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High resolution electrical imaging of fault zones
Physics of the Earth and Planetary Interiors 150 (2005) 93–105
High resolution electrical imaging of fault zones Malcolm Ingham∗ School of Chemical & Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Received 15 July 2003; received in revised form 4 November 2003; accepted 16 August 2004
Abstract Two-dimensional numerical modelling studies of a generic conductive fault zone indicate that very closely spaced TM mode magnetotelluric (MT) data are capable of providing high-resolution images of the fault structure. The degree of resolution that is possible is largely controlled by the conductance of the overburden and is also dependent on the spacing of measurement sites. In the absence of any overburden, or in the case of very thin overburden, the resolution is limited by the minimum electrode dipole length which is practicable. As an example, the results of two-dimensional inversion of closely spaced data measured across the Alpine Fault are compared with an earlier model derived from more widely spaced measurement sites. Much higher resolution of the fault zone is obtained from the closely spaced data and some features of the earlier model appear to arise due to the sparseness of the data. In the case of a very narrow fault zone, it is shown that the MT data exhibit very little lateral variation in phase and therefore independent control of static-shift is necessary. © 2004 Elsevier B.V. All rights reserved. Keywords: Fault zone; Magnetotellurics; Numerical modelling
1. Introduction Magnetotelluric (MT) measurements made across the Alpine Fault in the South Island of New Zealand have identified the existence of a zone of high electrical conductivity associated with the fault zone (Ingham and Brown, 1998). This result is similar to that of MT surveys over major transform and strike–slip faults elsewhere (e.g. Jones et al., 1992; Unsworth et al., 1999). In these studies, the high conductivity has generally been interpreted in terms of the presence of fluids ∗
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within the fault zone. Such an interpretation is significant because of the accumulated evidence that suggests that fluids play an important role in a variety of faulting processes (e.g. Hickman et al., 1995; Rice, 1992; Sibson et al., 1988; Crampin et al., 2002). In the study reported by Ingham and Brown (1998), the minimum spacing of measurement sites was ∼300 m, typical of the inter-site spacing in most studies of this nature. Intuitively, however, the ability of MT data to resolve lateral conductivity structure across a narrow fault zone should be limited only by the requirement that the length of the dipole used to measure the electrical field perpendicular to the fault zone should not be greater than the width of the fault zone itself.
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In this paper, therefore, the use of more closely spaced MT data to provide high-resolution images of conductive fault zones is investigated. First, the results of twodimensional numerical modelling studies, which illustrate the sensitivity of MT data to different features of generic models of fault zones, are discussed. This is followed by the presentation of a field example using very closely spaced dc resistivity and MT data collected across the Alpine Fault. The location of these measurements is very close to the original location of the study reported by Ingham and Brown (1998) and comparison of the results with those of the earlier study illustrates the limitations in resolution resulting from using measurement sites that are separated by distances similar to the width of the fault zone under study. MT sounding involves the measurement of time variations in horizontal components of the naturally occurring magnetic and electric fields. To image the structure of a fault zone from the surface down to several km depth, the typical frequency range for such measurements might range from 1000 to 0.1 Hz. At any individual frequency, the MT impedance tensor relates the amplitude and phase of the variations in the electric field components to those in the magnetic fields. Where measurements are made in a coordinate system oriented parallel and perpendicular to a fault zone, assuming two-dimensionality of the electrical structure associated with the fault, the diagonal elements of the impedance tensor should be zero. The off-diagonal elements relating to the electric fields perpendicular and parallel to the fault then correspond to the impedances associated with the TM and TE modes of electromagnetic induction, respectively. Due to its much sharper spatial variation, lateral changes in resistivity are often derived from solely the TM mode impedance (Wannamaker, 1999). Ledo et al. (2002) have also suggested that measurements made above a structure with a finite two-dimensional strike length are best interpreted two-dimensionally using only the TM mode responses. As a consequence, this paper concentrates solely on the ability of TM mode MT data to image fault structure.
2. Numerical modelling of a generic conductive fault zone A generic two-dimensional electrical resistivity model designed to investigate the ability of MT data to
Fig. 1. Generic model of a conductive fault zone.
detect the presence of a conductive vertical fault zone is shown in Fig. 1. The model is based upon the structure derived by Ingham and Brown (1998) for the Alpine Fault. The fault zone has a width , lies between units which have bulk resistivities of 500 and 1000 m, and is buried beneath a relatively conductive overburden (50 m) of thickness t. The resistivity ρ of the fault zone is taken to be less than the resistivities of the adjacent units, i.e. it is assumed, for example, that the presence of fluid within the fault zone has led to a reduction in bulk resistivity. The fault zone is shown to lose its definition below 10 km depth where a relatively conductive substratum of 100 m resistivity is included in the model. Many transform fault zones are believed to penetrate into the mid to lower crust, and the conductive substratum may be regarded as a representation of the fact that the continental lower crust is commonly observed to be relatively conductive compared to the upper crust (e.g. Hyndman et al., 1993). Shown in Figs. 2–4 are the results of forward model calculations used to assess the sensitivity of TM mode MT data to various features of this generic model. Fig. 2 shows, in the form of data pseudosections for frequencies from 1000 to 0.1 Hz, the expected TM apparent resistivity and phase responses across a fault
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Fig. 2. Simulated TM mode MT data across a conductive fault zone of resistivity 3 m and width with a conductive overburden of thickness 200 m.
zone with t = 200 m and ρ = 3 m, for different widths. The pseudosections cover measurements made up to distances of 1 km on either side of the centre of the fault zone and simulate the results obtained from measurement sites spaced 50 m apart. For a conductive fault zone of width m, the pseudosections clearly indicate the existence of the fault zone and show two main features associated with it—a depression of apparent resistivity throughout the entire period range and an increase in phase at high frequency (short period). However, with 200 m thickness of overburden, the TM mode data appear to be unable to resolve the presence of the conductive fault zone if its width is much less than 200 m. For m, the presence of the zone is indicated only by a
slight “pinching” of the contours of both ρa and ϕa and by a small high in phase at the shortest periods. For a fault zone of 100 m width, there are smooth changes in both ρa and φa which result from the resistivity contrast between the 500 and 1000 m units on either side of the fault zone. A residual slight high in phase persists at short period but, given the uncertainties inherent in real data, this is unlikely to be resolvable in practice. Although not shown here, data pseudosections for TE mode data also show features associated with the fault zone. In particular, a marked minimum in apparent resistivity exists and there is zone of enhanced phase at short period similar to that shown in Fig. 2 for
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Fig. 3. Simulated TM mode MT data across a conductive fault zone of resistivity 3 m and width 100 m with a conductive overburden of thickness t.
