Distortion Effects on Magnetotelluric Sounding Data Investigated by 3D Modeling of High-Resolution Topography

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1 GRC Transactions, Vol. 37, 2013 Distortion Effects on Magnetotelluric Sounding Data Investigated by 3D Modeling of High-Resolution Topography Mitchel A. Stark 1, Wolfgang Soyer 2, Stephen Hallinan 2, and M. Don Watts 2 1 Chevron Geothermal Services Company, Makati, Philippines 2 CGG Land General Geophysics, Milan, Italy Keywords LiDAR, time-domain electromagnetic, Kalinga, Philippines, static shift ABSTRACT Distortion of magnetotelluric (MT) sounding data due to nearstation topography and near-surface geologic inhomogeneities has long been recognized as an interpretational problem. The issue is particularly vexing in geothermal prospects where, in many cases, the terrain is rugged and the surface geology is dominated by laterally heterogeneous volcanics, with prominent areas of hydrothermal alteration. A number of remedial techniques have been developed, of which one of the most popular has been the use of controlled-source electromagnetic methods (most commonly Time Domain Electromagnetics or TDEM) to independently characterize the shallow resistivities. More recently, MT recording bandwidth for routine purposes has been extended up to 10,000 Hz, providing better coverage of the near-station structure, and 3D MT modeling and inversion capabilities have advanced to the point where small-scale topography and resistivity inhomogeneities can be addressed. These modeling methods are evaluated for an MT survey conducted in a geothermal prospect in the Central Cordillera region of Northern Luzon island, the Philippines, characterized by extremely rugged terrain and extensive hydrothermal and epithermal mineralization. In addition to MT and TDEM data sets, there is a digital elevation model resolved to ±15 cm for the prospect, derived from an airborne LiDAR survey. To isolate and understand the effects of terrain, we have run 3D models of the topography, assuming uniform earth resistivity. The models show that topography alone causes severe distortion of the MT sounding, including frequency-dependent distortions that persist to periods of >10s. Field data and 3D modeling of collocated TDEM and MT soundings indicate that TDEM data can also be distorted due to topography and therefore not suitable for correcting MT distortions under these conditions. The modeled MT soundings replicate features that might otherwise be attributed to subsurface resistivity variations, including: splits between TE and TM mode; frequency-dependent variations in apparent resistivity; departures from 45 phase; and high magnitudes of tipper, induction arrows, and impedance skew. Continuing investigation is aimed at largely eliminating the topographic effects and thereby facilitating more accurate interpretation of the subsurface resistivity variations. With high-resolution topographic data and modern 3D modeling capabilities, there should no longer be a need to accept uncertainty about the distortion of MT data due to terrain. Introduction Distortion of magnetotelluric (MT) sounding data due to small-scale topography and near-surface lateral geologic inhomogeneities has long recognized as an interpretational problem (Price, 1973; Berdichevsky and Dmitriev, 1976; Jiracek, 1990; Vozoff, 1991). A simplistic, therefore tractable portrayal of the distortion is known as static effects (Jones, 1988; Sternberg et al, 1988), i.e. frequency-independent shifting of one or both modes of the apparent resistivity curves. An example is shown in Figure 1a. Modeling and field observations indicate that static shifts of a decade or more are not uncommon, leading to 1D interpretations of earth resistivity that are incorrect in direct proportion to the shift. Moreover, interpreted depths will be erroneously displaced by the square root of the shift factor, according to the skin depth formula δ = 0.5 * (T*ρ) 0.5, where δ is skin depth in km, T is period in s, and ρ is apparent resistivity in ohm-m (Cagniard, 1954; Bostick, 1977). This illustrates that such distortion creates significant interpretational uncertainty. A number of remedial techniques have been developed, of which one of the most popular has been the use of controlledsource electromagnetic data (most commonly Time Domain Electromagnetics or TDEM) to independently characterize the shallow resistivities. Presuming the TDEM-derived shallow resistivity profile to be correct, the interpreter uses 1D modeling to calculate the MT response of that profile, and then simply shifts the MT curves along the y-axis to match the overlapping TDEMderived response (Andrieux and Wightman, 1984; Sternberg et al, 1988; Pellerin and Hohmann, 1990), thereby mitigating the static shift, in theory at least. Figure 1a shows an example where 521

2 Apparent Resistivity (ohm-m) Apparent Resistivity (ohm-m) TDEM MT Data! Station K Period (s)! MT Data (a) (b) TM mode, in which the electric field is oriented perpendicular to the topographic strike direction) is strongly shifted downward, and the valley effect, in which the TM mode is shifted upward. The distortion issue is particularly vexing in geothermal prospects where, in many cases, the terrain is rugged and the surface geology is dominated by laterally heterogeneous volcanics, with prominent areas of hydrothermal alteration. A prime example is the Kalinga geothermal prospect (Figure 2), located within a Philippines Department of Energy service contract block covering 260 km 2 in the Central Cordillera mountain range of northern Luzon island, the Philippines (Crisostomo et al, 2013). The area is deeply incised by the ENE-flowing Pasil River, from which elevation rises by 1,500 m to the young volcanic peaks, including Mt. Binulauan, located less than 5 km to the south. Locally, slopes exceeding 45 are commonly visible. The only vehicle access is by a 20-km narrow dirt road that traverses the slopes south of the