# Presentation: 763047

## Static Shift:

- The vertical offset of the entire MT (frequency vs. apparent resistivity) profile, caused by isolated horizontal resistivity gradients near the surface.

## surface

- 1

-             2

- J1

- J2

- Fig. 1:  Current crossing a boundary causes charge to accumulate.

- The charge on the vertical surface can be quantified using the boundary conditions from Maxwell’s equations.

## Boundary Conditions:

J1n=J2n  (The current perpendicular to the boundary is continuous across it.)
E1n - E2n = /0 (The difference between the perpendicular components of the electric field is proportional to the charge density on the surface.)

- J1n=J2n  (The current perpendicular to the boundary is continuous across it.)
- E1n - E2n = /0 (The difference between the perpendicular components of the electric field is proportional to the charge density on the surface.)

## The Surface Charge:

 E1n/1 = J1n = J2n = E2n/2                   (Ohm’s Law)

 E1n =1J ,  E2n = 2J

 (1  - 2)J = E1n - E2n = /0

 = 0(1  - 2)J 

The surface charge density on the interface and the electric field discontinuity across it is proportional to the difference in resistivity.

-  E1n/1 = J1n = J2n = E2n/2                   (Ohm’s Law)
-  E1n =1J ,  E2n = 2J
-  (1  - 2)J = E1n - E2n = /0
-  = 0(1  - 2)J
- The surface charge density on the interface and the electric field discontinuity across it is proportional to the difference in resistivity.

## Effect on TM Mode

- In the TM mode, we detect the electric field perpendicular to the boundary, which is discontinuous:  E1n - E2n = /0

- 1

- 2

- +

- +

- +

- +

- +

- Eprimary

- Esecondary

- Etotal

## Assuming 1>2 and current flowing from the resistive side to the less resistive side (left to right in diagram), there will be a positive charge accumulation on the boundary.  This charge will reduce the field to the left and augment the field to the right.
An MT station to the left will observe a reduced TM electric field, so the surface impedance and apparent resistivity measurements will be too low: 
	a  Zs =Ex/Hy
An MT station to the right will measure an apparent resistivity that is too high.

- An MT station to the left will observe a reduced TM electric field, so the surface impedance and apparent resistivity measurements will be too low:
- 	a  Zs =Ex/Hy
- An MT station to the right will measure an apparent resistivity that is too high.

## Effect on TE Mode

- In the TE mode, we measure the electric field component parallel to the boundary.  According to Maxwell, Ell is continuous regardless of surface charge, so Zs and a are continuous.  There is no shift.

## Slide 8

## The Problem

- TE and TM generally refer to field directions relative to geologic strike, not to the orientation of some unknown boundary.
- Either curve could be shifted in either direction, so it is not clear how to correct the shifts.

## Solution

- Correct static shift using TEM data from the same location (Pellerin, 1990)
-  From TEM data, create shallow resistivity profile, and then forward model the high frequency part of the MT curve.

## Slide 11

## Slide 12

## Slide 13

## Slide 14

## Slide 15

## Acknowledgements

- SAGE 2005 faculty, especially Louise and George.
- Fellow Team 4 students
- Jiracek, G., 1990, Near-surface and topographic distortions in electromagnetic induction, Surveys in Geophysics, 11, 163-203.
- Pellerin, L., and Hohmann, G.W., 1990, TEM: A remedy for MT static shift, Geophysics, 55, 1242-1250.
- Vozoff, K., 1991, The magnetotelluric method, _in_ Nabighian, M.N., Ed., Electromagnetic methods in applied geophysics, vol. II, part B:  Soc. Expl.  Geophys., 641-711.
- Ward, S.H., and Hohmann, G.W., 1988, Electromagnetic theory for geophysical applications, _in_ Nabighian, M.N, Ed., Electromagnetic methods in applied geophysics—theory, v.1, Soc. Expl. Geophys., Tulsa, 131-311.