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Topographic distortions of transient electromagnetic anomalies in resistive terrains
29
Geoexploration, 24 (1986) 29-42 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
TOPOGRAPHIC DISTORTIONS OF TRANSIENT ELECTROMAGNETIC ANOMALIES IN RESISTIVE TERRAINS R.D. OGILVY Regional Geophysics Research Group, British ~eo~~~‘ca~ Survey, Keyworth~ Nottingham (United Kingdom) (Received February 18, 1985; accepted after revision January 14, 1986)
ABSTRACT Ogilvy, RD., 1985. Topographic distortions of transient electromagnetic resistive terrains. Geoexploration, 24: 29-42.
anomalies
in
A theoretical model study has been undertaken to assess the influence of irregular terrain on common loop and separated loop transient electromagnetic anomalies. Hill, valley and step profiles over a dipping conductor exhibit free-space anomalies which differ markedly both in amplitude and shape from those which pertain in flat earth situations, Differences in loop-target distance and orientation can accentuate anomalies observed over hills and suppress those recorded in valleys. In many cases the diffusive behaviour of TEM anomalies provides the only basis for resolving interpretational ambiguities caused by topographic variations, since terrain-induced geometric distortions are spatially static with increasing delay time. The study confirms the importance of topography in any interpretative scheme based on response characteristics.
INTRODUCTION
The transient electromagnetic (TEN) survey technique is now widely used for mineral prospecting but the in~~retative guidelines available are invariably based on model studies in which the terrain is assumed to be a plane horizontal surface. Qualitative interpretation in which response characteristics such as maxima, minima, anomaly width and asymmetry are used to infer the location and attitude of a conductor will clearly be in error in rugged or mount~ous terrain. ~~to~ions in response shape will occur due to the irregular variations in distance between the loops and the conductive body. Even quantitative interpretation based on total curve-matching techniques (Oristaglio and Hohmann, 1984; Barn&t, 1984) will give erroneous results unless the topographic profile is taken into account. The purpose of this paper is to show, by theoretical modelling, how some simple free-space terrain features such as a hill, valley and step can distort the normal flat-earth response when profiling over a thin dipping sheet conductor. While it is a relatively simple exercise to incorporate topographic variations into a numerical modelling program, no topographic studies have been reported in the literature. It would appear also that practising field
0016-7142/861$03.50
0 1986 Elsevier Science Publishers B.V.
30
geophysicists (who may have limited access to powerful mainframe computing facilities) are still largely dependent on published or unpublished type curves (Buselli and Thorbum, 1983; Ogilvy, 1983, 1986; Ogilvy and Dabek, 1984; Rai and Verma, 1984) to assess the disposition and significance of a prospective target. It is important therefore to show how these pre-determined type curves might be distorted or transformed when profiling in rugged terrain. It should be emphasised that the study is intended to complement widely used free-space interpretative aids, and hence is concerned only with terrain-induced geometric distortions. Computational constraints did not allow the effect of a conductive host to be included. THEORY
AND COMPUTATION
Theoretical TEM model response curves have been computed for hill, valley and step type profiles using an exact integral equation solution (Weidelt, 1983). The solution assumes a step function excitation of the target conductor and that the receiver is an induction loop which measures the time derivative of the secondary magnetic field as in the commercially available SIROTEM, Crone PEM and Geonics EM37 systems. The target mineralisation is represented by a thin two-dimensional dyke of finite conductance. Many ore deposits closely resemble sheet-like or tabular bodies, and hence the dipping dyke is a simple but realistic model. The mathematical formulation used in the modelling program is based on integrating the effects of half-sheet free-decay mode eigencurrents. The formal solution is obtained using the Wiener-Hopf technique and then cast into a rapidly convergent integral suitable for numerical evaluation. For the sake of brevity the integral equation is not presented here. Interested readers should refer to Weidelt (1983) for a complete theoretical description. Being exact the formulation allows the true diffusive character of the transient process to be reproduced in the response curves, and to observe how early and late time anomalies behave with differing levels of sensitivity to specific terrain features. Model responses have been computed for the common loop geometry, in which a single conductor loop acts as both transmitter and receiver, and also for the separated inline or Slingram loop geometry. In both cases transmitter and receiver loop orientations are assumed to be horizontal, and that the vertical secondary field component has been observed irrespective of slope angle. This is a reasonable assumption in the case of separated loop profiling since both transmitter and receiver are effectively dipolar and the operator has some control over orientation. In the case of the larger common loop configuration horizontality may or may not be maintained, since loop orientation would depend on the areal extent or size of the loop relative to the topographic feature. For simplicity and to facilitate more readily a compari-
31