the TM mode data. However, the continuity across the fault zone of the electric field parallel to the resistivity boundaries results in these features not being as laterally constrained as are the features in the TM mode data which derive from an electric field which is discontinuous across the fault zone. This conclusion was reached in an earlier study by Schnegg et al. (1986) who explained the higher resolution of the TM mode in terms of the adjustment distance of the fields in the vicinity of a perturbing body being much shorter for the TM mode than for the TE mode. The ability of the TM data to resolve a narrow fault zone in fact depends upon both the thickness and the electrical resistivity of the overburden. For an overburden with lower resistivity than the units adjacent to the fault zone, if the thickness t of the overburden is signif-
icant, most of the electric current associated with the TM mode is concentrated in the overburden. Above the highly conductive fault zone, there is, in principle, a readjustment of current streamlines and current is pulled to greater depth. As described by Fischer (1985), this leads to an increase in the observed phase and, because of a concomitant decrease in surface electric field, to a decrease in apparent resistivity above the fault zone. The degree to which this occurs depends upon the adjustment distance of the TM electric field within the fault zone. For a fault zone of large width (in the case in question, roughly w > t), the adjustment distance is less than the width of the fault zone and the electric field is able to penetrate to sufficient depth into the conductive fault zone to yield an anomaly in both TM ρa and φa . In contrast, if w < t, the adjustment distance
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Fig. 4. Simulated TM mode MT data across a conductive fault zone of resistivity ρ ( m) and width 20 m with a conductive overburden of thickness 5 m.
is comparable to the width of the fault zone and current penetration to greater depth is unable to take place to a significant degree. As a result, no anomaly occurs in either ρa or φa . If the overburden is either thinner or more resistive (i.e. the skin-depth in the overburden is greater), a significant amount of current is present in the underlying units relative to the overburden and is therefore able to penetrate into the conductive fault zone. This allows much narrower fault zones to be resolved if the overburden is thinner or more resistive. In the absence of an overburden (t = 0), the surface electric field is discontinuous at the edges of the fault zone. This behaviour is illustrated in Fig. 3, which shows the TM mode data pseudosections that result from a fault zone of width 100 m for a series of different thick-
nesses of the conductive overburden. When, the fault zone is marked only by a small minimum in apparent resistivity. However, when the thickness of the overburden is significantly less than the width of the fault zone, the anomaly in apparent resistivity again delineates the location of the conductive fault zone. In contrast, as the skin-depth in the conductive overburden is comparable to its thickness t, the sensitivity of the phase data to the fault zone is much reduced. For a thinner thickness of the overburden, a small high in phase occurs at high frequency but does not define the fault zone to the same degree as does the apparent resistivity. The increased dependence on the apparent resistivity data to resolve a much narrower fault zone is further emphasised in Fig. 4. This shows the TM mode data pseudosections resulting from a fault zone of only 20 m
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width with an overburden of thickness t = 5 m for two different resistivities of the fault zone. In neither case does any anomaly in phase occur. The results shown in Fig. 4 also indicate the sensitivity of the resolution of the fault zone to its resistivity. When the contrast between the resistivity of the fault zone and the surrounding blocks is reduced, the magnitude of the anomaly is decreased. Although relatively pure water can have a resistivity of around 100 m, in reality, either through the presence within the fault zone of fine grained material resulting from fault action, or through heating of deeply circulating groundwater, the ambient resistivity of the fault zone is likely to be significantly less. Only in the case of faults penetrating into conductive sediments, may the lack of resistivity contrast prevent resolution of the fault zone. The pseudosections shown in Fig. 4 are based on data that simulate measurement sites that adjacent to, and within, the narrow fault zone are spaced only 5 m
apart. This separation implies a very much smaller electrode dipole length than is normally used in field measurements and, as a consequence, in a real situation would yield a much lower signal to noise ratio. Thus, although the theoretical implications drawn from Fig. 4 are valid, the practicalities of making real measurements over such a narrow fault zone are not straightforward. Apart from the pseudosections shown in Fig. 4, all of the other pseudosections are derived from theoretical measurement sites spaced continuously across the model with a separation of 50 m between adjacent sites. In practice, the ability of TM mode MT data to resolve a narrow fault zone is strongly dependent on the density of actual measurement sites. This is illustrated very clearly in Fig. 5 which shows the results that are obtained when theoretical TM mode data from particular distributions of sites are inverted using the twodimensional technique of Smith and Booker (1991).
Fig. 5. Resistivity structures derived by two-dimensional inversion of theoretical TM mode MT data generated from the generic fault model with m, ρ = 3 m, and t = 200 m. Arrows indicate the locations of simulated measurement sites used in each inversion. Dashed lines outline the actual resistivity boundaries of the generating model.
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The theoretical data have been generated from a forward model of the generic form shown in Fig. 1 with m, ρ = 3 m, and t = 200 m. In each part of Fig. 5, the locations of the actual resistivity boundaries in the model are shown by the dashed lines. The structure shown in Fig. 5(a) is that which is obtained when theoretical data from a sparse distribution of sites, of which none are located above the actual fault zone, are inverted. Although a slight minimum in resistivity, which is broadly coincident with the conductive fault zone, is observed below 2 km in depth, the recovered structure shows no clear indication of the existence of this zone. When data from a sparse distribution of sites, where one site is located above the fault zone, are inverted (Fig. 5(b)), the recovered resistivity structure does give an indication of the existence of the conductive zone. This is primarily evidenced in the minimum in resistivity that is observed at shallow depth. However, the overall resolution of the conductive fault zone is still poor and there is little control on either its true resistivity or its width. In contrast, as is shown in Fig. 5(c), inversion of data from a high density of sites, of which several are located above the fault zone, clearly resolves the existence of the narrow conductive zone. In this case although the true resistivity of the zone is still significantly overestimated, the width of the zone is well constrained. It is clear therefore that, theoretically at least, TM mode MT data have the potential to resolve and image very narrow conductive fault zones. Limits to the resolving ability depend largely upon the width of the fault zone relative to the thickness (or more correctly, the conductance) of any overburden, and on the density of measurement sites. The resistivity contrast between the fault zone and the surrounding material is also a controlling factor. In the limit of very thin overburden, however, a clear resolution of a very narrow fault zone would necessitate a reduction of the electrode spacing used to make the TM mode measurements so that the electric dipole does not completely straddle the fault zone.
