Les P Beard

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Interpretation of out of loop data in large fixed-loop TEM surveys  
Les P. Beard, Zonge International, Tucson, Arizona, USA 
 
Summary 
 
Interpretation of out-of-loop data from fixed-loop transient 
EM surveys can be enhanced by using plate models.  
Numerical modeling shows it is possible to distinguish flat-
lying conductors from vertical conductive sheets by 
comparing the appearance of the vertical and along-line out 
of loop measurements.  Where multiple, steeply-dipping 
conductive sheets exist, the sheet nearest the transmitter 
may diminish the responses of more distant sheets, but will 
usually not cause the more distant sheet to be undetectable.  
Screening increases with closeness of adjacent plates and 
with increasing conductance.  If the screening effect is not 
taken into account, estimates of conductance, and by 
inference, ore tonnage, may be underestimated  
 
Introduction 
 
In complex geological scenarios, conditions may exist 
whereby more than one conducting sheet is encompassed 
within the surveyed area (Figure 1).  Transient 
electromagnetic surveys employing large fixed loop 
transmitters have been used for wide area reconnaissance, 
and in such situations it is reasonable to ask how multiple 
conducting sheets would appear in the out-of-loop data 
collected in these surveys.  Modeling shows that the 
conductive sheet closest to the transmitter has a screening 
effect on the amplitude of anomalies produced by more 
distant, but otherwise identical conductive sheets, beyond 
the falloff produced simply by increased distance from the 
transmitter.  This is true for both the vertical (Z) and the 
along-line (X) components of the transient response.  More 
unexpected is that the magnitude of the anomaly of the 
proximate conductor is also decreased by the presence of 
the more distant conducting sheet.  We also see that when 
two conductive sheets are close together, the X component 
of the transient response is a better discriminator of two 
conductors than is the Z component.  In cases where the 
two conductors are near one another, there may be no 
crossover from positive to negative in the vertical 
component. 
 
Method 
 
The first step in interpreting out-of-loop data is to make a 
qualitative assessment of the anomaly based on the shape 
of the X- and Z-component transient decay curves.  Curve 
shape varies considerably depending on whether a 
conductor is flat-lying or vertical (Nabighian and Macnae, 
1991). 
 
 
 
Figure 1: dBx/dt anomalies for multiple conductors.  
Yellow circles indicate Z-crossovers.  Large loop 
transmitter is north of survey area. 
 
Figure 2 shows the difference in the responses of a 
horizontal conductive plate and one that is vertical, as 
computed by ZPLATE, a code which models a thin 
conductive plate in an otherwise resistive earth.  The 
modeled plate is 400 m long (parallel to the long edge of 
the transmitting loop) and 200 m wide/deep.  Its depth of 
burial is 50 m from the surface.  Its thickness is 
infinitesimal, but it has a 20 S conductance.  The plate is 
located 1200 m from the near edge of a 2000 m x 1000 m 
transmitting loop.  The modeled response shown is for 2 
ms.  The horizontal plate produces a large dBz/dt negative 
response and a positive-to-negative dBx/dt response, 
whereas the vertical plate has a positive-to-negative dBz/dt 
response and a positive dBx/dt anomaly directly over the 
plate.  
 
The data in Figure 1 indicate at least two steeply-dipping 
conductors, one in the north part of the survey area trending 
east-northeast, and another trending east-west through the 
central part.  Both have prominent Z-crossover anomalies 
(yellow circles) in addition to the X-component peaks.  But 
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Interpretation of fixed-loop TEM data 
is there a third conductor sandwiched between the two, 
shielded from responding by the conductors on either side?   
 
To address this, we used the LEROI code which allows 
modeling of multiple conductive plates in a layered earth.  
The model used was a two-layer earth with a 30 m thick, 20 
ohm-m layer overlying a 1000 ohm-m lower layer.  The 
vertical conductive plates are identical and are completely 
within the lower layer, but extend to the interface with the 
top layer.  The vertical plate’s conductance is 100 S.  The 
plates are 400 m apart at 800 m, 1200 m and 1600 m from 
the front edge of the 2000 m x 1000 m transmitting loop (to 
the left of the plot).  Shown in Figure 3 are transient decays 
from 5.1 ms to 12.4 ms.  The center plate has an obvious 
anomaly, indicating that the level of shielding is not 
extreme, and therefore, the minor anomalies between the 
two major bands of anomalies in Figure 1 are inconsistent 
with the presence of a third conductor.  (Note: In this study, 
the X-component response from LEROI was multiplied by 
-1 in order to produce a positive peak over conductors.) 
 
