm m)

20 200 2000

61246125Vault area ERT

January 2000

Distance from 6125 (m)

Dep

th (

m)

Gamma (cps) Gamma (cps)

Sand/Gravel

Sand/Silt

Silt/Clay

Sandstone

After Kemna, Binley & Slater (2004)

0 5 10

40

0

5

10

15

20

25

30

35

200

400

600

200

400

600 61246125

Vault area IPJanuary 2000

Distance from 6125 (m)

Dep

th (

m)

Gamma (cps) Gamma (cps)

Sand/Gravel

Sand/Silt

Silt/Clay

Sandstone

-20 -10 0

Phase (mrad)

After Kemna, Binley & Slater (2004)

Summary

DC resistivity and IP can be measured in varied geometrical arrangements.

Pseudosections have limited value as an image of the subsurface but are useful (for surface imaging) as a data check.

The choice of measurement scheme can have an effect on the final image.

IP will be sensitive to electrode material and other factors. Care must be taken in obtaining IP measurements.

Inverse methods can be used to determine images of resistivity and IP (next lecture).