3. Field example—the Alpine Fault 3.1. Study location The conductive structure derived by Ingham and Brown (1998) from their MT data measured in the
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vicinity of the Alpine Fault has a width which varies from around 300 m in the upper 1 km to over 1 km at approximately 2 km depth. However, of the eight measurements sites from which data were modelled, only two were within 200 m of the actual fault and these were separated by ∼ 300 m. Thus, although testing by Ingham and Brown (1998) of the various model parameters, indicated that the existence of the anomalous zone associated with the fault was unambiguous, uncertainty remains as to the width of the conductive region. To test the ability of very closely spaced TM mode MT data to provide a higher resolution image of the Alpine Fault and to better constrain the width of the conductive zone, new electrical measurements were made across the fault in February 2002. The study location is shown in Fig. 6, in which the MT sites discussed by Ingham (1996) and Ingham and Brown (1998) are also shown. The solid line indicates a transect of just under 1 km in length, oriented perpendicular to the trace of the Alpine Fault, along which direct current (dc) resistivity profiling was carried out and TM mode MT measurements were made. The aim of the dc measurements was to yield a model of the near surface electrical resistivity structure which might identify and aid any necessary correction for static-shift (Jones, 1988) of the MT data. The dc measurements were made using a Wenner array with a basic electrode separation of a = 10 m. Measurements were obtained along approximately 800 m of the transect using interelectrode spacings of 10, 20, 30, 40 and 50 m. MT data were collected using 50 m electric dipoles oriented perpendicular to the fault to measure variations in the TM mode electric field. Simultaneous magnetic field variations were measured using induction coil magnetometers in orientations parallel and perpendicular to the fault. Measurements were made at 11 locations over a lateral range of 700 m. Defining the x-direction as that perpendicular to the fault, the MT impedance tensor elements Zxx and Zxy were calculated for each of the locations. The nominal frequency range covered by the measurements was 1000–1 Hz, however, the presence of a power distribution line along a minor road adjacent to the transect (Fig. 6) restricted the useable frequency range to 100–1 Hz at some of the measurement sites. The near surface structure along the length of the transect consists of river gravels and moraine deposits. Slightly to the northeast of the transect, the line of the Alpine Fault is marked by a northwest facing scarp of
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Fig. 6. Location of dc and MT measurements across the Alpine Fault in the South Island of New Zealand. Solid line: location of transect of dc and MT data discussed in the text; dashed line: 1998 dc resistivity profile; crosses: 1998 Schlumberger dc resistivity soundings; dots: MT sites discussed by Ingham (1996) and Ingham and Brown (1998).
about 5 m height. However, the topography along the length of the transect is much more subdued with only a gradual drop in elevation of about 5 m over a distance of some 200 m in the centre of the transect. 3.2. dc resistivity data The dc resistivity data are shown in the form of a pseudosection of apparent resistivity in Fig. 7(a). The vertical axis of the pseudosection is in terms of a pseudodepth defined as the median depth of investigation (Edwards, 1977) for a given electrode separation a, and given by approximately 0.5a. Also shown in Fig. 7(a) are the two-dimensional resistivity model derived through two-dimensional numerical inversion of the data, and the apparent resistivity pseudosection calculated from the model. As can be seen from comparison of the measured and calculated pseudosections, the fit of the model to the data is excellent (rms misfit of 3.4%). The model is marked by a narrow zone of lower resistivity, centred on x = 560 m, that has a width at the surface of ∼70 m but which splays out with depth. Outside this zone, the surface resistivity values are high (>1000 m) with values dropping below this at depths below 20 m. The low resistivity zone coincides at the surface with the base of the topographic slope which is a direct continuation of the 5 m scarplet on the measurement line of Ingham and Brown (1998).
The dc resistivity data have been supplemented by earlier dc data measured at the same time, and along the same line, as the original MT data reported by Ingham and Brown (1998). These data include two Schlumberger depth soundings and a single Wenner profile using an electrode spacing of a = 100 m. The locations of both the soundings and the profile are shown in Fig. 6. Shown in Fig. 7(b) is a pseudosection of the combination of the two sets of profiling data. Also shown are the model and calculated response derived through two-dimensional inversion of the combined data. The addition of the profiling data at larger electrode separation shows that the narrow zone of lower resistivity observed at the surface connects to a deeper low resistivity zone which in the central part of the profile has a resistivity below 100 m beneath about 50 m in depth. Low surface resistivity values at the northwest end of the profile (Fig. 7(b)) are not well resolved as there are no data at small electrode separations in this region. These results are supported by the results of the Schlumberger soundings which are shown in Fig. 8 and summarised in Table 1. On the southeast side of the fault trace (sounding 1), one-dimensional modelling of the Schlumberger curve gives a resistivity of 117 m at a depth of around 50 m, in agreement with the traversing results. In contrast, sounding 2 (to the northwest of the end of the Wenner traverse) suggests that at comparable depths the resistivity remains between 500 and ∼3000 m.
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Fig. 7. (a) The 2002 measured Wenner dc resistivity traversing data, two-dimensional inversion model response, and two-dimensional inversion model. (b) Combined 1998 and 2002 measured Wenner dc resistivity traversing data, two-dimensional inversion model response, and twodimensional inversion model.
3.3. MT data Apparent resistivity and phase pseudosections of the TM mode MT data measured along the transect are
shown in Fig. 9. Most of the 11 measurement locations were uniformly spaced over the observed low in apparent resistivity noted in the dc traversing data. The apparent resistivity pseudosection shows that a distinct
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Fig. 8. Schlumberger dc resistivity soundings and one-dimensional model responses (smooth curves) at locations 1 and 2 as shown in Fig. 6.
