The document discusses using transient electromagnetic (TEM) data to correct for static shifts in magnetotelluric (MT) data. TEM data is less sensitive to near-surface heterogeneities and topography, which can cause vertical shifts in MT apparent resistivity curves. The document proposes converting TEM time-domain data to the frequency domain using modeling or time shifting, and shifting the MT data to match the TEM-derived data to correct for static effects. This allows 1D MT modeling techniques to be applied to TEM data as well.

1. Static shift correction of MT data using TEM data

2. Distortion of MT sounding curve  related to near-surface heterogeneities and topography  vertical shift of apparent resistivity curve for all period range, but no effect on phase curve Static shift of MT data

3. MT and TEM response modelling of a 3-D near-surface inhomogeneities in a 1-D medium (Sternberg et al, 1988)

4. MT response (apprent resistivity sounding curve)

5. 2-D MT modelling of a topographic effect (Chouteau & Bouchard, 1988)

6. Comparison of MT sounding curves with topographic effect (initial model), terrain-corrected data and flat earth model response

7. Distortion of MT sounding curve  related to near-surface heterogeneities and topography  vertical shift of apparent resistivity curve for all period range, but no effect on phase curve  no effect on TEM apparent resisitivity sounding curve Static shift of MT data

8. TEM response (apprent resistivity sounding curve)

9. Distortion of MT sounding curve  related to near-surface heterogeneities and topography  vertical shift of apparent resistivity curve for all period range, but no effect on phase curve Effect on 1-D modelling results  For a vertical shift of  a ( T ) curve characterized by a multiplication with a factor k then  layer resistivities are multiplied by k  layer thicknesses are multiplied by k 1/2 Static shift of MT data

10. 1-D synthetic models k = 4, k 1/2 = 2  k – 1  k

11. 1-D synthetic models k = 4, k 1/2 = 2  k – 1/2  k 1/2

12. Model response

13. Measurement of independent data at MT site, i.e. using sounding technique without electric field measurement (less sensitive to static effects)  Transient Electromagnetics (TEM) or Time Domain Electromagnetics (TDEM) Electromagnetic Array Mapping (EMAP) technique  Closely spaced MT along a traverse  Spatial filtering of electric field in an array Incorporate static shift parameter estimation in 2-D inversion modelling Static shift correction

14. EM sounding technique using artificial source  source: current injected into a horizontal wire loop receiver: coil magnetometer at the center of the source loop (central loop configuration)  measurements of secondary electric or magnetic field decay after transmitter switch-off (time domain)  the decay characterizes the conductivity of the subsurface early time – late time response ~ shallow – deep Transient Electromagnetics (TEM)

15. Transient Electromagnetics (TEM)

16. TEM signal

17. TEM data presentation

18. Zonge TEM system

19. Transient Electromagnetics (TEM)

20. TEM data are less sensitive to static effects  apparent resistivity from TEM data represents undistorted (correct value) of the shallow part of MT sounding curve  TEM sounding curve: apparent resisitivity vs. time (  a , t ) MT sounding curve: apparent resisitivity vs. period (  a , T )  "convert" TEM data to MT data by using modelling technique or time-shift technique TEM for static shift correction (Sternberg et al, 1988)

21. TEM modelling method

22. Eqivalence of TEM diffusion depth and MT skin depth  TEM diffusion depth: d = 48 (  t ) 1/2 MT skin depth:  = 500 (  T ) 1/2 at the same penetration depth of TEM and MT, t and T are equivalent  "conversion" of time (TEM) in msec. to period (MT) in sec. t T  ─── 200 TEM time shift technique

23. TEM time shift method

24. TEM response of 1-D synthetic models

25. Comparisons of TEM converted data with MT response of 1-D synthetic models

26. Comparisons of TEM converted data with MT response of 1-D synthetic models

27. "Convert" TEM data (  a , t ) to data in (  a , T ) by using time-shift technique Plot converted TEM data along with MT data exhibiting static shift or static effect Shift MT data by multiplying with a constant k to obtain a match with TEM-derived data TEM for static shift correction

28. Static shift correction of MT field data using TEM (Sternberg et al, 1988)

29. Summary MT static shift correction using TEM  TEM measurement at MT sounding sites  "Conversion" of TEM sounding data to MT data using time shift technique  Shift MT data to match to TEM-derived data A by product of "conversion" of TEM sounding data to MT data using time shift technique  to apply 1-D MT modelling technique to TEM data