Analysis and interpretation of anomalous ... - Semantic Scholar
JOURNAL Of
APPLIED GEOPHYSICS
ELSEVIER
Journal of Applied Geophysics 46 (200J) 217-233
www.elsevier.nlj/locateyja ppgeo
Analysis and interpretation of anomalous conductivity and
magnetic permeability effects in time domain
electromagnetic data
Part I: Numerical modeling
Dmitri y A. Pavlov, Michael S. Zhdanov * Department of Geology and Geophysics, College of Mines and Earth Sciences, University of Utah, Room 719, 135 5 1460 E, Salt Lake City,
UT 84 112-0111, USA
Received I I January 2000; accepted 8 March 200 1
Abstract Time domain electrom agnetic (TDEM) response is usually associ ated with edd y currents in conductive bodies. since this is the dominant effect. However. other effects. such as displacement currents from dielectric processes and magnetic fields associated with rock magnetizat ion. can contribute to TDEM response. In this paper we analyze the effect of magnetization on TDEM data. We use a 3-D code based on finite-difference method. developed by Wang and Hohmann [Geophysics 58 (I993) 797]. to study transient electrom agneti c field propagation through a medium containing bodies with both anomalous conductivity and anomalous magnetic permeab ility. The remarkable result is that the combination of anomalo us conductivity and permeability within the same body could increase significantl y the anomalous TDEM response in compa rison with purely conductive or purely magnetic anomalies. This effect has to be taken into account in interpretation of TDEM data over electrical inhomoge neous structures with potentially anomalous magnetic permeability. © 200 1 Elsevier Science B.V, All rights reserved, Keywords: Conductivity; Permeability; Modeling; Time-domai n electrom agnetic method
1. Introduction
Time domain electromagnetic (TDEM) method is one of the most widely used techniques in electro magnetic geophysical exploration, It is based on studying the response of the transient electromag netic field in a geological cross-section. This re sponse is usually associa ted with eddy currents in
• Corresponding author. Fax: + 1-801-581-7065 . E-mail address : mzhdan [email protected] (M.S. Zhdanov).
conductive bodies. since this is the dominant effect. However. other effects. such as displacement cur rents from dielectric processes and magnetic fields associa ted with rock magnetization. can contribute to TDEM response, Note that dielectric effec t may be of importance only at high frequencies, or at a very early time, This effect is usually neglected for the frequency and time ranges considered in traditional electromagnetic exploration methods, The effect of magnetization can be significant over a wide range of frequencies and time, The effect in frequency domain has been studied in sev
0926-985 1/0 1/ $ - see front mailer © 200 1 Elsevier Science B.V. All rights reserved. PH: 50 926 - 9 85 1(0 I ) 0 0 0 4 0- 4
D.A. Pavlov, M.S. Zhdanov / Journal ofApplied Geophysics 46 (2001) 217-233
218
Model 1 Earth surface
bodies with both anomalous conductivity and anoma lous magnetic permeability.
t
p=100 Ohm-m
Model 1. Conductive or magneticanomaly
10'..----.-~~r-r
: 100 m
I
background conductive anomaly, p=1 Ohmm conductive anomaly, p=1 0 Ohmm magnetic anomaly, !-!r=10
10°
10-'
"*:it+++++
200 m
_10- 2
. '!!lf
F .e
Model 2 Earth surfaee p=1000hm-m
~
s
s 10
t
+++++++++
*...
++++
++ +
~
-.<J
e~o
+ +
+
:I:
+
+
"tJ 10-5
+
:100m 10- 8
I I
10- 7
10- 8 , w~
W~
W~
Tlme(s)
200 m
Model 1. Combinedconductive and magneticanomaly
10'
10°
200m 40m Fig. 1. Geoelectrical models for numerical modeling.
+++~+~_.
-e ,
~+++ . . . ++-,")
s 10
~ ~10-.<J
++Z'",\.. +
e.N
+
:I:
"'0
-e, ' , .
+++++++1--t
2
F .e eral publications (Ward, 1959; Olhoeft and Strang way, 1974; Ward and Hohmann, 1988). Fraser (1981), discussed a method of using magnetic polar ization response for mapping magnetite with a multi coil, multifrequency airborne electromagnetic sys tem. Most of these results have been obtained based on a simple model of a conductive permeable sphere or conductive permeable cylinder in uniform space. In the recent paper by Zhang and Oldenburg (1999), more complex geoelectrical and geomagnetic models have been studied as well. The effect of magnetic permeability on the well logging measurements through metal casing has also been studied by sev eral authors (see, for example, Strack et al., 1996; Kaufman et al., 1996). The effect of magnetization on TDEM data as applied to mineral exploration problems, however, has not been discussed in the literature. At the same time this phenomenon could affect practical TDEM data. In this paper, we use a powerful tool of numerical modeling based on finite-difference method (Wang and Hohmann, 1993), to study transient electromag netic field propagation through a medium containing
~=5 ~=10 r
~~
10-'
_10-
background !l=1
¥, '+--+0:
-5
10
'\. +
'\.