low in apparent resistivity exists in the frequency range 100–1 Hz over about 200 m width between measurements sites C and G. Associated with this low, and in general agreement with the modelling results presented in Fig. 2, is a slight high in phase where values above 45◦ are observed at short periods. To the southeast of the low, the apparent resistivity is relatively constant both over most of the period range and laterally. In contrast, to the northwest there appears to be a strong lateral gradient in apparent resistivity. Although the depth to which the shortest period MT data are sensitive (∼200–500 m) is somewhat greater than the deepest penetration of the dc data (∼50 m), the observed MT responses are broadly consistent with the shallow resistivity structure derived from the inversion of the dc data. Over the range from 200 to 700 m along the transect, the observed MT apparent resistivity at short period is less than 100 m, in agreement with the resistivity value below 50 m depth obtained from the dc data. Similarly, the rise in MT apparent resistivity to the
northwest is consistent with the higher resistivity values determined by the Schlumberger soundings in this region. This correlation between the observed MT and dc data, and the phase features in the MT data, suggest that the MT data have not been significantly affected by static-shift (Jones, 1988). As a result, two-dimensional inversion of the MT data has been performed without any prior correction for static-shift. Shown in Fig. 10(a) is the result of a robust twodimensional inversion of the data using the technique of Smith and Booker (1991). The calculated apparent resistivity and phase responses of this model are shown in Fig. 9 and provide an excellent fit to the observed MT data. The derived model indicates the existence of a narrow (∼200 m wide) near vertical zone of low resistivity. This feature penetrates to a depth of at least 2 km beneath which resolution, as is evidenced by the minor oscillations in structure, is poor. This conductive zone is partially separated from a deeper region of low resistivity to the northwest by a vertical finger of higher resistivity. Immediately to the northwest of the low resistivity zone, the resistivity in the upper 1 km is high. For comparison, the appropriate part of the two-dimensional structure derived by Ingham and Brown (1998), also using the inversion technique of Smith and Booker (1991), is shown on the same scale in Fig. 10(b). Also marked are the locations of the two MT sites used by Ingham and Brown (1998) in this part of their transect of the Alpine Fault. Comparison of the two models confirms the theoretical results shown in Fig. 5 regarding the degree to which the resolution of the conductive zone depends upon the density of the measurement sites. It is clear that a lower density of sites results in an inability to discriminate between the two regions of low resistivity, and in the earlier model these appear as a single, much broader region of low resistivity. It can also be noted that the absence in the
Table 1 One-dimensional models of Schlumberger dc soundings Sounding 1
Sounding 2
Resistivity ( m)
Thickness (m)
Depth (m)
Resistivity ( m)
Thickness (m)
Depth (m)
1638 3804 867 117
0.27 6.8 45 –
– 0.27 7.1 52
782 4411 2389 610
0.35 2.4 52 –
– 0.35 2.8 55
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Fig. 9. Measured and model TM mode MT data.
earlier study of a measurement site within the range 500–700 m results not only in surface resistivities of much less than 100 m not being detected, but also in the surface projection of the deeper low resistivity region being located further to the southeast. The higher measurement density also suggests that the resistivity of the actual fault zone, although low compared to the surroundings, is considerably higher than was suggested by the earlier work. Both models shown in Fig. 10 identify the near surface region immediately to the northwest of the fault zone as resistive. In the case of the model derived by Ingham and Brown (1998), this partly results from the inclusion in the inversion of data from sites further to the northwest, and not indicated in Fig. 10(b). The identification of the resistive region in Fig. 10(a) is based only on the slightly higher apparent resistivity values observed at the northwesternmost sites I and J compared to the values at sites to the southeast. The feature is not therefore well resolved. Better resolution of this structure would necessitate additional sites to the northwest. Ingham and Brown (1998) interpreted this high resistivity as being associated with the Fraser Formation, a zone of cataclastic deformation and my-
lonitization (Reed, 1964) between the Alpine Fault and the Fraser Fault to the northwest (Fig. 6). Similarly, the existence of high resistivity to the southeast in the earlier model was based on data from sites located at some distance from the fault zone. Ingham and Brown (1998) interpreted this rise in resistivity as being due to the high grade schists to the southeast of the Alpine Fault. The results of the present, more detailed, study show that the resistivity immediately to the southeast of the fault zone is not as high as previously indicated.
4. Discussion The numerical modelling of a generic fault zone clearly suggests that the ability of MT data to resolve a conductive fault zone is limited primarily by the conductance (i.e. conductivity–thickness product) of any overburden. When either no, or only low conductance, overburden exists the limitations on resolution arise from the resistivity contrast between the fault zone and its surroundings, and the length of the dipole used to record electric field variations.
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Fig. 10. Two-dimensional inversion models of TM mode MT data across the Alpine Fault: (a) this study; (b) Ingham and Brown (1998).
The inversion and interpretation of field data from across the Alpine Fault confirms the ability of closely spaced MT data to provide high-resolution images of fault structure, as is suggested by the results of the numerical modelling. In the field example discussed, the fact that the surface moraines and gravels have a higher resistivity than that used in the generic fault model enables the field data to resolve a fault zone which has a width of only ∼200 m. The actual field situation is similar to that depicted in the central panels of Fig. 2, where, associated with the fault zone, there is a narrow, but distinct, low in the TM apparent resistivity data and a small high in phase. Comparison with the earlier model of the Alpine Fault, derived from more widely spaced data, shows that more closely spaced data give a much higher resolution of the fault zone.
In the present case a single conductive zone indicated by the earlier study is resolved as two narrower conductive zones. Not only is higher resolution of the fault structure obtainable, but comparison with the earlier model shows that interpretation of data from relatively widely spaced sites can in fact suggest the existence of features which more closely spaced data show not to exist. In this case, both the high resistivity immediately to the southeast of the fault, and the extremely low resistivity of the fault zone itself fall into this category. It is also clear that where variations in MT apparent resistivity occur on a very small spatial scale the absence of a significant phase signature requires that interpretation of these changes in terms of fault structure has supporting evidence that they are not purely
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the result of static-shift. Typically static-shift may be recognised by the existence of large lateral gradients in apparent resistivity coincident with laterally uniform phase. As can be seen, most clearly from Fig. 4, in the situation of a narrow conductive fault zone such an interpretation would be erroneous. In the field example of the Alpine Fault discussed above static-shift control is provided by shallow dc resistivity data obtained across the fault zone. With this type of constraint upon interpretation it is evident that high resolution electrically imagery is capable of providing new insights into fault structure.
Acknowledgements This work was supported by grant URF 1/52 from the Victoria University of Wellington University Research Fund. Help in the field from Simon Granville is gratefully acknowledged as are helpful reviews from Andreas Junge and an anonymous referee.
