geology 228/378 applied and environmental …geology 228/378 applied and environmental geophysics...
TRANSCRIPT
-
Geology 228/378Applied and
Environmental GeophysicsLecture 6
DC resistivity Surveys
-
Direct current (DC) Resistivity
1. Introduction 2. Current flow in the ground 3. Schlumberger, Wenner, dipole-dipole,
pole-dipole arrays 4. Field methods and instrumentation 5. Data interpretation 6. Field Examples
-
Ohms Law (discovered in 1827)
IRV = Georg Simon Ohm (1787-1854)
-
It's Resistivity, NOT Resistance
LRA
ALR
=
=
So the unit for resistivity is ohm-meter
-
For a point source in an infinity medium, we have the resistance R and potential V expressed as
====
===
S
jdsIrI
rIIRV
rrr
ALR
44
44 2
-
Furthermore
rVIor
IrV
44
==
This resistivity is called the apparent resistivity. Only when the material is uniform, the apparent resisitivityis equal to the constant, real resistivity.
jrV
rV
Ir
rIVr
IrV
a ====22 444
-
Most geophysical resistivity surveys have the measurements occurred at the surface of the earth. The air above the ground is literally an insulator (zero conductivity) and the current only flows in the ground. Thus, for calculating the current density on an equal-potential sphere, the surface area becomes from the closed spherical surface to the surface of the lower-hemisphere, and the potential changes to
rIV
2
=Electric field and current
VgradV ==EV== EJ
r
I
V
-
In practice, the field surveys usually measure the Voltage V, other than the potential itself. This voltage V is the difference of potential between 2 points. IN DC resistivity surveys the voltage is usually measured by two electrodes planted on the surface.
3131 22 r
IrIVVV
==
r1
r3
I
V3V1
-
For the current can be physically flowing through the ground, wehave to have 2 poles: one for current injected in (source) and one for the current flow out (sink). Thus, both the source and sink will generate an electric potential, but with opposite polarity.
22
11
2
2
rIV
andrIV
=
=
r1r2
Dipole
-
And the total potential for the two poles is
)11(222 2121
21 rrI
rI
rIVVV ==+=
And the total voltage between two points generated by the two poles is
)1111(2
)22
()22
(
4321
4231
21
rrrrI
rI
rI
rI
rI
VVV
+=
=
+=
-
APPARENT RESISTIVITY
because
)1111(2 4321 rrrr
IV a +=
IVk
IV
rrrra
=
+= 1
4321
)1111(2then
-
GEOMETRIC FACTOR
K is the geometric factor that describes the geometry of the electrode configuration being used:
A B
M NV
I
1
4321
11112
+=
rrrrK
111112
+=
NBANMBAMK
-
IV
rrrr
a
= )(2
13
31 Pole-Dipole array
-
Schlumberger Array
1
22211211
11112
+=
PCPCPCPCK
-
2l 2l(n-1) 2l
IVnnnla
+= )1)(1(2
-
Data plotting
-
Dipole-Dipole Array
-
path gpropagatin :couplingreceiver pattern,radiation :,
spreading lgeometrica :signals source received, :,
LPP
GSR
RS
-
Pole pole Pole - dipole
Dipole - dipole
Wenner Schlumberger
ELECTRODE ARRAYS
-
CHOICE OF THE BEST ARRAY
Depends on:1) type of structure to be mapped2) sensitivity of the resistivity meter 3) background noise level
Things to be considered:1) depth of investigation2) sensitivity of the array to vertical and horizontal structures 3) horizontal data coverage4) signal strength.