+
-,
+
\
\
.
\ \
10-
8
\
\
10- 7
10- 8
,
10-4
10-2
.
D.A. Pavlov. M.S. Zhdanoc / Journa l ofApp lied Geophysics 46 (2001) 217- 233
219
2. Physical background
The remarkable result is that the combination of anomalous conductivity and permeabilit y within the same body could increase significantly the anoma lous TOEM response, in comparison with purely conductive or purely magnetic anomalies. Thi s effec t has to be taken into account in interpre tation of the TOEM data over electrical inhomoge neous struct ures with potenti ally ano malous magnetic permeability. In a complimentary paper (Zhdanov and Pavlov, 2001) , we develop a meth od of joi nt inversion of TOEM data with respect to conduc tive and magnetic anoma lies.
Incident electromag netic fields generate seve ral processes in geo logica l target: eddy currents are induced in co nductive bodies, and magnetic polariza tion is induced in magnetic bodies. Acco rding to Lenz's law, eddy currents tend to cancel the changes in the incident magnetic field s'. The effec t of the induced eddy current is strong enough in the early stage of the time domain electromagnetic process and becomes negligible in the later stages . The sec ondary magnetic field assoc iated with magnetic
Model 1. Ey(t). t=100 microsec
~E f Q)
o
;:-r 50
__
c-----~=
200 250
Magnetic anoma ly ·J .
C
.
c .... ...
0 .1
(---'
-(:"~. '- - -~ '.... -.
0 .05
_--._-_~_ ==-
o
350
400 - t·-·-~ - ----- r---- 1-~-----,------ _,_ --- __._- - -_,_ ----.,..----, ----f ----~ -200 -150 -100 . 50 0 50 100 150 200 250 -250
0 .1
l
.l
0
AI(s 'm )
Coord inate (m) Fig . 6. Model l. Snap-shots of the vertical co mponents of magnetic field Hz ( I, x, z )j31 in the case of a horizontal plate for the time moment of 100 p.s. The top panel correspon ds to the case of a purely magnetic anomaly (P 2 = PI = 100 m) with the relative magnetic permeability of the plate Il r = 5. The bottom panel presents the results for the case of a purely conductive anomaly ( P2 = I n m, Il r = 0, and the middle panel demonstrates the combined effect of magnetic and conductive anomalies ( P2 = 1 n m, Il r = 5).
n
D.A. Parlor. M.S. Zhdanou / Journal of Applied Geophysics 46 (200 /) 2 /7-233
223
In order to co nduct the quantit ative co mparative analysis of the anomalous co nductiv ity and anoma lous perm eabil ity effects on TDEM response, we apply numerical modelin g techn ique.
perm eab ility and co nductivity, /-L, 0' . Th e first term reflects the effect of magnetic charges ca used by magnetic inhomogeneities. In the case of the piece wise constant distribution of /-L" these charges are co nce ntrated on the boundaries of the anoma lous bod y. The seco nd term reflects the co mbined effects of both relati ve perme ab ility and co nductivity. The increased perm eabi lity or increased co nductivity within same volume results in the same effect of increasing the volume den sity of eddy curre nts within an inhom ogene ous dom ain . Therefore, we ca n distin guish between the anoma lous perm eabi lity and anomalous conductivity effects only because of the boundar y effec t of the magnetic anoma ly.
3. Numerical modeling of tim e domain respon ses for 3-D bodies with anomalous conductivity and permeability We co nsider the two typica l models presented in Fig. I. Model 1 (Fig, I, top panel) co nsists of a horizont al rect angu lar plate wit h lateral dimensions
Model 1. dHz(t)/dt. t=1000 microsec Magnetic anomaly 50 100 :[1 50
s: 200 Ci
~ 250 300
350 -200
-150
-100
-50
o
50
100
150
..