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Ingham, M., 1996. Magnetotelluric soundings across the South Island of New Zealand: electrical structure associated with the orogen of the Southern Alps. Geophys. J. Int. 124, 134– 148. Ingham, M.R., Brown, C., 1998. A magnetotelluric study of the Alpine Fault, New Zealand. Geophys. J. Int. 135, 542– 552. Jones, A.G., 1988. Static shift of magnetotelluric data and its removal in a sedimentary environment. Geophysics 7, 967– 978. Jones, A.G., Kurtz, R.D., Boerner, D.E., Craven, J.A., McNeice, G.W., Gough, D.I., DeLaurier, J.M., Ellis, R.G., 1992. Electromagnetic constraints on strike–slip fault geometry—the Fraser River fault system. Geology 20, 561–564. Ledo, J., Queralt, P., Marti, A., Jones, A.G., 2002. Two-dimensional interpretation of three-dimensional magnetotelluric data: an example of limitations and resolution. Geophys. J. Int. 150, 127–139. Reed, J.J., 1964. Mylonites, cataclasites, and associated rocks along the Alpine Fault, South Island, New Zealand. N. Z. J. Geol. Geophys. 7, 645–684. Rice, J.R., 1992. Fault stress states, pore pressure distributions, and the weakness of the San Andreas Fault. In: Evans, B., Wang, T.F. (Eds.), Fault Mechanics and Transport Properties of Rocks. Academic, San Diego, USA, pp. 475–503. Schnegg, P.-A., Fischer, G., Le Quang, B.V., Weaver, J.T., 1986. Investigation of a buried vertical fault with natural and controlled source AMT. Ann. Geophys. 4 (B2), 139–144. Sibson, R., Robert, F., Poulsen, K.H., 1988. High-angle reverse faults, fluid-pressure cycling, and mesothermal gold-quartz deposits. Geology 16, 551–555. Smith, J.T., Booker, J.R., 1991. Rapid inversion of two- and three-dimensional magnetotelluric data. J. Geophys. Res. 96, 1168–1171. Unsworth, M., Egbert, G., Booker, J., 1999. High-resolution electromagnetic imaging of the San Andreas fault in central California. J. Geophys. Res. 104, 1131–1150. Wannamaker, P.E., 1999. Affordable magnetotellurics: interpretation in natural environments. In: Orastaglio, M., Spies, B. (Eds.), Three-Dimensional Electromagnetics. Geophysical Developments Series, vol. 7. Society of Exploration Geophysicists, Tulsa, p. 724.
High resolution electrical imaging of fault zones Malcolm Ingham∗ School of Chemical & Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Received 15 July 2003; received in revised form 4 November 2003; accepted 16 August 2004
Abstract Two-dimensional numerical modelling studies of a generic conductive fault zone indicate that very closely spaced TM mode magnetotelluric (MT) data are capable of providing high-resolution images of the fault structure. The degree of resolution that is possible is largely controlled by the conductance of the overburden and is also dependent on the spacing of measurement sites. In the absence of any overburden, or in the case of very thin overburden, the resolution is limited by the minimum electrode dipole length which is practicable. As an example, the results of two-dimensional inversion of closely spaced data measured across the Alpine Fault are compared with an earlier model derived from more widely spaced measurement sites. Much higher resolution of the fault zone is obtained from the closely spaced data and some features of the earlier model appear to arise due to the sparseness of the data. In the case of a very narrow fault zone, it is shown that the MT data exhibit very little lateral variation in phase and therefore independent control of static-shift is necessary. © 2004 Elsevier B.V. All rights reserved. Keywords: Fault zone; Magnetotellurics; Numerical modelling
1. Introduction Magnetotelluric (MT) measurements made across the Alpine Fault in the South Island of New Zealand have identified the existence of a zone of high electrical conductivity associated with the fault zone (Ingham and Brown, 1998). This result is similar to that of MT surveys over major transform and strike–slip faults elsewhere (e.g. Jones et al., 1992; Unsworth et al., 1999). In these studies, the high conductivity has generally been interpreted in terms of the presence of fluids ∗
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within the fault zone. Such an interpretation is significant because of the accumulated evidence that suggests that fluids play an important role in a variety of faulting processes (e.g. Hickman et al., 1995; Rice, 1992; Sibson et al., 1988; Crampin et al., 2002). In the study reported by Ingham and Brown (1998), the minimum spacing of measurement sites was ∼300 m, typical of the inter-site spacing in most studies of this nature. Intuitively, however, the ability of MT data to resolve lateral conductivity structure across a narrow fault zone should be limited only by the requirement that the length of the dipole used to measure the electrical field perpendicular to the fault zone should not be greater than the width of the fault zone itself.
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In this paper, therefore, the use of more closely spaced MT data to provide high-resolution images of conductive fault zones is investigated. First, the results of twodimensional numerical modelling studies, which illustrate the sensitivity of MT data to different features of generic models of fault zones, are discussed. This is followed by the presentation of a field example using very closely spaced dc resistivity and MT data collected across the Alpine Fault. The location of these measurements is very close to the original location of the study reported by Ingham and Brown (1998) and comparison of the results with those of the earlier study illustrates the limitations in resolution resulting from using measurement sites that are separated by distances similar to the width of the fault zone under study. MT sounding involves the measurement of time variations in horizontal components of the naturally occurring magnetic and electric fields. To image the structure of a fault zone from the surface down to several km depth, the typical frequency range for such measurements might range from 1000 to 0.1 Hz. At any individual frequency, the MT impedance tensor relates the amplitude and phase of the variations in the electric field components to those in the magnetic fields. Where measurements are made in a coordinate system oriented parallel and perpendicular to a fault zone, assuming two-dimensionality of the electrical structure associated with the fault, the diagonal elements of the impedance tensor should be zero. The off-diagonal elements relating to the electric fields perpendicular and parallel to the fault then correspond to the impedances associated with the TM and TE modes of electromagnetic induction, respectively. Due to its much sharper spatial variation, lateral changes in resistivity are often derived from solely the TM mode impedance (Wannamaker, 1999). Ledo et al. (2002) have also suggested that measurements made above a structure with a finite two-dimensional strike length are best interpreted two-dimensionally using only the TM mode responses. As a consequence, this paper concentrates solely on the ability of TM mode MT data to image fault structure.
2. Numerical modelling of a generic conductive fault zone A generic two-dimensional electrical resistivity model designed to investigate the ability of MT data to
Fig. 1. Generic model of a conductive fault zone.
detect the presence of a conductive vertical fault zone is shown in Fig. 1. The model is based upon the structure derived by Ingham and Brown (1998) for the Alpine Fault. The fault zone has a width , lies between units which have bulk resistivities of 500 and 1000 m, and is buried beneath a relatively conductive overburden (50 m) of thickness t. The resistivity ρ of the fault zone is taken to be less than the resistivities of the adjacent units, i.e. it is assumed, for example, that the presence of fluid within the fault zone has led to a reduction in bulk resistivity. The fault zone is shown to lose its definition below 10 km depth where a relatively conductive substratum of 100 m resistivity is included in the model. Many transform fault zones are believed to penetrate into the mid to lower crust, and the conductive substratum may be regarded as a representation of the fact that the continental lower crust is commonly observed to be relatively conductive compared to the upper crust (e.g. Hyndman et al., 1993). Shown in Figs. 2–4 are the results of forward model calculations used to assess the sensitivity of TM mode MT data to various features of this generic model. Fig. 2 shows, in the form of data pseudosections for frequencies from 1000 to 0.1 Hz, the expected TM apparent resistivity and phase responses across a fault
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Fig. 2. Simulated TM mode MT data across a conductive fault zone of resistivity 3 m and width with a conductive overburden of thickness 200 m.