son with flat-earth responses, the effects of changing loop orientation with slope have not been inco~o~ted. Further, to avoid slope averaging effects the common loop diameter used in the study was made small relative to the topographic model. For the separated loop geometry the computations were performed for circular transmitter and receiver loops of radius 2.5 m and for a fixed horizontal intercoil spacing of 50 m. It should be noted that the positivity of the separated loop response is consistent with data acquired by the S~~~TE~ system and that an opposite convention and different normalisation procedure is used by the Crone PEM system. The common loop radius was taken to be 5.0 m. In al1 cases the target conductance was kept constant at 50 S. Computation of the vertical field anomalies was carried out on a Honeywell 66/DPS-300 mainframe computer and then plotted off-line on a CALCOMP plotter. Each case, which consisted of 41 stations evaluated at 10 delay times (0.8, 1.2, 1.6, 2.0, 2.6, 3.4, 4.2, 5.0, 5.8, 7.0 ms), required between 2.5-3 hours of CPU time. Computations of this length are expensive and underline the desirability of having reliable interpretative guidelines based on response characteristics rather than total curve-matching, as shown by Ogilvy and Dabek (1985) and Ogilvy (1986). To economise on computer time, the vertical component was computed at fixed 10 m increments along the profile irrespective of response shape. Abrupt variations in response shape caused by inadequate sampling were subsequently smoothed out using a cubic spline curve fitting routine. The results are shown as plots of vertical field &V/A) normalised for transmitter and receiver turns, against distance in metres. For greater clarity some computed time-curves have been omitted from the final presentation_ Case parameters are given together with a cross-section to show the relative disposition of the conductor. MODEL RESPONSES
Narrow hilt. A comparison of Fig. la and lb illustrates bow a normal flatearth response might be transformed by a narrow ridge or hill profile. The most obvious effects are a broadening of the anomaly width, a higher up-dip and down-dip amplitude and a flattening of the down-dip maxima. All these effects can be attributed to the increased proximity of the loop to the conductor on the hill slopes and the persistence of that proximity especially on the down-dip side of the conductor. A flattened down-dip peak response at early time would normally indicate, as in Fig. lb, that the terrain slopeand theconductor dip are similar for that profile. It will also be observed that the sharp breaks in the topo~aphic profile at +60 m have a distorting influence on the shape of the anomaly flanks. With increasing dip (Fig. le) the topographic relief has Iess influence on anomaly shape as expected. With decreasing dip the
32 TEfl
RODEL
RESPONSE
-
COnnON
LOOP
DELAY
TIPiES
DELAY
TInES
_,OO.~ TEn
nOOEL
DEPTH STRIKE
R /
RESPONSE
OF
-
PLATE
COntlON
LOOP
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90.0°
mm 7
DISTANCE
(p1)
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E
33 TErl
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RESPONSE
-. COflnON
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TEf’
nODEI_
RESPONSE
-
COWlON
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OISTANCE
tn)
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LOOP
Fig. 1. a. Flat earth TEM model response, common loop, 6 = 45”. b. Narrow hill TEM model response, common loop, 6 = 45”. c. Narrow hill TEM model response, common loop, 6 = 75” d. Narrow hill TEM model response, common loop, 6 = 36”.
34
distortion is more accentuated as shown in Fig. Id. Such distortions whether in the form of local maxima or minima could compromise any shape analysis unless recognised by the interpreter as being terrain-induced. In the case of dipping conductors, terrain breaks of this kind can be identified by their static behaviour with increasing time. True minima associated with minimal coupling above the conductor will invariably show a time-dependent lateral migration downdip as the eddy currents diffuse inwards. Narrow valley. A more serious deformation of the flatearth response can occur in the case of a narrow valley profile (Fig. 2). Unlike the preceding case the overall response shape bears little resemblance to the flat-earth response given in Fig. la, and in addition there is a marked divergence in early and late time behaviour. At early time the normal asymmetric twin peak anomaly is depressed by the topographic minima at f 60 m, to give the appearance of a single centrally located peak response close to the conductor’s top edge. For a delay of 7 ms, the principal components of the response are a central minimum and flanking shoulders which, if viewed locally, might be mis-interpreted in terms of a conductor of near vertical disposition or even with a reversed dip, since the up-dip peak is more clearly defined. The central diagnostic minimum is poorly developed at early times due to the topographic compression of the valley slopes. In a practical field situation it could be difficult to differentiate between topographic minima such as those observed at f 60 m and time varying dynamic lows which are widely used to infer the position and dip of the conductor. For example, the response at 4.2 ms displays three minima of near-equal prominence and the geometric low at -60 m could readily be misinterpreted as reflecting the top edge of a shallow dipping conductor. The problem could be accentuated for deep-seated or poor conductors, as the target response may only be evident at late time or at signal levels which are close to the geologic noise threshold. If the target response is only observed in a single channel, anomaly analysis may have to be based on a single time-curve and without the insight afforded by multichannel data. In these circumstances, it could be difficult to differentiate between topographic distortions and genuine diffusive effects. ~though the results of the study are meant to be used in a qu~itative fashion, it is clear that peak amplitude and hence detec~bility will also be affected by the type of terrain profile. For the common loop geometry and a dipping sheet conductor, the twin maxima are normally off-set from the vertical projection of the conductor’s top edge, Hence any major topographic feature in these critical regions will significantly influence the peak response. For stations which are located on approaching valley slopes the target-loop propagation distance will exceed that which pertains in the flatearth case, and both anomaly width and peak response will be reduced. In contrast, a hill profile may increase the lateral proximity and enhance width and peak amplitude response. These observations are more readily demonstrated by plotting the 0.8 ms response for a hill, valley and flat-earth profile
35 TEtl
-1
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2 ”
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.00
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-
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STRIKE
90.0”
CONOUCTANCE
50.0
S
~,.,,,,,,.,,,,.,..,,,,,,,,,,,.,,,,,,,,,,,,,,, :
I
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DISTANCE
Fig. 2. a. Narrow valley TEM model response, and valley response curves for 0.8 ms, common
,,,, i
8 -
common loop, loop, 6 = 45”.