river, and many of the 48 MT stations were located on man-made rice terraces cut into steep hillsides. The prospect is dotted with thermal manifestations and associated hydrothermal alteration, and hosts a number of older epithermal mineral deposits. In addition to the Quaternary volcanics, the surface geologic map shows broad exposures of Tertiary units which are likely to have contrasting resistivities: a metavolcanic basement complex, intrusive rocks, and the Mabaca River Group, a strongly folded marine sedimentary formation. The area is crisscrossed by mapped and inferred fault traces. Overall, the prospect is practically an open-air laboratory of features known to cause distortion of MT soundings. Station K Period (s) Figure 1. Examples of collocated TDEM / MT soundings where static shifting is feasible (1a), and where it is not (1b). this method appears viable, because the two MT modes and the TDEM-derived response are all parallel. In many cases, however, the viability of this method is much less apparent; Figure 1b shows a case where none of the three curves are parallel with any of the others. This means that the static assumption is not valid, and the correction exercise described above becomes a matter of considerable uncertainty. MT recording bandwidth has expanded in recent years, so data to periods of s is routinely recorded, whereas in the past the spectrum was limited to about 0.01 s. Sometimes, as in Figure 1b, the shortperiod data reveal the frequency-dependent portion of distortions that appear static at lower frequency. In recent years, computer hardware and software advances have greatly improved the practicality, resolution and accuracy of 3D modeling and inversion of MT and TDEM data (Nam et al, 2008). Modeling results of Watts et al (2013) based on Yavich and Scholl (2012) demonstrate that static shifting based on TDEM data is not a valid approach in extreme terrain situations. The ability to model fine-scale features has led to greater understanding of MT distortion effects. In particular, distortions due to topography alone have been studied. Some well-understood simple examples (Avdeev, 2005; Nam et al, 2008; Miensopust, 2013) include the ridge effect, in which one MT mode (the Transverse Magnetic or Figure 2. Geologic map of Kalinga geothermal prospect showing 2012 MT stations. Geophysical field work was carried out in 2011 and In the 2011 campaign, only two collocated MT / TDEM soundings (shown in Figure 1) were acquired, providing a small pilot data set. After studying those two soundings, performing preliminary modeling, and consulting with outside experts (W. Cumming and G. Ussher, personal communication), the decision was made to conduct the 2012 MT survey with no TDEM. The decision was based not only on the technical considerations discussed above, but also because the terrain and logistical challenges of the prospect make TDEM data acquisition relatively expensive, thus diminishing its benefit-cost ratio. 522

3 2012 Data Acquisition Figure 2 shows the 46 stations occupied during May and June, The distribution extends along the Pasil River valley, with most stations located within 2 km of the river, and less than halfway up the slopes leading up to the highlands to the south and north. Station locations included all four of the major mapped geologic units. Data quality was generally quite good through the recorded bandwidth of to 300 s (examples shown in Figures 3 through 6). An airborne LiDAR survey was also conducted in May 2012 (Crisostomo et al, 2013), providing a digital elevation model (DEM) to a resolution of ±15 cm. This was the basis for the finescale topographic modeling conducted to investigate how the terrain distorts the MT data. 3D Topographic Modeling Three-dimensional modeling and inversion were performed using proven finite difference code (Mackie et al, 1994; Mackie and Watts, 2012). For general interpretative purposes, the entire MT and topographic dataset was inverted to produce a 3D resistivity model (Crisostomo et al, 2013). In addition, higher-resolution 3D finite difference modeling was conducted, based on the LiDARderived DEM and using a mesh grading to a minimum size of 50 * 50 * 10 m. A uniform subsurface resistivity of 50 ohm-m was assumed in the high-resolution model, so there was no attempt to match the data. Rather, the purpose of the high-resolution model was to isolate the distorting effects due to the terrain alone. These effects were then compared qualitatively with the observed data. For simplicity, all observed and modeled impedance tensors were rotated to a frequency-independent orientation of N15 W, consistent with the predominant strike direction in the area. Note, however, that allowing each station to rotate to its principal directions would more effectively bring out the differences between modes. At some stations, local topographic strike direction may be very different from N15 W, and therefore the full effects of topography may be muted on the rotated soundings. Notwithstanding the fine mesh and DEM resolution, the modeling of the terrain was still based on stair-step approximation of slopes, and did not account for the layout of 50- to 100-m dipole lengths laid out in the field. Inaccuracies attributable to these modeling limitations are unknown. Experiments with even finer mesh sizes, as small as 10 * 10 * 2m, showed that the finer-mesh models produce more intense and localized distortion. The best hope is that the 50-m mesh creates a smoothing effect similar to the effect of the finite dipole lengths, in which case the modeling will produce a reasonably accurate prediction of the MT data and distortion effects. Examples of 3D Topo Modeling of Specific Stations Figures 3 through 6 show representative examples of observed and modeled apparent resistivity soundings. The station locations Figure 3. Station K-16 observed and modeled apparent resistivities. 523 Figure 4. Station K-24 observed and modeled apparent resistivities.

