NARYN MT/GDS transect

1. NEW APPROACHES IN THE INTERPRETATION OF DEEP EM SOUNDING DATA ALONG THE “NARYN” TRANSECT IN KYRGYZ TIAN-SHAN NARYN Work Group: Moscow State University (Golubtsova N.S., Pushkarev P.Yu.) GEMRC, Inst. of Physics of the Earth RAS, Troitsk, Russia (Sokolova E.Yu., Baglaenko N.V., Martanus E.R., Varentsov I.M.) Scientific Station, United Inst. of High Temperatures RAS, Bishkek, Kyrgizia (Rybin A.K., Batalev V.Yu., Safronov I.V., Schelochkov G.G.) presented by Elena Sokolova

2. NARYN MT/GDS transect ```~  The NARYN transect of EM soundings, crossing for 700 km the Kyrgyz and China Tian Shan, became a promising object for investigation of the deep conductivity structure of this active region.The data collection acquired by Kyrgyz and American teams includes 19 long period MT sites (large circles) simultaneously observed in 3 groups with perfect quality by LIMS equipment (S.K. Parks, 1999-2000)and tens of local AMTs (small dark circles, CES-2 soundings, Scientific Station of UIHT RAS, Bishkek). A valuable experience of EM data interpretation on the transect has been obtained in (Rybin et al., 2002) and (Bielinski et al., 2003). The presented paper describes new developments in the rational complex of NARYNsounding data analysis, which are elaborated to increase the resolving power of the EM method and finally to get new assumptions on the deep geoelectric structure and geodynamics of the Tian Shan region. The primary step done to achieve higher resolution consisted in compiling of a new data ensemble, adequate in precision, comprehension and dimensionality to the modern requirements of the profile inversion quality. The new methods of synchronous sounding data processing (Varentsov et al., 2003) were applied to construct the horizontal magnetic tensor (M) responses and to verify and improve available long period impedance (Z) and tippers (Wz) estimates, which together with conditional local prospecting Z and Wz data have completed the broadband multi-component profile data set. The dimensionality and principal directions analysis for these transfer operators were done with a help of the phase tensor (Caldwell et al., 2003) and horizontal magnetic tensor (Varentsov, Sokolova, 2004)decompositions. It was approved that the most part of the impedance and horizontal magnetic tensor data are well satisfied to 2D approach in the whole frequency range considered, while the tipper data above 2,5-3 hour seriously deviate from the general direction of EM field polarization correspondent to the sub-latitude regional tectonics. The general strategy of the 2D inversion implies the combination of the interactive successive partial inversions approach (Dmitriev et al., 2003) and joint weighted multi-component inversion (Varentsov, 2002), verifying and complimenting each other. The both approaches are based on the assumption of the priority of the geomagnetic and impedance phase data role in the inversion course.

3. 1. Outlook of the previous EM studies N S The trial and error fitting of the real tipper data 0.1-1600 s (broadband soundings with CES-2 instruments in the sites shown by numbers). The favor of the performed interpretation: it has revealed for the first time the correlation of the regional crustal conductors with the low velocity zones detected by seismic tomography, that can be reasonably explained by fluids saturating deep horizons and fault zones. Disadvantages: restricted frequency range and component ensemble. Fig. 1.1.Geoelectric and seismic cross-sections along a profile crossing Kyrgyz Tien Shan 300 km east from NARYN line (Trapeznikov et al., 1997) : a – the resistivity profile (shown inside the model are resistivities in Ohmm; NL – Nikolaev’s faults line, AIF – Atbashi-Inylchek faults); b – the Vp velocity (in km/s) profile from seismic tomography according to (Roecker et al., 1993), the low velocity zones are hatched. N S NL AIF The successive partial inversionsof long period (20-20000s) bimodal MV and MT data (14 LIMS stations, 1999) with automatic regularized inversion code (Varentsov, 2002). The main advantage of the approach: the priority of the MV data, getting free from the distorting influence of near-surface inhomogeneties with the lowering frequencies and clearing the information on the deep structures. The rough block approximation of the conductivity section (40 blocks) and low period range limitedthe resolution. Fig. 1.2. Geoelectric and seismic cross-sections along the NARYN profile (Rybin et al., 2001). a,b – see the legend for Fig.1.1 (Rodi and Mackie’s, 2001) regularized bimodal inversion of 19 MT and MV long period LIMS (1999-2000) and 30 broadband CES-2 soundings’ data looks like the most comprehensive approach. However, resulted “mosaic”structure of high and low conductive bodies still contains artifacts and instabilities of this preliminary inverse problem solution, starting from a homogeneous conductivity distribution in the upper 150 km. Fig.1.3. Geoelectric cross-sections along the “NARYN” profile (Bielinski et al.,2003).