z
ri
6 = 45”.
b. Hill, flat earth,
TEn
tlOOEL
RESPONSE
-
COflfION
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i
DISTANCE
7
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5
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1
DISTANCE
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Fig. 3. a. Step TEM model response, conductor at base, common TEM model response, conductor at top, common loop, 6 = 45”.
@
loop, 6 = 45”. b. Step
37
at the same scale (see Fig. 2b). All other factors being equal a conductor located beneath a valley floor is less likely to be detected, in a relative sense, than one located in a hill or beneath a mountain slope. It should be emphasised that the study is intended to highlight, by way of example, some of the more unusual response behaviour which can occur in rugged terrain. Clearly these observations relate to the specific circumstances of the model used, and may not necessarily apply to all hill or valley profiles or to all dispositions of the conductor. In general the study showed that the distorting influence of terrain on anomaly shape diminished with increasing dip, as illustrated in Figs. lb, c and d. Step. For completeness, the response of a step profile is shown in Fig. 3. A conductor at the base of a step profile is in fact a composite of previous cases since it consists of one half of a valley response (Fig. 2a) and one half of a flat-earth response (Fig. la). Similarly Fig. 3b is a composite of a hill and flat-earth profile. Perhaps a less obvious geometric distortion occurs when a vertical conductor is positioned immediately below the break in slope. Fig. 4 shows that in such a case an asymmetric anomaly is observed - a characteristic which would normally suggest a shallow dipping conductor. Only by taking the terrain into account, and noting the spatially static behaviour of the central TEII
nODEi
RESPONSE
-
Fz 7
c i
COnnON
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1
I
-100.3
Fig. 4. Step
TEM
model
response,
conductor
I
E -
‘-’ ““’ at top, common
” loop, 6 = 90”.
38
minima with increasing time would the correct vertical disposition of the conductor be established. Separated
inline loops
Narrow hill. Fig. 5 shows that the response obtained with the separated loop geometry is similarly enhanced by a hill profile. For a dip S = 90” the positive shoulders are depressed but the central negative trough is increased six-fold by the greater loop proximity. For a dip 6 = 45” (Fig. 6) the down-dip positive shoulder of the normal flat-earth response (see Fig. 8) is distorted by the development of a second negative trough in the early time response. This behaviour is not unlike the frequency-domain anomalous response of two closely spaced conductors (Ketola and Puranen, 1967) and could be mis-interpreted as such, using flatearth criteria. Similar distortions were noted by Villegas-Garcia and West (1983) when studying ridge and valley discontinuities in a conductive overburden. It will be noted that at later time the down-dip flank of the anomaly, although depressed, remains positive, suggesting that late time behaviour is less affected by abrupt variations in topography. Narrow ualley. The effect of a valley profile is to attenuate the peak-topeak amplitude response as shown in Fig. 7. It will be observed that provided
TEnnODEL RESPONSE
TX-RX OEPTH
SPACING OF PLATE
STRIKE
90.0’
-
SEPARATED
INLINE
LOOPS
50. tl 25.n
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.l-----f
5. Narrow
hill and
flat-earth
TEM
model
response,
separated
inline
loops,
6 = 90”.
39 TEt,
tlODEL
RESPONSE
SEPARATED
INLINE
LOOPS
I”“““‘,“““’
E /
Fig. 6. Narrow
hill TEM
Ttn
rlODEL
DISTANCE
7
model
RESPONSE
response, -
SEPARATED
z-
tn,
separated INLINE
inline
loops,
6 = 45”.
LOOPS
G TX-RX
4 ,,,,
SPACING
CONDUCTANCE ,,,,,,,
50.
50.0
f
S
m
E 7
DISTANCE
ttll
d -
Fig. 7. Narrow valley TEM model response, comparison of constant distance intercoil spacing, separated inline loops, 6 = 90”.
horizontal
and slope
40 TEn
nOOEL
RESPONSE
-
SEPARATED
INLINE
LOOPS
.0100; DELAY .0075< \
.0050-
z 6
.00?5-
TInES
tnS1 .8
;
Fig.
