The Magnetovariational Response Functions - Models and Methods of Magnetotellurics - page 138

the base site to the observation site. The most convenient is parameter G because

it is less subjected to distorting influence of inhomogeneity around the base site.
Consider the “normal” base site B N located in a horizontally homogeneous zone
and an arbitrary “anomalous” base site B A located in a horizontally inhomogeneous
zone. According to (4.42),

H τ ( r ) = [ M ( r | r B A )] H τ ( r B A )

= [ M ( r | r B A )][ M ( r B A | r B N )] H τ ( r B N ) = [ M ( r | r B N )] H τ ( r B N ) ,

where

[ M ( r | r B N )] = [ M ( r | r B A )][ M ( r B A | r B N )] ,

whence

det [ M ( r | r B N )] = det [ M ( r | r B A )] det [ M ( r B A | r B N )] .

Thus,

1
det [ M ( r B A

det [ M ( r | r B A )] = det [ M ( r | r B N )]

| r B N )]

and

1
G ( r B A

G ( r | r B A ) =

r B N ) G ( r

| r B N ) .

|

At a given frequency the values of G ( r | r B A ) and G ( r | r B N ) obtained with the

B A B N

anomalous and normal base sites and differ by the constant factor

|

G ( r 1 | G ( r 2 | 1 / G ( r B A r B N ). Hence, the ratio between parameters r B A ) and r B A )

determined at the observation sites O( r 1 ) and O( r 2 ) does not depend on position

of the base site B A :

G ( r 1 | r B A )

r B A ) = G ( r 1 |

r B N )

r B N ) .

G ( r 2 | G ( r 2 |

The relative variations of the geometric-mean parameter G are invariant under

position of the base site B A . So, the parameter G can give the most reliable image of

configuration of geoelectric structures. By analogy with effective electric intensity
D eff , it referred to as the effective magnetic intensity

| | .

M eff = det [ M ( r | r B )] (4 . 62)