4 of those four soundings are highlighted in Figure 2. Note that the model assumed a uniform subsurface resistivity of 50 ohm-m; there was no effort to match the observed data. In the absence of topographic distortion, all the modeled soundings would show the two modes equal and flat at 50 ohm-m. Departures from that result represent purely the distorting effects of topography. The purpose of the comparison is to understand qualitatively the nature of the distorting effects of topography at each site. Station K-16 (Figure 3) was located at the base of a steep slope leading up to a WNW-trending ridge. In the short-period band 10-4 to 10-1 s both the observed and topo-modeled soundings show a split between the modes, increasing at longer period, as well as a generally decreasing resistivity trend. The major features of the observed sounding are replicated in the modeled sounding, indicating that much (though not all) of the apparent resistivity variations can be attributed to distortions induced by topography. At high frequency, the modeled yx mode curve asymptotes to the correct 50 ohm-m apparent resistivity, while the xy curve is offset upwards. This behavior is generally consistent with the station location at the base of a steep slope. Station K-24 (Figure 4) was located in a valley formed where a young volcanic dome collapsed or exploded, leaving a broad area covered by debris, descending gently northward toward the Pasil River. This is one of the few stations for which the 3D topo model predicts very little distortion; the modeled modes are nearly identical and flat at 50 ohm-m. The observed sounding shows the same pattern to about 10-2 s period. At lower frequencies the observed sounding shows some moderate splitting and both modes indicate a relatively resistive unit, presumably consistent with the intact remains of the resistive dome lavas, buried under the debris, and not an artifact of terrain-induced distortion. Station K-32 (Figure 5) was located further south, at higher elevation atop the curving rim of the remnants of the dome. The modeled sounding is a classic example of the ridge or peak effect, with both modes shifted downwards relative to the uniform model resistivity. This effect is fully manifested at periods > 10-2 s, asymptoting to apparent resistivity values of about 20 ohm-m. Despite the simple one-dimensional appearance of this modeled sounding, interpreting a 20 ohm-m layer at depth would be erroneous, based entirely on distortions induced by topography. The observed sounding shows similar behavior, but continues to lower apparent resistivity values at longer periods, suggesting a real subsurface resistivity contrast. There were not many soundings for which the topo modeling showed the classic ridge or peak effect, because the data acquisition contractor avoided such sites. Station K-99 (Figure 6) was located on a flat mesa. Such mesas are found in various locations around the prospect where young volcanics outcrop, and are believed to be formed by competent lavas or welded tuffs that are resistive to erosion. In the case of K-99, the electrically conductive marine sedimentary formation is mapped directly below the mesa-forming volcanics. The distortion expected due to the mesa would be similar to the ridge Figure 5. Station K-32 observed and modeled apparent resistivities. Figure 6. Station K-99 observed and modeled apparent resistivities. 524

5 effect, but at longer period. The modeled sounding shows exactly that. In the band 0.02 to 2 s, both modes descend from the correct 50 ohm-m apparent resistivity to about 25 ohm-m. Again, if the observed sounding was identical to this model, it would be erroneous to interpret a subsurface 25 ohm-m layer. In this case, the observed sounding indicates near-surface resistivity of about 500 ohm-m, likely reflecting the competent, unaltered mesa-forming volcanic rock. The lower apparent resistivities at longer periods can be only partly attributed to the topographic distortion; a real lower resistivity body must exist at depth to explain the dramatic descent of the curves, and probably reflects the conductive marine sediments mapped immediately below the mesa. These four examples show that the local topography (within tens to hundreds of m of the station) distorts the MT apparent resistivities, with a pronounced frequency-dependent component of distortion found in a band that correlates with the spatial scale of the local topography. The modeled curves reach distortion factors ranging to about 5, relative to the modeled resistivity of 50 ohm-m. At longer periods, the distortion appears static. Distortion was most commonly downward, with