 
 
Figure 2: Different responses for horizontal (top) and 
vertical (bottom) conductive plates.  Transmitting loop 
is on the left side. 
 
 
 
 
Figure 3: dBz/dt anomalies from three identical vertical 
conductors separated by 400m. 
 
The question remains as to what degree of shielding takes 
place.  Using LEROI and the same layered earth model, we 
computed the responses of three separate models, having 
single 100 S conductors located at 800m, 1200m, and 
1600m.  Table 1 shows the peak-to-peak anomaly from 
channel 4 (5.1 ms) for the three conductor case and the 
single conductor models. 
 
Table 1. Three vs one conductor Z responses (t=100 S) 
Location Single Plate Three Plates Pct Diff 
800m 32.67 31.83 2.6 
1200m 22.24 20.82 6.4 
1600m 14.92 13.95 6.5 
 
Table 1 reveals that the single plate response is larger than 
the three plates together, even for the plate nearest the 
transmitting loop.  However, the amount of shielding is 
marginal, no more than 6.5% for any given plates. 
 
We performed the same analysis with plates having a 
smaller conductance, 20 S instead of 100 S.  Table 2 shows 
that even though responses are smaller, the amount of 
screening is still no more than 5%.  As an aside, we note 
that these anomalies had small “negative” lobes, and some 
of them failed to go below zero.  If an interpreter is looking 
for Z-crossovers, such anomalies may be overlooked 
because they fail to go negative, or do so only weakly. 
 
Table 2. Three vs one conductor Z responses  
               (t=20 S, 400 m separation) 
Location Single Plate Three Plates Pct Diff 
800m 2.95 2.88 2.4 
1200m 2.98 2.83 5.0 
1600m 2.66 2.56 3.8 
 
In the previous models, the plates were 400 m apart.  If the 
separation is narrowed to 200 m, the shielding effect is 
enhanced, partly from superposition of the anomalies, as 
shown in Figure 4.  Table 3 shows that the degree of 
shielding has increased to more than 13%.  However, even 
in this more extreme case, the central plate still produces a 
strong, easily detectable anomaly. 
 
©  2011 SEG
SEG San Antonio 2011 Annual Meeting 1243
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Interpretation of fixed-loop TEM data 
 
Figure 4: dBz/dt anomalies from three identical vertical 
conductors separated by 200m. 
 
Table 3. Three vs one conductor Z responses  
              (t=100 S, 200m separation) 
Location Single Plate Three Plates Pct Diff 
800m 32.67 30.03 8.1 
1000m 26.93 22.82 9.1 
1200m 22.24 19.30 13.2 
 
The X-component response for the 800m-1000m-1200m 
model is shown in Figure 5, and the shielding effect is 
listed in Table 4.  The X-component appears to be shielded 
somewhat less than the Z-component (Table 3), but that is 
partly because of superposition effects.  If the positive X-
component peak is taken without regard for the negative 
lobe, the shielding is increased, as shown in Table 5. 
 
 
Figure 5: dBx/dt anomalies from three identical vertical 
conductors separated by 200m. 
 
Table 4. Three vs one conductor X responses with 
superposition (t=100 S) 
Location Single Plate Three Plates Pct Diff 
800m 30.14 27.91 7.4 
1200m 24.81 22.27 10.2 
1600m 20.49 19.98 2.5 
 
Table 5. Three vs one conductor X responses, positive 
peaks only(t=100 S) 
Location Single Plate Three Plates Pct Diff 
800m 28.10 24.69 7.9 
1200m 23.45 18.10 22.8 
1600m 19.63 15.81 19.5 
 
To simulate actual field parameters we modeled a 
rectangular loop that was 2000 m x 1000 m.  The long edge 
of the rectangle was perpendicular to the survey lines, and 
parallel to the strike of the conductive plates.  We 
computed the peak response of the Z- and X-components 
for a single 100 S plate at varying distances from the loop 
center (Figure 6).  The falloff in response with distance for 
the Z component and the X component are nearly identical.  
The decay exponent for the dBz/dt is 1.7 and for dBx/dt, 
1.6.  The low falloff exponent is a result of the large loop 
size (Parasnis, 1991). 
 
