Intrduction to the Magnetotellurics.pptx

This presentation explains brief introduction of the magnetotelluric studies with a case study

1. 1 Introduction to Magnetotellurics

2. Magnetotelluric and its Theory 2 Source of MT signals Low frequency signals (< 1 Hz) from the interaction of the solar wind with the earth’s magnetic field. High frequency signal (> 1 Hz) is generated by universal thunderstorm activity, generally near to the equator. The MT method records time variations of the Earth’s magnetic and electric fields over a wide frequency range at arrays of ground sites to measure Earth’s electrical resistivity (conductivity ) structure with depth. Ratio of Electric to Magnetic provides resistivity measurement of the body ( Tikhonov,1950; Cagniard , 1953) . Figure 1: Schematic depiction of the magnetosphere, showing the outflow of plasma from the sun, the diversion by the bowshock around the magnetosphere, and the general structure of the magnetosphere. The broad blue arrows within the magnetosphere show the direction of the magnetic field.

3. Passive surface measurement of time varying the Earth’s natural electric (E) and magnetic (H) fields. Assume planar horizontal magnetic source field (reasonable assumption in mid-latitudes, far from external source regions ). This is a diffusive process, the physics based on Maxwell’s equations of electromagnetic induction. Frequency range 10 KHz to .0001 Hz (0.0001 s to 10000 s). Ratio of E / H used to derive resistivity structure of the sub-surface. 3 Figure 2: Typical Resistivity values of different Rocks and Minerals.

4. The penetration depths of electromagnetic fields within the Earth depend on the sounding period and the resistivity structure . High period has high penetration depth where as low period has less penetration depth . 4 and = apparent resistivity   Skin Depth /Penetration Depth Figure 3: Skin depth relation with different periods in homogeneous earth resistivity model

5. Natural electric/magnetic field variations used to image the deep electrical resistivity structure of the Earth. The MT method, samples a volume of the medium. The penetration depth depends on the frequency of the EM waves. The MT method is very powerful at detecting subsurface conductivity/resistivity structure. MT data can directly determine the thickness of the lithosphere by identifying the boundary between the lithosphere and asthenosphere, which is associated with a decreased electrical resistivity. 5 Why Magnetotelluric (MT)?

6.   Commonly, the horizontal electric and magnetic field components (Ex , E y , B x and B y) can be related to one another through the impedance tensor Z . In the frequency domain, Z is defined as: E (  ) = Z (  ) B (  )   In Cartesian coordinates, where x denotes magnetic north and y magnetic east, this tensor can be written using matrix notation as , = The impedance tensor Z contains information about the geoelectric dimensionality (1-D, 2-D, or 3-D) and directionality of the subsurface .   6 Impedance tensor and spatial dimensionality

7. 7 Figure 4: N-layered Earth resistivity model . Figure 5: Basic 2-D resistivity model consisting of two quarter-spaces. The structure is invariant along strike (x-direction) and the EM fields can be decomposed into two modes. (1) The electric field parallel to strike induces magnetic fields perpendicular to strike and in the vertical plane, referred as Transverse Electric mode (TE mode). (2) Magnetic fields parallel to the strike induces electric fields perpendicular to strike and in the vertical plane, referred as Transverse Magnetic mode (TM mode ). Apparent resistivity over a layered Earth Magnetotelluric over 2-D resistivity model

8. MT instrument and field setup Figure 6: MT system and sensors (a), MT field model layout (b) and MT field photographs (c) site from IGP and (d) site from Higher Himalaya. 8

9. Electric and Magnetic field installation 9 Figure 7: Installation of electric and magnetic field sensors. Non-polarized electrode in figure (a) and which buried with mud in figure (b), figure (c) and (d ) are burial of horizontal and vertical magnetic field sensors, respectively. The MFS 06 and MFS 06e induction coils used to record the temporal magnetic field variations in three orthogonal directions N-S ( Hx ), E-W ( Hy ) and vertical (Hz). Non-polarizing electrodes were used to measure the electric field in the orthogonal directions (Ex and Ey ), generally magnetic north-south and east-west.

10. Figure 8: Flow chart of MT time series processing (Borah et al., 2015). 10 Flow chart illustrate the MT initial time series processing methods adopted in this study. Data Processing

11. Figure 09: Figure shows the time varying electric (Ex, and Ey ) and Magnetic ( Hx , Hy , and Hz) fields from top to bottom, respectively at site no. 09. 11 Time series plot

12. 12 Figure 10: Time series data with 64 Hz sampling frequency in figure (a) affected by cultural noise (spikes and trend) and figure (b) is showing the same data after trend elimination and spike masked. Spike and Trend Removal

13. 13 Remote referencing method Figure 11:Apparent resistivity and phase curve of 09 lng MT station a. robust linear regression method of a single station and b. remote reference method.

14. Dimensionality and Distortion analysis of MT data Determination of geoelectric dimensionality and directionality is an important step in MT data analysis and interpretation. The dimensionality information are preserved within the impedance tensor. . Diagonal components contain the information on lateral conductivity and off-diagonal components indicate mainly the vertical conductivity effects ( Berdichevskey , 1999). For 1D earth, the diagonal components of impedance tensor are zero in all coordinate systems, while the non-diagonal components are equal in modulus but with opposite signs. When one of the measurement axes coincides with the strike direction of the structure, the diagonal components of impedance vanish to zero and the non-diagonal components differ and the structure can be 2D. In a 3D earth, all the components of impedance tensor never vanish in any direction, and there is no strike ( Dobrin and Savit , 1988). However increased sounding period and large skin depth, the body act as a local near inhomogeneity, which gives (non-inductive) galvanic response. 14

15. MT Field planning Figure 12: MT site location (inverted black triangles) projected on tectonic map of the northwest Himalaya along the Satluj valley. Map modified after Thakur and Rawat , 1992; Vannay et al., 2004; Thiede et al., 2006 and Webb et al., 2011 15

16. Figure 13: Resistivity cross section along the N45  W profile extending from IGP to TH in the Himachal Himalaya. The various high and low resistive structures are marked in English alphabetic letters. Upper panel shows the generalized geological cross section of the Himachal Himalaya adopted from Webb et al., (2011). 16 Intra Crustal Low Resistive layer (IC-LRL ) with 4- 30  m . A dome shaped high resistive body (500  m) interpreted as the high resistive Indian basement also mapped by Israil et al., (2008) in Garhwal Himalaya. Surface traces of the LHS/HHCS marked with high resistivity around 500  m. Near surface high conducting layer marked as a result of sediment squeezed fluids of the overriding Himalayan wedge (Molnar, 1993 and Hazarika et al., 2017). Resistivity structure of the study area

17. 17 Thank You