upward shifts rarer and less pronounced. In a case where the local topography is flat, but steep slopes exist 500 to 1000 m from the station, the 1922 distortion was smaller in magnitude (maximum factor of 2), and was manifested at longer periods, from.02 to about 2s. In all these cases, the frequency-dependent 1920 distortion extended to periods somewhat longer than might be expected based on the skin depth formula. with topographic features oriented in the perpendicular (N75 E) direction, where the xy resistivity is the Transverse Magnetic (TM) mode. The spatial extent of the distortion anomalies is governed by local topography, even though the skin depth for 10s period and 50 ohm-m resistivity would be 11 km. The modeled apparent resistivities range as high as 500 ohm-m in the valleys, and lower than 5 ohm-m along the high ridges -- a factor-of-10 distortion in both directions. The yx apparent resistivity map (not shown) shows a similar range of distortion factors, but the anomalous areas are oriented in the perpendicular direction. Figure 8 shows xy impedance phase at a period of 0.1s. In general the phase is entirely determined by the apparent resistivity spectrum (Weidelt, 1972), and vice versa, thus providing no independent information. However, for the uniform resistivity model, departure of the phase from 45 is a convenient indicator of the amount and bandwidth of frequency-dependent distortion. The modeled anomalous phases range from about 35 on the low side, to almost 60 on the high side. Maps of phase at shorter and longer periods (not shown) show that the spatial extents of the phase anomalies scale with period. In Figure 8 the anomalies are Mapped 3D Topographic Modeling Results To investigate the modeled topographic effects in a more general sense, the model results were mapped on the topographic base. Figures 7 through 9 show maps of apparent resistivity, impedance phase, and tipper, at a single period. Again, the subsurface model was a uniform 50 ohm-m resistivity, so in the absence of topographic distortion, the apparent resistivities should all be 50 ohm-m, phases should all be 45, and tippers should all be 0. Departures from those values are purely due to terrain effects. Figure 7 shows the modeled apparent resistivities at 10 s period. As illustrated in the individual sounding examples shown in Figures 3 6, frequency-dependent distortion usually persists to periods of 0.1s, and occasionally to longer periods, but in all cases, for 10s the distortion is essentially static and fully developed. For the xy mode shown in Figure 7, in which the E field is oriented N15 W, the map shows maximum distortion associated Hz, XY apparent resistivity Figure 7. Topographic model response of xy (E field N15W) apparent resistivity at 10s Hz, XY impedance phase Figure 8. Topographic model response of xy (E field N15W) impedance phase at 0.1s. ohm.m degrees

6 Figure 9. Topographic model response of tipper magnitude at 0.001s. on the order of 1 km, consistent with the skin depth of 1.1 km, and are much broader than the apparent resistivity anomalies. This behavior shows that the frequency-dependent component of distortion is manifested in a band consistent with the spatial extent of the distorting topography. In this case, the implication is that significant frequency-dependent distortion can be attributed to topography at distances on the order of 1 km. Thus, even stations located on flat terrain may be affected by fairly distant topography. The frequency-dependent distortion (and hence phases departing from 45 ) decreases at longer periods, but in some areas remains evident out to periods of 10s (not shown). Figure 9 shows modeled tipper magnitude at s. Tipper is a function of vertical magnetic fields, which are zero in the case of a 1D horizontally layered earth. Thus, non-zero tipper magnitudes are sometimes interpreted as evidence of lateral resistivity variations. However Figure 9 shows that tippers ranging up to 1.0 can be induced purely by topography. These values are actually higher than tippers measured at any period at any of the 46 stations. In most of the mapped area, the modeled tipper magnitude exceeds 0.5. The terrain-induced tippers seem to be primarily a function of very fine-scale near-station topography. At longer periods (not shown) the modeled tipper magnitudes are lower, subsiding to < 0.2 for periods 0.1 s. Figure 9 also indicates that tipper magnitude is a function primarily of topographic asymmetry. In the river valleys, where slopes rise in both directions perpendicular to the river, modeled tipper magnitudes are low. Similarly low tipper magnitudes also prevail