4. 2. New approaches to NARYN EM data processing site 407 The original tipper and impedance estimates on LIMS data were obtained with a standard equipment software and adapted A. Chave code (Rybin et al., 2000) . Basing on the analysis of multi-site data records we reprocessed the observations estimating local impedances Z and tippers Wz as well as synchronous geomagnetic operators (Fig.4) in Remote Reference and Multi-Remote Reference modes (Varentsov et al., 2003). Multi-RR stacking offered an extra statistic dimension and improved the stability of responses at the longest periods. In general, NARYN data are characterized by low level of noises, but in a few sites anew sorting strategy based on the control of horizontal magnetic field spatial variability (RRMC method,Varentsov, Sokolova, This Workshop) increased the accuracy of the estimates. Fig. 2.1. The comparison of the original Z processing results (1999) (Rybin et al., 2000) for site 407 with the new ones (2004), obtained with a help of PRC_MTMV system in RRMC mode (Varentsov et al., 2003) : upper panel - the impedances (left – main; right – additional); and the lower panels: the corresponding phases. Fig. 2.2. The comparison of the original Wz processing results (1999) for site 406 (upper panel) and site 404 (lower panel) with the corresponding new ones (2004_WZ – tipper;2004_SZ – synchronous tipper with the base site 410). Fig. 2.3. The apparent resistivity curves for long period MT sounding site 401-414 (Ro_xy, NS – upper left panel and Ro_yx, EW – lower left one) and correspondent main impedance phases (right panels) (PRC_MTMV processing system (2004, Varentsov et al., 2003).

5. 3. MT/GDS data set on NARYN transect : local transfer operators Fig.3, 4illustrate the representativeness of NARYN data set and reflect a wide spectrum of inhomogeneties on the different structural levels of geoelectric section. a b Fig.3.Pseudosections of local transfer operators along “NARYN” profile. LIMS and CES-2 data 0.1-32680 s: phases of Z_det (Rybin et al., 2000) (a); LIMS and Broad Band data 0.1-32680 s: amplitudes of the real and imaginary induction vectors (Rybin et al., 2000) (b); LIMS data 20-32680 s: phases of the main impedances (Rybin et al., 2000) (c) and processing results of NARYN WG_2004 (d). c d

6. 4. Extension of the data set : inter-station transfer operators 411 410 Horizontal M:H(r) =M(r,r´)H(r’) , and vertical Sz (regional tipper): Hz(r)=Sz(r,r´)H(r´) synchronous operators were reliably estimated for the periods up to 46000 sfor 12 LIMS sites with RRMC technique (Varentsov, Sokolova, This Workshop) for two reference bases: 410 ( for 10 sites of southern synchronous array) and 411 (for 4 northern ones). Fig.4.2.Pseudosection of HMT invariant AMP_Mma (amplitude of M tensor component in max direction) according to Swift (top), and regional tipper Sz: amplitudes of the real induction vectors (middle) and imaginary ones (bottom). Fig.4.1.Pseudosections of amplitudes (top) and phases (bottom) of Mxx component of the horizontal magnetic tenso (HMT)r.