zone with t = 200 m and ρ = 3 m, for different widths. The pseudosections cover measurements made up to distances of 1 km on either side of the centre of the fault zone and simulate the results obtained from measurement sites spaced 50 m apart. For a conductive fault zone of width m, the pseudosections clearly indicate the existence of the fault zone and show two main features associated with it—a depression of apparent resistivity throughout the entire period range and an increase in phase at high frequency (short period). However, with 200 m thickness of overburden, the TM mode data appear to be unable to resolve the presence of the conductive fault zone if its width is much less than 200 m. For m, the presence of the zone is indicated only by a
slight “pinching” of the contours of both ρa and ϕa and by a small high in phase at the shortest periods. For a fault zone of 100 m width, there are smooth changes in both ρa and φa which result from the resistivity contrast between the 500 and 1000 m units on either side of the fault zone. A residual slight high in phase persists at short period but, given the uncertainties inherent in real data, this is unlikely to be resolvable in practice. Although not shown here, data pseudosections for TE mode data also show features associated with the fault zone. In particular, a marked minimum in apparent resistivity exists and there is zone of enhanced phase at short period similar to that shown in Fig. 2 for
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Fig. 3. Simulated TM mode MT data across a conductive fault zone of resistivity 3 m and width 100 m with a conductive overburden of thickness t.
the TM mode data. However, the continuity across the fault zone of the electric field parallel to the resistivity boundaries results in these features not being as laterally constrained as are the features in the TM mode data which derive from an electric field which is discontinuous across the fault zone. This conclusion was reached in an earlier study by Schnegg et al. (1986) who explained the higher resolution of the TM mode in terms of the adjustment distance of the fields in the vicinity of a perturbing body being much shorter for the TM mode than for the TE mode. The ability of the TM data to resolve a narrow fault zone in fact depends upon both the thickness and the electrical resistivity of the overburden. For an overburden with lower resistivity than the units adjacent to the fault zone, if the thickness t of the overburden is signif-
icant, most of the electric current associated with the TM mode is concentrated in the overburden. Above the highly conductive fault zone, there is, in principle, a readjustment of current streamlines and current is pulled to greater depth. As described by Fischer (1985), this leads to an increase in the observed phase and, because of a concomitant decrease in surface electric field, to a decrease in apparent resistivity above the fault zone. The degree to which this occurs depends upon the adjustment distance of the TM electric field within the fault zone. For a fault zone of large width (in the case in question, roughly w > t), the adjustment distance is less than the width of the fault zone and the electric field is able to penetrate to sufficient depth into the conductive fault zone to yield an anomaly in both TM ρa and φa . In contrast, if w < t, the adjustment distance
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Fig. 4. Simulated TM mode MT data across a conductive fault zone of resistivity ρ ( m) and width 20 m with a conductive overburden of thickness 5 m.
is comparable to the width of the fault zone and current penetration to greater depth is unable to take place to a significant degree. As a result, no anomaly occurs in either ρa or φa . If the overburden is either thinner or more resistive (i.e. the skin-depth in the overburden is greater), a significant amount of current is present in the underlying units relative to the overburden and is therefore able to penetrate into the conductive fault zone. This allows much narrower fault zones to be resolved if the overburden is thinner or more resistive. In the absence of an overburden (t = 0), the surface electric field is discontinuous at the edges of the fault zone. This behaviour is illustrated in Fig. 3, which shows the TM mode data pseudosections that result from a fault zone of width 100 m for a series of different thick-
nesses of the conductive overburden. When, the fault zone is marked only by a small minimum in apparent resistivity. However, when the thickness of the overburden is significantly less than the width of the fault zone, the anomaly in apparent resistivity again delineates the location of the conductive fault zone. In contrast, as the skin-depth in the conductive overburden is comparable to its thickness t, the sensitivity of the phase data to the fault zone is much reduced. For a thinner thickness of the overburden, a small high in phase occurs at high frequency but does not define the fault zone to the same degree as does the apparent resistivity. The increased dependence on the apparent resistivity data to resolve a much narrower fault zone is further emphasised in Fig. 4. This shows the TM mode data pseudosections resulting from a fault zone of only 20 m
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width with an overburden of thickness t = 5 m for two different resistivities of the fault zone. In neither case does any anomaly in phase occur. The results shown in Fig. 4 also indicate the sensitivity of the resolution of the fault zone to its resistivity. When the contrast between the resistivity of the fault zone and the surrounding blocks is reduced, the magnitude of the anomaly is decreased. Although relatively pure water can have a resistivity of around 100 m, in reality, either through the presence within the fault zone of fine grained material resulting from fault action, or through heating of deeply circulating groundwater, the ambient resistivity of the fault zone is likely to be significantly less. Only in the case of faults penetrating into conductive sediments, may the lack of resistivity contrast prevent resolution of the fault zone. The pseudosections shown in Fig. 4 are based on data that simulate measurement sites that adjacent to, and within, the narrow fault zone are spaced only 5 m
apart. This separation implies a very much smaller electrode dipole length than is normally used in field measurements and, as a consequence, in a real situation would yield a much lower signal to noise ratio. Thus, although the theoretical implications drawn from Fig. 4 are valid, the practicalities of making real measurements over such a narrow fault zone are not straightforward. Apart from the pseudosections shown in Fig. 4, all of the other pseudosections are derived from theoretical measurement sites spaced continuously across the model with a separation of 50 m between adjacent sites. In practice, the ability of TM mode MT data to resolve a narrow fault zone is strongly dependent on the density of actual measurement sites. This is illustrated very clearly in Fig. 5 which shows the results that are obtained when theoretical TM mode data from particular distributions of sites are inverted using the twodimensional technique of Smith and Booker (1991).
Fig. 5. Resistivity structures derived by two-dimensional inversion of theoretical TM mode MT data generated from the generic fault model with m, ρ = 3 m, and t = 200 m. Arrows indicate the locations of simulated measurement sites used in each inversion. Dashed lines outline the actual resistivity boundaries of the generating model.