We compared the responses shown in Figure 6 with the 
responses of a shielded plate in a two-plate model.  We 
adjusted the location of the second plate so that it was 100, 
200, 400, 600, 800, and 1000 m from the first plate.  Figure 
7 shows the shielded response for the X-component and 
Figure 8 shows shielding for the Z-component.  In both 
cases, the shielding has become small, only a few percent, 
by the time the plates are 400 m apart.  When the plates are 
less than that distance, the shielding effect may exceed 
20%.  When the plates are very close together, the shielding 
may be enhanced by superposition whereby the proximate 
sheet’s negative lobe cancels the more distant sheet’s 
positive anomaly, distorting the Z-crossover.  Figure 9 
illustrates this effect with two vertical sheets separated by 
only 100 m.  The negative lobe of the first sheet and the 
positive lobe of the second are barely visible.  In real data, 
and anomaly such as this easily could be interpreted as a 
single body.  Drilling vertically at the “Z-crossover” would 
produce a dry hole, as the ore bodies would be on either 
side.  In such circumstance, the X-component might be the 
best data set from which to interpret as the peaks may be 
preserved despite superposition (Figure 10). 
 
 
Figure 6: Z- and X-component anomalies for a 100 S 
plate at varying distances from the transmitting loop 
center. 
 
 
 
 
 
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Interpretation of fixed-loop TEM data 
 
Figure 7: X-component anomalies for a single 100 S 
plate at varying distances from the transmitting loop 
center (unshielded) and the shielded anomaly of the 
second plate of a two-plate model. 
 
 
Figure 8: Z-component anomalies for a single 100 S 
plate at varying distances from the transmitting loop 
center (unshielded) and the shielded anomaly of the 
second plate of a two-plate model. 
 
 
Figure 9: Z-component anomalies for two 100 S plates 
spaced 100 m apart.  Superposition causes the positive 
lobe of the second plate to be small. 
 
 
Figure 10: X-component anomalies for two 100 S plates 
spaced 100 m apart.  Two anomaly peaks are still 
clearly seen. 
 
Conclusions 
 
Numerical modeling is useful in obtaining a better 
understanding any set of geophysical data, but it is 
indispensible when the particular data type is infrequently 
encountered.  With out-of-loop TEM data, the extra 
information obtained by having both Z- and X-component 
data was well worth the marginal extra cost in obtaining 
two components rather than one.  With the out-of-loop data 
encountered in this field study, modeling helped us 
determine whether the conductors were flat-lying or 
vertical, and helped in understanding the degree of 
screening that might be taking place, and how 
superposition can effect interpretation of Z-component data 
by obscuring the expected crossover.  We also were able to 
rule out a third conductor sandwiched between the two 
conductive bands.  The level of screening increases as plate 
conductance increases and as plates get closer to one 
another.  Although screening may diminish an anomaly’s 
response only marginally, if not taken into account it can 
reduce the estimate of conductance and by inference, ore 
grade or tonnage.   
 
Acknowledgments 
 
The modeling programs used in this work were ZPLATE, 
developed for Zonge by Scott MacInnes, and LEROI, 
developed by Art Raiche and others, and made available 
through AMIRA.  
 
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EDITED REFERENCES 
Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2011 
SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for 
each paper will achieve a high degree of linking to cited sources that appear on the Web. 
 
REFERENCES 
Nabighian, M. N., and J. C. Macnae, 1991, Time domain electromagnetic prospecting methods, in M. N. 
Nabighian, ed., Electromagnetic methods in applied geophysics — Applications, vol. II, SEG 
Investigations in Geophysics No. 3, 271–283. 
Parasnis, D. S., 1991, Large layout harmonic field systems, in M. N. Nabighian, ed., Electromagnetic 
methods in applied geophysics — Applications, vol. II, SEG Investigations in Geophysics No. 3, 
427–520. 
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Interpretation of out of loop data in large fixed-loop TEM surveys

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Interpretation of out of loop data in large fixed-loop TEM surveys

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