7. N S Fig.5.2. Pseudosections of the maximal impedance phases (left) and impedance Skew (right): amplitude (top) and phase tensor (bottom) transformations. Recently suggested scheme of “phase tensor” and horizontal magnetic tensor (HMT) decomposition (Caldwell et al., 2004; Varentsov, Sokolova, This Worckshop) became the preferable instruments of the invariant analysis. Their results are essentially less dependent from the galvanic effects (compare with conventional “amplitude” transformations in Fig. 5.1-5.2). The impedance data are rather well satisfied to 2D approach in the whole frequency range considered, while the tipper data above 2,5-3 hour deviate from the general sub-latitude polarization correspondent to the regional tectonics. The HMT principle directions are fitting well to correspondent impedance “phase tensor” invariants. Strikes are almost profile-perpendicular; skews in most of sites are quite low for periods from hundreds of seconds till 4-6h and justify two-dimensionality of impedance and HMT responses in a broad period range. 5. Invariant analysis Fig.5.1. Vector diagram of max (blue)and min (red) impedances according to (Eggers, 1982) (top panel)and (Caldwell et al., 2004)(bottom). Horizontal axes show the periods and the vertical ones – sounding sites along NARYN profile from the North to the South. The linear scale length are also shown on the bottom of the figures. Fig.5.3. Real (blue) and imaginary (red) induction vectors (left) and anomalous horizontal magnetic perturbation vector diagrams (right). The legend similar to Fig. 5.1.

8. 6. Problems of the data set at long periods T=32768 s Fig. 6.2. The map of real (black) and imaginary (red) induction vectors at NARYN profile and in the surrounding Kyrgyz Tian Shan area. Fig.6.1. The frequencydistribution of the real (blue) and imaginary (red) induction vectors (top panel) and graphs of the amplitudes (middle panel) and azimuths (bottom) of Real ones for 7 GDS sites in the region of Kyrgyz Tian Shan (look at the map of NARYN profile to find sites’ location). The deviation of the induction vectors (T> 2,5-3 hour) from the general sub-meridional direction and their regular sub-latitude pointing at daily harmonics is most probably produced by Sq source effect. It prevents the usage of long period (T>2.5 h) tipper data in the conventional 2D inversion. The presence of significant vertical component in the external field (like in the case of polar source of BEAR array sounding (Sokolova et al., This Workshop) put also under question the perspectives of long period NARYN MT data interpretation in the traditional plane-wave paradigm despite their obvious 2D character, regular behavior and even approaching the MV global curve (s. 403, for example). The same may be addressed to HMT data at Sq harmonics. The question definitely needs further investigation. The estimation of the generalized (6-component) impedancetensor (Dmitriev, Berdichevsky, 2002) could be the next step to clarify the situation with long period NARYN data interpretation. Site 403 Site 412 Fig. 6.3. The frequency distributions of the real (blue) and imaginary (red) induction vectors (bottom row of plots) and MT curves (amplitude, top panels, and phases, middle panels) for sites 403 (left) and 412 (right). For s. 403 the comparison of new MT processing results (RRMC method, Varentsov and Sokolova, This WS) with the previous ones (Rybin et al. 2000) are shown, while for s. 412 – the cloud (just simply “line”) of estimates for different spectral windows. In the vector plot for s. 403 Sz and Wz (RRMC and SS, single site estimates) are compared, while for s. 412 – only RRMC estimates of Wz and Sz vectors.

9. 7. Inversion with the priority of the geomagnetic data a b A The main advantage of MV data is that with lowering frequency the near-surface distortions of the magnetic field attenuate and vanish, and hence do not spoil the information on the deep structures. So, we avoid the diffi-culty associated with static shift of the apparent resisti-vity curves, and the geoelectric const-ructions become more reliable.Our experience demon-strates the efficiency of an interpretation scheme based on successive partial inversions with the priority of geomag-netic data and reasonably chosen staring model. d c e f Fig. 7.1.The tests simulating the integrated interpretation of MV and MT data for “Tian Shan”- type models in the course of successive partial inversions (SPI) for different data component. (a) - “true” model and (b) - blocky starting model for the SPI-interpretation. (c ) – resistivity section obtained from tipper inversion with REBOCC code with starting model xxx (resistivity isolines in Omm); (d) – results of blocky inversion of tippers with starting model (b) with (Varentsov, 2002) code (resistivity values inside blocks in Omm); (e) – the same results for transverse ap. resistivity inversion; (f) – longitudinal impedance phases and (g) combined data ensemble inversion. g Fig.7.2.Resistivity model (Omm) along NARYN profile: inversion of the observedl Re Wz data ( 13 periods from 25. to 1600 s at 19 sites) with REBOCC code (6 iterations, RMC=0.5). The results are shown for: (a) homogeneous half space (100 Omm) starting model; (b) layered Earth with gradient interlayer boundaries, including crustal conductor at depths 30-50 km.