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The theoretical data have been generated from a forward model of the generic form shown in Fig. 1 with m, ρ = 3 m, and t = 200 m. In each part of Fig. 5, the locations of the actual resistivity boundaries in the model are shown by the dashed lines. The structure shown in Fig. 5(a) is that which is obtained when theoretical data from a sparse distribution of sites, of which none are located above the actual fault zone, are inverted. Although a slight minimum in resistivity, which is broadly coincident with the conductive fault zone, is observed below 2 km in depth, the recovered structure shows no clear indication of the existence of this zone. When data from a sparse distribution of sites, where one site is located above the fault zone, are inverted (Fig. 5(b)), the recovered resistivity structure does give an indication of the existence of the conductive zone. This is primarily evidenced in the minimum in resistivity that is observed at shallow depth. However, the overall resolution of the conductive fault zone is still poor and there is little control on either its true resistivity or its width. In contrast, as is shown in Fig. 5(c), inversion of data from a high density of sites, of which several are located above the fault zone, clearly resolves the existence of the narrow conductive zone. In this case although the true resistivity of the zone is still significantly overestimated, the width of the zone is well constrained. It is clear therefore that, theoretically at least, TM mode MT data have the potential to resolve and image very narrow conductive fault zones. Limits to the resolving ability depend largely upon the width of the fault zone relative to the thickness (or more correctly, the conductance) of any overburden, and on the density of measurement sites. The resistivity contrast between the fault zone and the surrounding material is also a controlling factor. In the limit of very thin overburden, however, a clear resolution of a very narrow fault zone would necessitate a reduction of the electrode spacing used to make the TM mode measurements so that the electric dipole does not completely straddle the fault zone.
3. Field example—the Alpine Fault 3.1. Study location The conductive structure derived by Ingham and Brown (1998) from their MT data measured in the
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vicinity of the Alpine Fault has a width which varies from around 300 m in the upper 1 km to over 1 km at approximately 2 km depth. However, of the eight measurements sites from which data were modelled, only two were within 200 m of the actual fault and these were separated by ∼ 300 m. Thus, although testing by Ingham and Brown (1998) of the various model parameters, indicated that the existence of the anomalous zone associated with the fault was unambiguous, uncertainty remains as to the width of the conductive region. To test the ability of very closely spaced TM mode MT data to provide a higher resolution image of the Alpine Fault and to better constrain the width of the conductive zone, new electrical measurements were made across the fault in February 2002. The study location is shown in Fig. 6, in which the MT sites discussed by Ingham (1996) and Ingham and Brown (1998) are also shown. The solid line indicates a transect of just under 1 km in length, oriented perpendicular to the trace of the Alpine Fault, along which direct current (dc) resistivity profiling was carried out and TM mode MT measurements were made. The aim of the dc measurements was to yield a model of the near surface electrical resistivity structure which might identify and aid any necessary correction for static-shift (Jones, 1988) of the MT data. The dc measurements were made using a Wenner array with a basic electrode separation of a = 10 m. Measurements were obtained along approximately 800 m of the transect using interelectrode spacings of 10, 20, 30, 40 and 50 m. MT data were collected using 50 m electric dipoles oriented perpendicular to the fault to measure variations in the TM mode electric field. Simultaneous magnetic field variations were measured using induction coil magnetometers in orientations parallel and perpendicular to the fault. Measurements were made at 11 locations over a lateral range of 700 m. Defining the x-direction as that perpendicular to the fault, the MT impedance tensor elements Zxx and Zxy were calculated for each of the locations. The nominal frequency range covered by the measurements was 1000–1 Hz, however, the presence of a power distribution line along a minor road adjacent to the transect (Fig. 6) restricted the useable frequency range to 100–1 Hz at some of the measurement sites. The near surface structure along the length of the transect consists of river gravels and moraine deposits. Slightly to the northeast of the transect, the line of the Alpine Fault is marked by a northwest facing scarp of
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Fig. 6. Location of dc and MT measurements across the Alpine Fault in the South Island of New Zealand. Solid line: location of transect of dc and MT data discussed in the text; dashed line: 1998 dc resistivity profile; crosses: 1998 Schlumberger dc resistivity soundings; dots: MT sites discussed by Ingham (1996) and Ingham and Brown (1998).
about 5 m height. However, the topography along the length of the transect is much more subdued with only a gradual drop in elevation of about 5 m over a distance of some 200 m in the centre of the transect. 3.2. dc resistivity data The dc resistivity data are shown in the form of a pseudosection of apparent resistivity in Fig. 7(a). The vertical axis of the pseudosection is in terms of a pseudodepth defined as the median depth of investigation (Edwards, 1977) for a given electrode separation a, and given by approximately 0.5a. Also shown in Fig. 7(a) are the two-dimensional resistivity model derived through two-dimensional numerical inversion of the data, and the apparent resistivity pseudosection calculated from the model. As can be seen from comparison of the measured and calculated pseudosections, the fit of the model to the data is excellent (rms misfit of 3.4%). The model is marked by a narrow zone of lower resistivity, centred on x = 560 m, that has a width at the surface of ∼70 m but which splays out with depth. Outside this zone, the surface resistivity values are high (>1000 m) with values dropping below this at depths below 20 m. The low resistivity zone coincides at the surface with the base of the topographic slope which is a direct continuation of the 5 m scarplet on the measurement line of Ingham and Brown (1998).
The dc resistivity data have been supplemented by earlier dc data measured at the same time, and along the same line, as the original MT data reported by Ingham and Brown (1998). These data include two Schlumberger depth soundings and a single Wenner profile using an electrode spacing of a = 100 m. The locations of both the soundings and the profile are shown in Fig. 6. Shown in Fig. 7(b) is a pseudosection of the combination of the two sets of profiling data. Also shown are the model and calculated response derived through two-dimensional inversion of the combined data. The addition of the profiling data at larger electrode separation shows that the narrow zone of lower resistivity observed at the surface connects to a deeper low resistivity zone which in the central part of the profile has a resistivity below 100 m beneath about 50 m in depth. Low surface resistivity values at the northwest end of the profile (Fig. 7(b)) are not well resolved as there are no data at small electrode separations in this region. These results are supported by the results of the Schlumberger soundings which are shown in Fig. 8 and summarised in Table 1. On the southeast side of the fault trace (sounding 1), one-dimensional modelling of the Schlumberger curve gives a resistivity of 117 m at a depth of around 50 m, in agreement with the traversing results. In contrast, sounding 2 (to the northwest of the end of the Wenner traverse) suggests that at comparable depths the resistivity remains between 500 and ∼3000 m.
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Fig. 7. (a) The 2002 measured Wenner dc resistivity traversing data, two-dimensional inversion model response, and two-dimensional inversion model. (b) Combined 1998 and 2002 measured Wenner dc resistivity traversing data, two-dimensional inversion model response, and twodimensional inversion model.