8. Narrow
valley
and flat-earth
TEM
model
response,
separated inline loops, 6 = 45”.
the horizontal intercoil spacing is kept constant the zero cross-over width is unaffected by the terrain and can still be used as a diagnostic for locating the top edge of the conductor. This simple rule of thumb is not applicable if slope distance is used to measure loop separation. Using slope distance in a valley or hill situation will have the effect of laterally compressing the anomalous response as shown. The influence on response shape is analogous to changing the transmitter-receiver spacing in a flat earth situation (Ogilvy and Dabek, 1985), except of course, in rugged terrain, the effects are combined into a single profile. A simple post-survey cosine correction can rectify the lateral compression but there will still be a difference in observed amplitude, (cf. Fig. 5). For this reason, and in spite of the operational inconvenience, it may be advantageous to maintain a constant horizontal loop separation when profiling in hilly terrain. For a dip 6 = 45” (Fig. 8) a valley profile severely attenuates the peak amplitude response but has relatively little influence on anomaly shape, unlike the common loop geometry.
CONCLUSIONS
Some theoretical TEM model responses have been computed to show the effect of irregular terrain on common loop and separated loop anomalies.
41
The results show that substantial distortions in response shape can occur for hill-, valley- or step-type profiles due to the irregular variations in target-loop distance. Standard flat-earth type curves or characteristic curves will give erroneous results unless the influence of topography on response shape and amplitude is recognised. For the common loop geometry, abrupt changes in slope can introduce secondary minima which although spatially static with time might be misinterpreted as being diffusive minimal coupling lows associated with the top edge of a conductive sheet. It is shown that, by virtue of difference in loop proximity to the areal extent of the target, hill profiles can enhance the peak amplitude response, whereas anomalies observed in valleys are suppressed. Normal twin peak anomalies are unlikely to be observed in narrow valleys except at late time, since topographic compression tends to produce a singlepeak, centrally located anomaly. In many cases, the diffusive behaviour of TEM anomalies with increasing delay time provides the only basis for resolving interpretational ambiguities caused by topographic variations. When profiling with the separated loop configuration, valley profiles result in a predictable decrease in peak-to-peak amplitude but terrain breaks do not significantly influence response shape. In contrast, a hill profile can substantially enhance response magnitude and transform the normal flatearth response beyond recognition. The study confirms that topography is an important factor, both in survey design, in that it affects detectability, and in interpretation since normal flat-earth characteristics can be substantially altered. Traditional flatearth TEM criteria should be used with caution whenever an observed anomaly occurs in conjunction with a major topographic feature. ACKNOWLEDGEMENTS
The model study was undertaken using a modified and extended version of a computer program originally developed by Dr P. Weidelt, formerly of the Bundesanstalt fur Geowissenschaften und Rohstoffe, F.R.G. The author is appreciative of the BGS-BGR information exchange agreement that made this co-operation possible. Thanks are also due to Mr Z.K. Dabek for his assistance. The work was supported by a contract between the Natural Environment Research Council (NERC) and the Commission of the European Communities (Contract No. MSM 097(H)). The paper is published with the permission of the Director, British Geological Survey, NERC. REFERENCES Barnett, C.T., 1984. Simple inversion of time-domain electromagnetic 49: 925-933. Buselli, G. and Thorburn, M., 1983. SIROTEM model suite. CSIRO report (unpublished).
data. Geophysics, Div. of Min. Physics
42 Ketola, M. and Puranen, M., 1967. Type curves for the interpretation of Slingram (horizontal loop) anomalies over tabular bodies. Report of Investigations 1 and 2, Geol. Surv. of Finland (unpublished). Ogilvy, R.D., 1983. A model study of the transient electromagnetic coincident loop technique. Geoexploration, 21: 231-264. Ggilvy, R.D., 1986. Theoretical transient electromagnetic response curves for a thin dipping dyke in free space - separated inline loop configuration. Geophys. Prospec. In press. 1984. Theoretical transient electromagnetic response Ogilvy, R.D. and Dabek, Z.K., curves over a thin dipping dyke - Common loop configuration. Brit. Geol. Surv., Regional Geophys. Res. Group, Rep. 168, 22 pp. Ogilvy, R.D. and Dabek, Z.K., 1985. First order numerical approach to the interpretation of transient electromagnetic anomalies. Trans. Inst. Min. Metall., Section B, Appl. Earth Sci., 92: B20-B30. Diffusion of electromagnetic fields into a Oristaglio, M.L. and Hohmann, G.W., 1984. two-dimensional earth: A finite-difference approach. Geophysics, 49: 870-894. Rai, S.S. and Verma, S.K., 1984. Nomograms to interpret Crone PEM data using a dipping sheet model. Geophys. Prospect., 32: 740-749. Villegas-Garcia, C.J. and West, G.F., 1983. Recognition of electromagnetic overburden anomalies with horizontal loop electromagnetic survey data. Geophysics, 48: 42-51. Weidelt, P., 1983. The harmonic and transient electromagnetic response of a thin dipping dike. Geophysics, 48: 934-952.