10. Conclusions Basing on the improved transfer operators estimates and results of their invariant analysis, the quasi two-dimensional multi-component data ensemble for profile bi-modal inversion on NARYN transect have been formed. The ensemble incorporates amplitude and phase impedance, tipper and horizontal magnetic tensor components. Each data component are supplied with the mask of weights, reflecting the objective measure of its deviation from 2D (skew and angle parameters) and accuracy. The general weighting scheme suggests the priority of the phase invariants in the whole frequency diapason and geomagnetic data - at the short and middle period range, where they are more protected from disturbing subsurface galvanic effects. The application of different 2D inversion strategies is in the progress, including the inversion of pure geomagnetic ensembles, the successive inversion of single components, and the use of static shift correcting routines within bi-modal multi-component inversion solutions. The first experiments with separate TM/TE impedance and single tipper inversions confirm the importance of geomagnetic responses in the resolution of deep structures.The progress in the NARYN EM data inversion we connect with the application of the powerful stabilized inversion technique using the piece-wise continuous conductivity structure approximation (Varentsov, 2002; Varentsov, 2004)and combining the advantages of the traditional “block” parameterization with the abilities of modern “scanning” schemes for the arbitrary conductivity distribution in the selected “windows” of the section. Acknowledgements To the fruitful atmosphere of the NARYN WG collaboration. This study was supported by grant RFBR 04-05-64970 References Berdichevsky, M.N., Dmitriev V.I., et al., 2003, On magnetovariational sounding – new possibilities, Phys. Solid Earth, 39 (09), 701-727. Bielinski R.A., et al., 2003, Lithospheric heterogeneity in the Kyrgyz Tien Shan imaged by magnetotelluric studies. GRL, V. 30, N15, 1806. Caldwell T.G. et al., 2004, The magnetotelluric phase tensor. GJI, 158, 457-469. Dmitriev, V.I., Berdichevsky, M.N., 1979, The fundamental model of magnetotelluric prospecting, IEEE Proc., 67,1034-1044. Dmitriev V.I., Berdichevsky, M.N., 2002, A generalized impedance model, Izvestya, Phys. Solid Earth,38 (10), 897-903. Eggers D.E. An Eigenstate formulation of the magnetotelluric impedance tensor.Geophysics. 1982. V.47. Р. 1204 - 1214. Roecker S.W. et al., 1993, Three-dimensional Elastic Wave Velocity Structure of the Western and Central Tien Shan, JGR. V. 98. № B9., 15779 – 15795. Rybin A., et al. 2000, Magnetotelluric and magnetovariation studies of the Estern Kyrgyz Tien-Shan, Abstracts of Inernational Workshop “Geodynamics of the Tien-Shan”, Bishkek, Kyrgyzstan, p.70-71. Sokolova E.Yu., Varentsov Iv.M., 2004. The Effective Source for the BEAR EM Array Sounding on the Baltic Shield. This Workshop. Trapesnikov et al., 1997, Magnetotelluric soundings in the mountains of the Kyirgyz Tien Shan, Physics of the Earth, 1, 3-20. Varentsov Iv.M., 2002, A General Approach to the MT Data Inversion in a piecewise-continuous medium, Izvestya, Phys. Solid Earth, 38(11), 913-934. Varentsov Iv.M. et al., 2003. System of EM Field Transfer Operators for the BEAR Array of Simultaneous Soundings: Methods and Results. Izvestya, Phys. Solid Earth, 39(2),118-148. Varentsov Iv.M., Sokolova E.Yu., BEAR WG, 2003. Diagnostics and Suppression of Auroral Distortions in the Transfer Operators of the EM Field in the BEAR Experiment. Izvestya, Phys. Solid Earth, 39(4), 283-307. Varentsov Iv.M., Sokolova E.Yu., 2004. The Multi-site Estimation of MT/GDS Transfer Functions with Horizontal Magnetic Control. This Workshop. Varentsov Iv.M., EMTESZ-Pomerania WG, 2004. The Estimation and Analysis of Horizontal Magnetic Inter-station Transfer Functions in the EMTESZ-Pomerania project. This Workshop.