1. Extraction of surface impedance from magnetotelluric data
Magnetotelluric Practitioner: BS Khanyile
Supervisor: P Cilliers

2. Introduction
The magnetotellurics (MT) method is based on measuring time variations of orthogonal components of electric and magnetic fields at the surface of the earth (Weckmann et al., 2005).
The time invariant quantity called MT impedance tensor is the response of the Earth to electromagnetic induction and carries information about the conductivity distribution of the subsurface.
The electromagnetic fields are assumed to be linearly related by the impedance tensor in the frequency domain (Chave, 2004):

4. Flowchart: Processing MT data
Processing Algorithm
Raw data
Local site
Visual inspection of time series
Selection time window
Fourier transform
Complex Fourier transform
Frequency averaging
Compute
MT TRANSFER FUNCTIONS
Flowchart: Processing MT data

5. Theory In order to analysis MT data, the robust estimate was adopted.
The first equation was rewritten in terms of auto- and cross spectral densities:

6. The component of the impedance tensor is estimated as

7. Using the impedance tensor equation above, and assuming that the earth is homogenous and isotropic (Hermance, 1973),
and then one can derive the resistivity and phase impedance of the earth in the frequency domain as:
Where ῶ is the angular frequency and Im and Re indicate the imaginary and real parts, respectively.

8. Results and Discussions
For each of the site, the raw time series recorded at 1 s interval were reprocessed using of the robust method (Smirnov, 2003).
This method of robust is based least squares solution for the usual MT response function relating the Fourier transforms of the horizontal electric and magnetic fields.
No remote reference processing was considered.
Figs. 1-3 show the apparent resistivity and phase of both off diagonal components at three stations.
In Fig. 1. The and apparent resistivity and phase plots appear disturbed at around low periods less than 100s.
Figs. 2-3 are strongly disturbed; no conclusions that can be made because of appear as the lot of noise in these sites.

9. Results
Figure 1. The solid line shows the estimate from robust processing of all the data set. Apparent resistivity (top) and impedance phase (bottom) as function of period for Middelpos site.

10. Results
Figure 2. The solid line shows the estimate from robust processing of all the data set. Apparent resistivity (top) and impedance phase (bottom) as function of period for Vaalputs site.

11. Results
Figure 3. The solid line shows the estimate from robust processing of all the data set. Apparent resistivity (top) and impedance phase (bottom) as function of period for Hermanus site.

12. Conclusions
The importance of high-quality horizontal magnetic field components for a meaningful estimation of geomagnetic
and magnetotelluric response functions need to be emphasized.
Further analysis need to be done especially statistical analysis to qualify the results.

13. Thank you!
Ngiyabonga!