Geoexploration, 24 (1986) 29-42 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
TOPOGRAPHIC DISTORTIONS OF TRANSIENT ELECTROMAGNETIC ANOMALIES IN RESISTIVE TERRAINS R.D. OGILVY Regional Geophysics Research Group, British ~eo~~~‘ca~ Survey, Keyworth~ Nottingham (United Kingdom) (Received February 18, 1985; accepted after revision January 14, 1986)
ABSTRACT Ogilvy, RD., 1985. Topographic distortions of transient electromagnetic resistive terrains. Geoexploration, 24: 29-42.
anomalies
in
A theoretical model study has been undertaken to assess the influence of irregular terrain on common loop and separated loop transient electromagnetic anomalies. Hill, valley and step profiles over a dipping conductor exhibit free-space anomalies which differ markedly both in amplitude and shape from those which pertain in flat earth situations, Differences in loop-target distance and orientation can accentuate anomalies observed over hills and suppress those recorded in valleys. In many cases the diffusive behaviour of TEM anomalies provides the only basis for resolving interpretational ambiguities caused by topographic variations, since terrain-induced geometric distortions are spatially static with increasing delay time. The study confirms the importance of topography in any interpretative scheme based on response characteristics.
INTRODUCTION
The transient electromagnetic (TEN) survey technique is now widely used for mineral prospecting but the in~~retative guidelines available are invariably based on model studies in which the terrain is assumed to be a plane horizontal surface. Qualitative interpretation in which response characteristics such as maxima, minima, anomaly width and asymmetry are used to infer the location and attitude of a conductor will clearly be in error in rugged or mount~ous terrain. ~~to~ions in response shape will occur due to the irregular variations in distance between the loops and the conductive body. Even quantitative interpretation based on total curve-matching techniques (Oristaglio and Hohmann, 1984; Barn&t, 1984) will give erroneous results unless the topographic profile is taken into account. The purpose of this paper is to show, by theoretical modelling, how some simple free-space terrain features such as a hill, valley and step can distort the normal flat-earth response when profiling over a thin dipping sheet conductor. While it is a relatively simple exercise to incorporate topographic variations into a numerical modelling program, no topographic studies have been reported in the literature. It would appear also that practising field
0016-7142/861$03.50
0 1986 Elsevier Science Publishers B.V.
30
geophysicists (who may have limited access to powerful mainframe computing facilities) are still largely dependent on published or unpublished type curves (Buselli and Thorbum, 1983; Ogilvy, 1983, 1986; Ogilvy and Dabek, 1984; Rai and Verma, 1984) to assess the disposition and significance of a prospective target. It is important therefore to show how these pre-determined type curves might be distorted or transformed when profiling in rugged terrain. It should be emphasised that the study is intended to complement widely used free-space interpretative aids, and hence is concerned only with terrain-induced geometric distortions. Computational constraints did not allow the effect of a conductive host to be included. THEORY
AND COMPUTATION
Theoretical TEM model response curves have been computed for hill, valley and step type profiles using an exact integral equation solution (Weidelt, 1983). The solution assumes a step function excitation of the target conductor and that the receiver is an induction loop which measures the time derivative of the secondary magnetic field as in the commercially available SIROTEM, Crone PEM and Geonics EM37 systems. The target mineralisation is represented by a thin two-dimensional dyke of finite conductance. Many ore deposits closely resemble sheet-like or tabular bodies, and hence the dipping dyke is a simple but realistic model. The mathematical formulation used in the modelling program is based on integrating the effects of half-sheet free-decay mode eigencurrents. The formal solution is obtained using the Wiener-Hopf technique and then cast into a rapidly convergent integral suitable for numerical evaluation. For the sake of brevity the integral equation is not presented here. Interested readers should refer to Weidelt (1983) for a complete theoretical description. Being exact the formulation allows the true diffusive character of the transient process to be reproduced in the response curves, and to observe how early and late time anomalies behave with differing levels of sensitivity to specific terrain features. Model responses have been computed for the common loop geometry, in which a single conductor loop acts as both transmitter and receiver, and also for the separated inline or Slingram loop geometry. In both cases transmitter and receiver loop orientations are assumed to be horizontal, and that the vertical secondary field component has been observed irrespective of slope angle. This is a reasonable assumption in the case of separated loop profiling since both transmitter and receiver are effectively dipolar and the operator has some control over orientation. In the case of the larger common loop configuration horizontality may or may not be maintained, since loop orientation would depend on the areal extent or size of the loop relative to the topographic feature. For simplicity and to facilitate more readily a compari-
31