-
DIPOLE-DIPOLE ADVANTAGES
Low EM coupling between current and potential circuits
Good for depth penetration
High resolution and is sensitive to vertical resistivity boundaries (e.g. dykes and cavities)
-
DIPOLE-DIPOLE DISADVANTAGES
Poor for vertical resolution of horizontal structures (e.g. sills or sedimentary layers)
Data collected from dipole-dipole array are easily affected by near-surface resistivity variations and therefore can produce noisy data at sites with cultural relics
Small signal strength for large values of n
-
Apparent Resistivity Pseudo-section for a Block model
-
Wenner
Pole-pole
Dipole - dipole
Pole-dipole
Block model response
-
Near Surface Layer
-
Near Surface Layer Response, Plan View
-
Near Surface Layer Response, Pseudosection
-
Buried Vertical Contact
-
Buried Vertical Contact Response, Plan View
-
Buried Vertical Contact Response, Pseudosection
-
3D Prism
-
3D Prism Response, Plan View
-
3D Prism Response, Pseudosection
-
Pole-pole array sensitivity
-
Pole-dipole array sensitivity
-
Dipole-dipole array sensitivity
-
Wenner array sensitivity
-
Schlumberger array sensitivity
-
Resistivity Surveys
-
AGI Sting R-1 and the Swift automotive switchbox
-
DC Resistivity Interpretation
path gpropagatin :couplingreceiver pattern,radiation :,
spreading lgeometrica :signals source received, :,
LPP
GSR
RS
-
Electric current in layered media
The current flow in the layered media deviates from that observed in the homogeneous media. In particular, notice that in the layered media the current flow lines are distorted in such a way that current preferentially seems to be attracted to the lower-resistivity portion of the layered media. In the model on the left, current appears to be pulled downward into the 50 ohm-m layer. In the model on the right, current appears to be bent upward, trying to remain within the lower resistivity layer at the top of the model. This shouldn't be surprising. What we are observing is the current's preference toward flowing through the path of least resistance. For the model on the left, that path is through the deep layer. For the model on the right, that path is through the shallow layer.
-
Sting/Swift prg: DIP-DIP title2 unit electrode spacing3 array No. dip-dip=3193 No. of data points1 1-middle point used0 0-no IP1st: apparent rho-location 2nd: P1P2 spacing3rd: dipole separation factor n4th: apparent resistivity3.000 2.000 1 2961.0005.000 2.000 1 2769.0007.000 2.000 1 1040.3009.000 2.000 1 2994.30011.000 2.000 1 779.580
.45.000 2.000 1 10305.00047.000 2.000 1 6955.20049.000 2.000 1 5515.00051.000 2.000 1 4435.9004.000 2.000 2 2168.8006.000 2.000 2 1696.4008.000 2.000 2 1233.200
-
DC Resistivity Interpretation
path gpropagatin :couplingreceiver pattern,radiation :,
spreading lgeometrica :signals source received, :,
LPP
GSR
RS
-
CURRENT CONDUCTION IN ROCKS
Electrolytic conduction occurs by the relatively slow movement of ions within an electrolyte
Electronic conduction is the process by which metals, for example, allow electrons to move rapidly, so carrying the charge
This is applicable in zero and low frequency case
-
crystalline rock can lead to low resistivities if they are filled with fluids.
The resistivities of various earth materials are shown below.
Material Resistivity (Ohm-meter)Air Pyrite 3 x 10^-1Galena 2 x 10^-3Quartz 4 x 10^10 - 2 x 10^14Calcite 1 x 10^12 - 1 x 10^13Rock Salt 30 - 1 x 10^13Mica 9 x 10^12 - 1 x 10^14Granite 100 - 1 x 10^6Gabbro 1 x 10^3 - 1 x 10^6Basalt 10 - 1 x 10^7Limestones 50 - 1 x 10^7Sandstones 1 - 1 x 10^8Shales 20 - 2 x 10^3Dolomite 100 - 10,000Sand 1 - 1,000Clay 1 - 100Ground Water 0.5 - 300Sea Water 0.2
-
Archies law:In the ground, and in low frequencies, electricity is essentially conducted through the interstitial water in pores by ionic transport
wnmSa =
effective formation resistivity;wpore water resistivity; porosity;S saturation;a 0.5-2.5;m 1.3-2.5;n ~2.
-
E C V ar i at i on wi t h Dept h
0
5
1 0
1 5
2 0
2 5
5 0 1 0 0 1 5 0
E C ( mi cr omhos/ cm)
-
EC Variation w ith Depth
0
5
10
15
20
25
50 100 150
EC (micromhos/cm)
Dept
h (f
t)
Cr Variation with Depth
0
5
10
15
20
25
0 5 10 15 20
Cr (mg/l)
Dept
h (f
t)
EC vs Cr
y = 0.2827x - 19.9R2 = 0.8969
0
5
10
15
20
0 50 100 150
EC ( micromhos/cm)
Cr
( mg/
l)
-
DC Resistivity Case Studies
=
L
ds
RS ePPGSR)(
)()(
path gpropagatin :couplingreceiver pattern,radiation :,
spreading lgeometrica :signals source received, :,
LPP
GSR
RS
-
Detection of Saltwater Intrusion along the Noyo
River, California
-
Resistivity and Seismic Survey Results
-
N
NEX Gas Station Site
ESTCP LTM Test Cell
GeoVIS/Piezocone Facility
Plume Control and Containment System NVBC Port Hueneme In-Situ BioBarrier
(Leading Edge)
ESTCP NFESC/ASU In-Situ BioBarrier
NVBC Port Hueneme In-Situ BioBarrier
(Mid Plume)
Patterson Rd.