along high ridge tops. The highest tipper magnitudes are found along steep slopes. The reason the high short-period tippers produced by the model are not observed in the data is probably that the stations were not located on the steepest slopes. Conclusions The high-resolution 3D modeling exercise demonstrates that terrain causes very significant distortion of MT data in this area of extreme topography. Comparison with field data shows that many of the features of individual soundings can be largely explained by topographic effects alone. The modeled distortion departs from the 1000Hz, tipper magnitude 526 simplistic concept of static shifting, with frequency-dependent effects persisting commonly to periods of 0.1 s, and in some cases to periods exceeding 1.0s. Modeling of TDEM, supported by field data, shows that under these conditions, static shifting to match the overlap does not correct the problem. The saving grace of terrain-induced distortion is that the topography is completely known, and with modern 3D modeling techniques, the effects can be mitigated The key is to recognize the strength of the terrain-induced distortion, understand qualitatively the types of effects that can 0.00 be expected, and avoid over-interpreting features observed on the sounding curves. This caution is especially applicable during field data acquisition, when 3D modeling results are not normally available, and the survey supervisor endeavors to use preliminary results to choose the remaining station locations. One continuing difficulty is that very fine-scale terrain can cause distortion of the MT sounding. Modern 3D topographic modeling capability, coupled with a high-resolution DEM, has brought analysis of topographic effects to a scale smaller than the field dipole lengths. The code does not replicate the precise field layout, and that may be a significant remaining limitation. With continued code improvements and further investigation, it may eventually be possible to strip out topographic effects from the data, leaving corrected soundings that allow more accurate in-field interpretation of subsurface resistivity variations. References Andrieux, P. and W. Wightman, The so-called static corrections in magnetotelluric measurements. Society of Exploration Geophysicists Technical Program Expanded Abstracts, p Avdeev, D.B., Three-dimensional electromagnetic modelling and inversion from theory to application. Surveys in Geophysics, v. 26, p Berdichevsky, M., and V.I. Dmitriev, Distortion of magnetic and electrical fields by near-surface lateral inhomogeneities. Acta Geod. Geophys. Mont., Acad. Sci. Hung., v. 11, p Bostick, F. X., A simple almost exact method of magnetotelluric analysis. In: Ward, S., Ed., Workshop of Electrical Methods in Geothermal Exploration, Univ. of Utah Res. Inst., U. S. Geol. Surv. Contract g-359. 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7 Mackie, R. L., T.J. Smith, and T.R. Madden, Three dimensional electromagnetic modeling and inversion using conjugate gradients. Geophysics, v. 59, p Mackie, R.L. and M.D. Watts, Detectability of 3-D sulphide targets with AFMAG. Society of Exploration Geophysicists, 82nd annual meeting. Miensopust, M.P. Queralt, P., Jones, A.G., and the 3D MT modellers, Magnetotelluric 3-D inversion a review of two successful workshops on forward and inversion code testing and comparison. Geophys. J. Intern., accepted for publication. Nam, M.J, H.J. Kim, Y. Song, T.J. Lee, and J.H. Suh, J.H., Threedimensional topography corrections of magnetotelluric data. Geophysics Journal International, v. 174, p Pellerin, L., and G. Hohmann, Transient electromagnetic inversion: A remedy for magnetotelluric static shifts. Geophysics v. 55, p Price, A.T., The theory of geomagnetic induction. Phys. Earth planet. Inter. v. 7, p Sternberg, B. K., Washburne, J.C., and L. Pellerin, L., Correction for the static shift in magnetotellurics using transient electromagnetic soundings. Geophysics, v. 53, p Vozoff, K., The magnetotelluric method. in Electromagnetic methods in applied geophysics, Soc. Expl. Geophys., v. 2B, p Watts, M.D., S. Hallinan, R. Mackie, G. Nordquist, C. Scholl, and M. Stark, Limitations of MT static shift corrections using time-domain EM data. Society of Exploration Geophysicists Technical Program Expanded Abstracts (submitted). Weidelt, P., The inverse problem of geomagnetic induction. J. Geophys. (Z. Geophysik), v. 38, p Yavich, N., and Scholl, C., 2012, Advances in Multigrid Solution of 3D Forward MCSEM Problems, Extended abstracts, 5th Saint Petersburg International Conference & Exhibition Geosciences. 527

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