son with flat-earth responses, the effects of changing loop orientation with slope have not been inco~o~ted. Further, to avoid slope averaging effects the common loop diameter used in the study was made small relative to the topographic model. For the separated loop geometry the computations were performed for circular transmitter and receiver loops of radius 2.5 m and for a fixed horizontal intercoil spacing of 50 m. It should be noted that the positivity of the separated loop response is consistent with data acquired by the S~~~TE~ system and that an opposite convention and different normalisation procedure is used by the Crone PEM system. The common loop radius was taken to be 5.0 m. In al1 cases the target conductance was kept constant at 50 S. Computation of the vertical field anomalies was carried out on a Honeywell 66/DPS-300 mainframe computer and then plotted off-line on a CALCOMP plotter. Each case, which consisted of 41 stations evaluated at 10 delay times (0.8, 1.2, 1.6, 2.0, 2.6, 3.4, 4.2, 5.0, 5.8, 7.0 ms), required between 2.5-3 hours of CPU time. Computations of this length are expensive and underline the desirability of having reliable interpretative guidelines based on response characteristics rather than total curve-matching, as shown by Ogilvy and Dabek (1985) and Ogilvy (1986). To economise on computer time, the vertical component was computed at fixed 10 m increments along the profile irrespective of response shape. Abrupt variations in response shape caused by inadequate sampling were subsequently smoothed out using a cubic spline curve fitting routine. The results are shown as plots of vertical field &V/A) normalised for transmitter and receiver turns, against distance in metres. For greater clarity some computed time-curves have been omitted from the final presentation_ Case parameters are given together with a cross-section to show the relative disposition of the conductor. MODEL RESPONSES
Narrow hilt. A comparison of Fig. la and lb illustrates bow a normal flatearth response might be transformed by a narrow ridge or hill profile. The most obvious effects are a broadening of the anomaly width, a higher up-dip and down-dip amplitude and a flattening of the down-dip maxima. All these effects can be attributed to the increased proximity of the loop to the conductor on the hill slopes and the persistence of that proximity especially on the down-dip side of the conductor. A flattened down-dip peak response at early time would normally indicate, as in Fig. lb, that the terrain slopeand theconductor dip are similar for that profile. It will also be observed that the sharp breaks in the topo~aphic profile at +60 m have a distorting influence on the shape of the anomaly flanks. With increasing dip (Fig. le) the topographic relief has Iess influence on anomaly shape as expected. With decreasing dip the
32 TEfl
RODEL
RESPONSE
-
COnnON
LOOP
DELAY
TIPiES
DELAY
TInES
_,OO.~ TEn
nOOEL
DEPTH STRIKE
R /
RESPONSE
OF
-
PLATE
COntlON
LOOP
25.n
.a 2.0 4.2 7.0
90.0°
mm 7
DISTANCE
(p1)
% -
E
33 TErl
tlOOEt
RESPONSE
-. COflnON
zi 7
TEf’
nODEI_
RESPONSE
-
COWlON
l.OOP
OISTANCE
tn)
0 -
LOOP
Fig. 1. a. Flat earth TEM model response, common loop, 6 = 45”. b. Narrow hill TEM model response, common loop, 6 = 45”. c. Narrow hill TEM model response, common loop, 6 = 75” d. Narrow hill TEM model response, common loop, 6 = 36”.
34
distortion is more accentuated as shown in Fig. Id. Such distortions whether in the form of local maxima or minima could compromise any shape analysis unless recognised by the interpreter as being terrain-induced. In the case of dipping conductors, terrain breaks of this kind can be identified by their static behaviour with increasing time. True minima associated with minimal coupling above the conductor will invariably show a time-dependent lateral migration downdip as the eddy currents diffuse inwards. Narrow valley. A more serious deformation of the flatearth response can occur in the case of a narrow valley profile (Fig. 2). Unlike the preceding case the overall response shape bears little resemblance to the flat-earth response given in Fig. la, and in addition there is a marked divergence in early and late time behaviour. At early time the normal asymmetric twin peak anomaly is depressed by the topographic minima at f 60 m, to give the appearance of a single centrally located peak response close to the conductor’s top edge. For a delay of 7 ms, the principal components of the response are a central minimum and flanking shoulders which, if viewed locally, might be mis-interpreted in terms of a conductor of near vertical disposition or even with a reversed dip, since the up-dip peak is more clearly defined. The central diagnostic minimum is poorly developed at early times due to the topographic compression of the valley slopes. In a practical field situation it could be difficult to differentiate between topographic minima such as those observed at f 60 m and time varying dynamic lows which are widely used to infer the position and dip of the conductor. For example, the response at 4.2 ms displays three minima of near-equal prominence and the geometric low at -60 m could readily be misinterpreted as reflecting the top edge of a shallow dipping conductor. The problem could be accentuated for deep-seated or poor conductors, as the target response may only be evident at late time or at signal levels which are close to the geologic noise threshold. If the target response is only observed in a single channel, anomaly analysis may have to be based on a single time-curve and without the insight afforded by multichannel data. In these circumstances, it could be difficult to differentiate between topographic distortions and genuine diffusive effects. ~though the results of the study are meant to be used in a qu~itative fashion, it is clear that peak amplitude and hence detec~bility will also be affected by the type of terrain profile. For the common loop geometry and a dipping sheet conductor, the twin maxima are normally off-set from the vertical projection of the conductor’s top edge, Hence any major topographic feature in these critical regions will significantly influence the peak response. For stations which are located on approaching valley slopes the target-loop propagation distance will exceed that which pertains in the flatearth case, and both anomaly width and peak response will be reduced. In contrast, a hill profile may increase the lateral proximity and enhance width and peak amplitude response. These observations are more readily demonstrated by plotting the 0.8 ms response for a hill, valley and flat-earth profile
35 TEtl
-1
c \
2 ”
5
.00
tlOOEL
RESPONSE
-
COllllON
LOOP
I
4.2 7.0
-2.00-i
Ttl?