3.3. MT data Apparent resistivity and phase pseudosections of the TM mode MT data measured along the transect are
shown in Fig. 9. Most of the 11 measurement locations were uniformly spaced over the observed low in apparent resistivity noted in the dc traversing data. The apparent resistivity pseudosection shows that a distinct
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Fig. 8. Schlumberger dc resistivity soundings and one-dimensional model responses (smooth curves) at locations 1 and 2 as shown in Fig. 6.
low in apparent resistivity exists in the frequency range 100–1 Hz over about 200 m width between measurements sites C and G. Associated with this low, and in general agreement with the modelling results presented in Fig. 2, is a slight high in phase where values above 45◦ are observed at short periods. To the southeast of the low, the apparent resistivity is relatively constant both over most of the period range and laterally. In contrast, to the northwest there appears to be a strong lateral gradient in apparent resistivity. Although the depth to which the shortest period MT data are sensitive (∼200–500 m) is somewhat greater than the deepest penetration of the dc data (∼50 m), the observed MT responses are broadly consistent with the shallow resistivity structure derived from the inversion of the dc data. Over the range from 200 to 700 m along the transect, the observed MT apparent resistivity at short period is less than 100 m, in agreement with the resistivity value below 50 m depth obtained from the dc data. Similarly, the rise in MT apparent resistivity to the
northwest is consistent with the higher resistivity values determined by the Schlumberger soundings in this region. This correlation between the observed MT and dc data, and the phase features in the MT data, suggest that the MT data have not been significantly affected by static-shift (Jones, 1988). As a result, two-dimensional inversion of the MT data has been performed without any prior correction for static-shift. Shown in Fig. 10(a) is the result of a robust twodimensional inversion of the data using the technique of Smith and Booker (1991). The calculated apparent resistivity and phase responses of this model are shown in Fig. 9 and provide an excellent fit to the observed MT data. The derived model indicates the existence of a narrow (∼200 m wide) near vertical zone of low resistivity. This feature penetrates to a depth of at least 2 km beneath which resolution, as is evidenced by the minor oscillations in structure, is poor. This conductive zone is partially separated from a deeper region of low resistivity to the northwest by a vertical finger of higher resistivity. Immediately to the northwest of the low resistivity zone, the resistivity in the upper 1 km is high. For comparison, the appropriate part of the two-dimensional structure derived by Ingham and Brown (1998), also using the inversion technique of Smith and Booker (1991), is shown on the same scale in Fig. 10(b). Also marked are the locations of the two MT sites used by Ingham and Brown (1998) in this part of their transect of the Alpine Fault. Comparison of the two models confirms the theoretical results shown in Fig. 5 regarding the degree to which the resolution of the conductive zone depends upon the density of the measurement sites. It is clear that a lower density of sites results in an inability to discriminate between the two regions of low resistivity, and in the earlier model these appear as a single, much broader region of low resistivity. It can also be noted that the absence in the
Table 1 One-dimensional models of Schlumberger dc soundings Sounding 1
Sounding 2
Resistivity ( m)
Thickness (m)
Depth (m)
Resistivity ( m)
Thickness (m)
Depth (m)
1638 3804 867 117
0.27 6.8 45 –
– 0.27 7.1 52
782 4411 2389 610
0.35 2.4 52 –
– 0.35 2.8 55
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Fig. 9. Measured and model TM mode MT data.
earlier study of a measurement site within the range 500–700 m results not only in surface resistivities of much less than 100 m not being detected, but also in the surface projection of the deeper low resistivity region being located further to the southeast. The higher measurement density also suggests that the resistivity of the actual fault zone, although low compared to the surroundings, is considerably higher than was suggested by the earlier work. Both models shown in Fig. 10 identify the near surface region immediately to the northwest of the fault zone as resistive. In the case of the model derived by Ingham and Brown (1998), this partly results from the inclusion in the inversion of data from sites further to the northwest, and not indicated in Fig. 10(b). The identification of the resistive region in Fig. 10(a) is based only on the slightly higher apparent resistivity values observed at the northwesternmost sites I and J compared to the values at sites to the southeast. The feature is not therefore well resolved. Better resolution of this structure would necessitate additional sites to the northwest. Ingham and Brown (1998) interpreted this high resistivity as being associated with the Fraser Formation, a zone of cataclastic deformation and my-
lonitization (Reed, 1964) between the Alpine Fault and the Fraser Fault to the northwest (Fig. 6). Similarly, the existence of high resistivity to the southeast in the earlier model was based on data from sites located at some distance from the fault zone. Ingham and Brown (1998) interpreted this rise in resistivity as being due to the high grade schists to the southeast of the Alpine Fault. The results of the present, more detailed, study show that the resistivity immediately to the southeast of the fault zone is not as high as previously indicated.
4. Discussion The numerical modelling of a generic fault zone clearly suggests that the ability of MT data to resolve a conductive fault zone is limited primarily by the conductance (i.e. conductivity–thickness product) of any overburden. When either no, or only low conductance, overburden exists the limitations on resolution arise from the resistivity contrast between the fault zone and its surroundings, and the length of the dipole used to record electric field variations.
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Fig. 10. Two-dimensional inversion models of TM mode MT data across the Alpine Fault: (a) this study; (b) Ingham and Brown (1998).
The inversion and interpretation of field data from across the Alpine Fault confirms the ability of closely spaced MT data to provide high-resolution images of fault structure, as is suggested by the results of the numerical modelling. In the field example discussed, the fact that the surface moraines and gravels have a higher resistivity than that used in the generic fault model enables the field data to resolve a fault zone which has a width of only ∼200 m. The actual field situation is similar to that depicted in the central panels of Fig. 2, where, associated with the fault zone, there is a narrow, but distinct, low in the TM apparent resistivity data and a small high in phase. Comparison with the earlier model of the Alpine Fault, derived from more widely spaced data, shows that more closely spaced data give a much higher resolution of the fault zone.
In the present case a single conductive zone indicated by the earlier study is resolved as two narrower conductive zones. Not only is higher resolution of the fault structure obtainable, but comparison with the earlier model shows that interpretation of data from relatively widely spaced sites can in fact suggest the existence of features which more closely spaced data show not to exist. In this case, both the high resistivity immediately to the southeast of the fault, and the extremely low resistivity of the fault zone itself fall into this category. It is also clear that where variations in MT apparent resistivity occur on a very small spatial scale the absence of a significant phase signature requires that interpretation of these changes in terms of fault structure has supporting evidence that they are not purely
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the result of static-shift. Typically static-shift may be recognised by the existence of large lateral gradients in apparent resistivity coincident with laterally uniform phase. As can be seen, most clearly from Fig. 4, in the situation of a narrow conductive fault zone such an interpretation would be erroneous. In the field example of the Alpine Fault discussed above static-shift control is provided by shallow dc resistivity data obtained across the fault zone. With this type of constraint upon interpretation it is evident that high resolution electrically imagery is capable of providing new insights into fault structure.
Acknowledgements This work was supported by grant URF 1/52 from the Victoria University of Wellington University Research Fund. Help in the field from Simon Granville is gratefully acknowledged as are helpful reviews from Andreas Junge and an anonymous referee.
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