200
250
Conductive and magnetic anomaly 50 100
... ( ( (r1\r;J~') "="·-:.V '
: [ 150
s:
200
~ 250 o
300
- - ----
350
· 0.02
400 -'-----.,.---,-----~--~"'-_,..'.:--.,._'-_,..-o 50 -250 -200 -150 -100 -50 100 150 200 250
- 0.03
Conductive anomaly
l
50 ~ l 00 -V
E 150
APPLIED GEOPHYSICS
ELSEVIER
Journal of Applied Geophysics 46 (200J) 217-233
www.elsevier.nlj/locateyja ppgeo
Analysis and interpretation of anomalous conductivity and
magnetic permeability effects in time domain
electromagnetic data
Part I: Numerical modeling
Dmitri y A. Pavlov, Michael S. Zhdanov * Department of Geology and Geophysics, College of Mines and Earth Sciences, University of Utah, Room 719, 135 5 1460 E, Salt Lake City,
UT 84 112-0111, USA
Received I I January 2000; accepted 8 March 200 1
Abstract Time domain electrom agnetic (TDEM) response is usually associ ated with edd y currents in conductive bodies. since this is the dominant effect. However. other effects. such as displacement currents from dielectric processes and magnetic fields associated with rock magnetizat ion. can contribute to TDEM response. In this paper we analyze the effect of magnetization on TDEM data. We use a 3-D code based on finite-difference method. developed by Wang and Hohmann [Geophysics 58 (I993) 797]. to study transient electrom agneti c field propagation through a medium containing bodies with both anomalous conductivity and anomalous magnetic permeab ility. The remarkable result is that the combination of anomalo us conductivity and permeability within the same body could increase significantl y the anomalous TDEM response in compa rison with purely conductive or purely magnetic anomalies. This effect has to be taken into account in interpretation of TDEM data over electrical inhomoge neous structures with potentially anomalous magnetic permeability. © 200 1 Elsevier Science B.V, All rights reserved, Keywords: Conductivity; Permeability; Modeling; Time-domai n electrom agnetic method
1. Introduction
Time domain electromagnetic (TDEM) method is one of the most widely used techniques in electro magnetic geophysical exploration, It is based on studying the response of the transient electromag netic field in a geological cross-section. This re sponse is usually associa ted with eddy currents in
• Corresponding author. Fax: + 1-801-581-7065 . E-mail address : mzhdan [email protected] (M.S. Zhdanov).
conductive bodies. since this is the dominant effect. However. other effects. such as displacement cur rents from dielectric processes and magnetic fields associa ted with rock magnetization. can contribute to TDEM response, Note that dielectric effec t may be of importance only at high frequencies, or at a very early time, This effect is usually neglected for the frequency and time ranges considered in traditional electromagnetic exploration methods, The effect of magnetization can be significant over a wide range of frequencies and time, The effect in frequency domain has been studied in sev
0926-985 1/0 1/ $ - see front mailer © 200 1 Elsevier Science B.V. All rights reserved. PH: 50 926 - 9 85 1(0 I ) 0 0 0 4 0- 4
D.A. Pavlov, M.S. Zhdanov / Journal ofApplied Geophysics 46 (2001) 217-233
218
Model 1 Earth surface
bodies with both anomalous conductivity and anoma lous magnetic permeability.
t
p=100 Ohm-m
Model 1. Conductive or magneticanomaly
10'..----.-~~r-r
: 100 m
I
background conductive anomaly, p=1 Ohmm conductive anomaly, p=1 0 Ohmm magnetic anomaly, !-!r=10
10°
10-'
"*:it+++++
200 m
_10- 2
. '!!lf
F .e
Model 2 Earth surfaee p=1000hm-m
~
s
s 10
t
+++++++++
*...
++++
++ +
~
-.<J
e~o
+ +
+
:I:
+
+
"tJ 10-5
+
:100m 10- 8
I I
10- 7
10- 8 , w~
W~
W~
Tlme(s)
200 m
Model 1. Combinedconductive and magneticanomaly
10'
10°
200m 40m Fig. 1. Geoelectrical models for numerical modeling.
+++~+~_.
-e ,
~+++ . . . ++-,")
s 10
~ ~10-.<J
++Z'",\.. +
e.N
+
:I:
"'0
-e, ' , .
+++++++1--t
2
F .e eral publications (Ward, 1959; Olhoeft and Strang way, 1974; Ward and Hohmann, 1988). Fraser (1981), discussed a method of using magnetic polar ization response for mapping magnetite with a multi coil, multifrequency airborne electromagnetic sys tem. Most of these results have been obtained based on a simple model of a conductive permeable sphere or conductive permeable cylinder in uniform space. In the recent paper by Zhang and Oldenburg (1999), more complex geoelectrical and geomagnetic models have been studied as well. The effect of magnetic permeability on the well logging measurements through metal casing has also been studied by sev eral authors (see, for example, Strack et al., 1996; Kaufman et al., 1996). The effect of magnetization on TDEM data as applied to mineral exploration problems, however, has not been discussed in the literature. At the same time this phenomenon could affect practical TDEM data. In this paper, we use a powerful tool of numerical modeling based on finite-difference method (Wang and Hohmann, 1993), to study transient electromag netic field propagation through a medium containing
~=5 ~=10 r
~~
10-'
_10-
background !l=1
¥, '+--+0:
-5
10
'\. +
'\.