flOOEL
RESPONSE
-
COPlflON
LOOP
DELAY
TiMES
cnS) .8
STRIKE
90.0”
CONOUCTANCE
50.0
S
~,.,,,,,,.,,,,.,..,,,,,,,,,,,.,,,,,,,,,,,,,,, :
I
iz 7
DISTANCE
Fig. 2. a. Narrow valley TEM model response, and valley response curves for 0.8 ms, common
,,,, i
8 -
common loop, loop, 6 = 45”.
z
ri
6 = 45”.
b. Hill, flat earth,
TEn
tlOOEL
RESPONSE
-
COflfION
LOOP
(
ns ) .8 2.0 4.2 7.0
i
DISTANCE
7
TEll
DODEL
RESPONSE
-
COntlON
tz -
tfl,
LOOP
.8 2.0 4.2 7.0
z 7
‘7 !
5
z
z
-100.
1
DISTANCE
-\
I ( /
E
tfl,
-
N__. \ j_
~
*
Fig. 3. a. Step TEM model response, conductor at base, common TEM model response, conductor at top, common loop, 6 = 45”.
@
loop, 6 = 45”. b. Step
37
at the same scale (see Fig. 2b). All other factors being equal a conductor located beneath a valley floor is less likely to be detected, in a relative sense, than one located in a hill or beneath a mountain slope. It should be emphasised that the study is intended to highlight, by way of example, some of the more unusual response behaviour which can occur in rugged terrain. Clearly these observations relate to the specific circumstances of the model used, and may not necessarily apply to all hill or valley profiles or to all dispositions of the conductor. In general the study showed that the distorting influence of terrain on anomaly shape diminished with increasing dip, as illustrated in Figs. lb, c and d. Step. For completeness, the response of a step profile is shown in Fig. 3. A conductor at the base of a step profile is in fact a composite of previous cases since it consists of one half of a valley response (Fig. 2a) and one half of a flat-earth response (Fig. la). Similarly Fig. 3b is a composite of a hill and flat-earth profile. Perhaps a less obvious geometric distortion occurs when a vertical conductor is positioned immediately below the break in slope. Fig. 4 shows that in such a case an asymmetric anomaly is observed - a characteristic which would normally suggest a shallow dipping conductor. Only by taking the terrain into account, and noting the spatially static behaviour of the central TEII
nODEi
RESPONSE
-
Fz 7
c i
COnnON
LOOP
DISTANCE
1
I
-100.3
Fig. 4. Step
TEM
model
response,
conductor
I
E -
‘-’ ““’ at top, common
” loop, 6 = 90”.
38
minima with increasing time would the correct vertical disposition of the conductor be established. Separated
inline loops
Narrow hill. Fig. 5 shows that the response obtained with the separated loop geometry is similarly enhanced by a hill profile. For a dip S = 90” the positive shoulders are depressed but the central negative trough is increased six-fold by the greater loop proximity. For a dip 6 = 45” (Fig. 6) the down-dip positive shoulder of the normal flat-earth response (see Fig. 8) is distorted by the development of a second negative trough in the early time response. This behaviour is not unlike the frequency-domain anomalous response of two closely spaced conductors (Ketola and Puranen, 1967) and could be mis-interpreted as such, using flatearth criteria. Similar distortions were noted by Villegas-Garcia and West (1983) when studying ridge and valley discontinuities in a conductive overburden. It will be noted that at later time the down-dip flank of the anomaly, although depressed, remains positive, suggesting that late time behaviour is less affected by abrupt variations in topography. Narrow ualley. The effect of a valley profile is to attenuate the peak-topeak amplitude response as shown in Fig. 7. It will be observed that provided
TEnnODEL RESPONSE
TX-RX OEPTH
SPACING OF PLATE
STRIKE
90.0’
-
SEPARATED
INLINE
LOOPS
50. tl 25.n
‘: ,,,,,,,,,, DISTANCE
ttl)
~,IT
P -
‘L
‘1
E x-
Fig.
-100.
.l-----f
5. Narrow
hill and
flat-earth
TEM
model
response,
separated
inline
loops,
6 = 90”.
39 TEt,
tlODEL
RESPONSE
SEPARATED
INLINE
LOOPS
I”“““‘,“““’
E /
Fig. 6. Narrow
hill TEM
Ttn
rlODEL
DISTANCE
7
model
RESPONSE
response, -
SEPARATED
z-
tn,
separated INLINE
inline
loops,
6 = 45”.
LOOPS
G TX-RX
4 ,,,,
SPACING
CONDUCTANCE ,,,,,,,
50.
50.0
f
S
m
E 7
DISTANCE
ttll
d -
Fig. 7. Narrow valley TEM model response, comparison of constant distance intercoil spacing, separated inline loops, 6 = 90”.
horizontal
and slope
40 TEn
nOOEL
RESPONSE
-
SEPARATED
INLINE
LOOPS
.0100; DELAY .0075< \
.0050-
z 6
.00?5-
TInES
tnS1 .8
;
Fig.