Plea
sant
Val
ley
Rd.
23rd
Ave
.
Pacific Ave.
NN
NEX Gas Station Site
ESTCP LTM Test Cell
GeoVIS/Piezocone Facility
Plume Control and Containment System NVBC Port Hueneme In-Situ BioBarrier
(Leading Edge)
ESTCP NFESC/ASU In-Situ BioBarrier
NVBC Port Hueneme In-Situ BioBarrier
(Mid Plume)
Patterson Rd.
Plea
sant
Val
ley
Rd.
23rd
Ave
.
Pacific Ave.
Piezocone GeoVIS Demonstration
-
Utility PoleUtilityShed
WaterStorageTanks
20V
ehic
leG
ate
20V
ehic
leG
ate
4 Pe
rson
nel
Gat
e100
60
W1(Inject)
W3
W2
Utility PoleUtilityShed
WaterStorageTanks
20V
ehic
leG
ate
20V
ehic
leG
ate
4 Pe
rson
nel
Gat
e100100
60
W1(Inject)
W3
W2
N
Note: Layout displayed with 10 x 10 grid
NN
Note: Layout displayed with 10 x 10 grid
N
Electrode locations for DC resistivity Surveys
East of Building 401East of Building 401Lot
AGI
1
2
3
4
56
7
8
9
10
1112
13
14
15
16
1718
19
20
21
22
23
24
25
26
27
28
Wells from previous extraction systemHydraulic Test WellsWells from previous extraction systemHydraulic Test WellsWells from previous extraction systemHydraulic Test WellsProposed electrode location
-
2/28/2005
-
4/1/2005
-
4/1 - 2/28, 2005
-
Day 00Day 01Day 02Day 03Day 04Day 07Day 11Day 16Day 23Day 31Day 37Day 41
-
0 5 10 15 20 25 30 350
0.5
1
1.5
00 0000 00
0000
0000 00 00 00
0101
01
01
01
01
01
01
0101
01
0202 02 02
02
02
02
0202
02 02
0303
0303
03
03
03
0303 03 03
04 0404 04
04
04
04
04
0404
04
0707 07 07
0707
0707
07 07 07
1111 11
11 1111
1111
11 11 11
16 16 16 1616
1616 16 16
1616
2323
23 23 23 23 23 23 23 23 2331 31
31 31 31 31 31 3131
313137 37
3737
3737 37
3737
373741 41 41 41
4141 41 41 41
4141
Y distance (ft)
DEC
(S/m
)
Z=12 ft
0 5 10 15 20 25 30 350
0.5
1
1.5
00 0000 00 00
0000 00 00 00 00
0101
01
01
01
01
01
0101 01 01
0202 02 02
0202
02
0202
0202
03 03 0303
0303
0303
0303 03
04 0404 04
04
04
04
0404 04 04
07 0707 07 07
0707
0707 07 07
1111 11
11 1111
1111
11 11 11
16 1616 16 16 16 16 16 16 16
1623
2323 23 23 23 23 23 23 23 23
3131
31 3131 31
31 3131
3131
37 37 37 37 3737 37
3737
3737
4141
41 4141 41 41 41 41 41 41
Y distance (ft)
DEC
(S/m
)
Z=15 ft
-
Homework:
The averaged electric conductivity of the groundwater found at the National Chromium site is about 100 microSiemensper centimeter.It is equivalent to 10 milliSiemens per meter.The averaged formation conductivity found by the DC resistivity tomography is 1milliSiemens per meter.
By assuming the porosity is 35%, 100 % saturation, and a = 1.0, m=2.0 and by Archies law, what is the estimated formation conductivity?
Geology 228/378Applied and Environmental GeophysicsLecture 6Direct current (DC) ResistivityAPPARENT RESISTIVITYGEOMETRIC FACTORDIPOLE-DIPOLE ADVANTAGESDIPOLE-DIPOLE DISADVANTAGESResistivity SurveysCURRENT CONDUCTION IN ROCKSDetection of Saltwater Intrusion along the Noyo River, CaliforniaResistivity and Seismic Survey Results