+
-,
+
\
\
.
\ \
10-
8
\
\
10- 7
10- 8
,
10-4
10-2
.
D.A. Pavlov. M.S. Zhdanoc / Journa l ofApp lied Geophysics 46 (2001) 217- 233
219
2. Physical background
The remarkable result is that the combination of anomalous conductivity and permeabilit y within the same body could increase significantly the anoma lous TOEM response, in comparison with purely conductive or purely magnetic anomalies. Thi s effec t has to be taken into account in interpre tation of the TOEM data over electrical inhomoge neous struct ures with potenti ally ano malous magnetic permeability. In a complimentary paper (Zhdanov and Pavlov, 2001) , we develop a meth od of joi nt inversion of TOEM data with respect to conduc tive and magnetic anoma lies.
Incident electromag netic fields generate seve ral processes in geo logica l target: eddy currents are induced in co nductive bodies, and magnetic polariza tion is induced in magnetic bodies. Acco rding to Lenz's law, eddy currents tend to cancel the changes in the incident magnetic field s'. The effec t of the induced eddy current is strong enough in the early stage of the time domain electromagnetic process and becomes negligible in the later stages . The sec ondary magnetic field assoc iated with magnetic
Model 1. Ey(t). t=100 microsec
~E f Q)
o
;:-r 50
__
c-----~=
200 250
Magnetic anoma ly ·J .
C
.
c .... ...
0 .1
(---'
-(:"~. '- - -~ '.... -.
0 .05
_--._-_~_ ==-
o
350
400 - t·-·-~ - ----- r---- 1-~-----,------ _,_ --- __._- - -_,_ ----.,..----, ----f ----~ -200 -150 -100 . 50 0 50 100 150 200 250 -250
0 .1
l
.l
0
AI(s 'm )
Coord inate (m) Fig . 6. Model l. Snap-shots of the vertical co mponents of magnetic field Hz ( I, x, z )j31 in the case of a horizontal plate for the time moment of 100 p.s. The top panel correspon ds to the case of a purely magnetic anomaly (P 2 = PI = 100 m) with the relative magnetic permeability of the plate Il r = 5. The bottom panel presents the results for the case of a purely conductive anomaly ( P2 = I n m, Il r = 0, and the middle panel demonstrates the combined effect of magnetic and conductive anomalies ( P2 = 1 n m, Il r = 5).
n
D.A. Parlor. M.S. Zhdanou / Journal of Applied Geophysics 46 (200 /) 2 /7-233
223
In order to co nduct the quantit ative co mparative analysis of the anomalous co nductiv ity and anoma lous perm eabil ity effects on TDEM response, we apply numerical modelin g techn ique.
perm eab ility and co nductivity, /-L, 0' . Th e first term reflects the effect of magnetic charges ca used by magnetic inhomogeneities. In the case of the piece wise constant distribution of /-L" these charges are co nce ntrated on the boundaries of the anoma lous bod y. The seco nd term reflects the co mbined effects of both relati ve perme ab ility and co nductivity. The increased perm eabi lity or increased co nductivity within same volume results in the same effect of increasing the volume den sity of eddy curre nts within an inhom ogene ous dom ain . Therefore, we ca n distin guish between the anoma lous perm eabi lity and anomalous conductivity effects only because of the boundar y effec t of the magnetic anoma ly.
3. Numerical modeling of tim e domain respon ses for 3-D bodies with anomalous conductivity and permeability We co nsider the two typica l models presented in Fig. I. Model 1 (Fig, I, top panel) co nsists of a horizont al rect angu lar plate wit h lateral dimensions
Model 1. dHz(t)/dt. t=1000 microsec Magnetic anomaly 50 100 :[1 50
s: 200 Ci
~ 250 300
350 -200
-150
-100
-50
o
50
100
150
..
200
250
Conductive and magnetic anomaly 50 100
... ( ( (r1\r;J~') "="·-:.V '
: [ 150
s:
200
~ 250 o
300
- - ----
350
· 0.02
400 -'-----.,.---,-----~--~"'-_,..'.:--.,._'-_,..-o 50 -250 -200 -150 -100 -50 100 150 200 250
- 0.03
Conductive anomaly
l
50 ~ l 00 -V
E 150