8. Narrow
valley
and flat-earth
TEM
model
response,
separated inline loops, 6 = 45”.
the horizontal intercoil spacing is kept constant the zero cross-over width is unaffected by the terrain and can still be used as a diagnostic for locating the top edge of the conductor. This simple rule of thumb is not applicable if slope distance is used to measure loop separation. Using slope distance in a valley or hill situation will have the effect of laterally compressing the anomalous response as shown. The influence on response shape is analogous to changing the transmitter-receiver spacing in a flat earth situation (Ogilvy and Dabek, 1985), except of course, in rugged terrain, the effects are combined into a single profile. A simple post-survey cosine correction can rectify the lateral compression but there will still be a difference in observed amplitude, (cf. Fig. 5). For this reason, and in spite of the operational inconvenience, it may be advantageous to maintain a constant horizontal loop separation when profiling in hilly terrain. For a dip 6 = 45” (Fig. 8) a valley profile severely attenuates the peak amplitude response but has relatively little influence on anomaly shape, unlike the common loop geometry.
CONCLUSIONS
Some theoretical TEM model responses have been computed to show the effect of irregular terrain on common loop and separated loop anomalies.
41
The results show that substantial distortions in response shape can occur for hill-, valley- or step-type profiles due to the irregular variations in target-loop distance. Standard flat-earth type curves or characteristic curves will give erroneous results unless the influence of topography on response shape and amplitude is recognised. For the common loop geometry, abrupt changes in slope can introduce secondary minima which although spatially static with time might be misinterpreted as being diffusive minimal coupling lows associated with the top edge of a conductive sheet. It is shown that, by virtue of difference in loop proximity to the areal extent of the target, hill profiles can enhance the peak amplitude response, whereas anomalies observed in valleys are suppressed. Normal twin peak anomalies are unlikely to be observed in narrow valleys except at late time, since topographic compression tends to produce a singlepeak, centrally located anomaly. In many cases, the diffusive behaviour of TEM anomalies with increasing delay time provides the only basis for resolving interpretational ambiguities caused by topographic variations. When profiling with the separated loop configuration, valley profiles result in a predictable decrease in peak-to-peak amplitude but terrain breaks do not significantly influence response shape. In contrast, a hill profile can substantially enhance response magnitude and transform the normal flatearth response beyond recognition. The study confirms that topography is an important factor, both in survey design, in that it affects detectability, and in interpretation since normal flat-earth characteristics can be substantially altered. Traditional flatearth TEM criteria should be used with caution whenever an observed anomaly occurs in conjunction with a major topographic feature. ACKNOWLEDGEMENTS
The model study was undertaken using a modified and extended version of a computer program originally developed by Dr P. Weidelt, formerly of the Bundesanstalt fur Geowissenschaften und Rohstoffe, F.R.G. The author is appreciative of the BGS-BGR information exchange agreement that made this co-operation possible. Thanks are also due to Mr Z.K. Dabek for his assistance. The work was supported by a contract between the Natural Environment Research Council (NERC) and the Commission of the European Communities (Contract No. MSM 097(H)). The paper is published with the permission of the Director, British Geological Survey, NERC. REFERENCES Barnett, C.T., 1984. Simple inversion of time-domain electromagnetic 49: 925-933. Buselli, G. and Thorburn, M., 1983. SIROTEM model suite. CSIRO report (unpublished).
data. Geophysics, Div. of Min. Physics
42 Ketola, M. and Puranen, M., 1967. Type curves for the interpretation of Slingram (horizontal loop) anomalies over tabular bodies. Report of Investigations 1 and 2, Geol. Surv. of Finland (unpublished). Ogilvy, R.D., 1983. A model study of the transient electromagnetic coincident loop technique. Geoexploration, 21: 231-264. Ggilvy, R.D., 1986. Theoretical transient electromagnetic response curves for a thin dipping dyke in free space - separated inline loop configuration. Geophys. Prospec. In press. 1984. Theoretical transient electromagnetic response Ogilvy, R.D. and Dabek, Z.K., curves over a thin dipping dyke - Common loop configuration. Brit. Geol. Surv., Regional Geophys. Res. Group, Rep. 168, 22 pp. Ogilvy, R.D. and Dabek, Z.K., 1985. First order numerical approach to the interpretation of transient electromagnetic anomalies. Trans. Inst. Min. Metall., Section B, Appl. Earth Sci., 92: B20-B30. Diffusion of electromagnetic fields into a Oristaglio, M.L. and Hohmann, G.W., 1984. two-dimensional earth: A finite-difference approach. Geophysics, 49: 870-894. Rai, S.S. and Verma, S.K., 1984. Nomograms to interpret Crone PEM data using a dipping sheet model. Geophys. Prospect., 32: 740-749. Villegas-Garcia, C.J. and West, G.F., 1983. Recognition of electromagnetic overburden anomalies with horizontal loop electromagnetic survey data. Geophysics, 48: 42-51. Weidelt, P., 1983. The harmonic and transient electromagnetic response of a thin dipping dike. Geophysics, 48: 934-952.