Models of the Near-Surface Distortions - Models and Methods of Magnetotellurics - page 271

Fig. 7.42 Model of a
three-dimensional horst

7.4.2 The Graben Model

A two-dimensional model of the graben is shown in Fig. 7.45. Similar to the horst
model, the upper layer (

1 ) simulates the conductive sedimentary strata underlaid
by the resistive lithosphere ( 3 ). The

graben model is a counterpart of the three-segment conductance model with a con-
ductive central segment (Fig. 7.16). Let us compare these two models.
Figure 7.46 presents the longitudinal and transverse apparent-resistivity and
impedance-phase curves,

2 ) resting on the highly conductive mantle (

, ⊥ , observed outside and over the graben.
They closely resemble the curves for

, ⊥

and

, ⊥ , ⊥ and obtained in the three-

⊥ − ⊥ − segment model (Figs. 7.21 and 7.22). Outside the graben the and curves

− −

are hardly distorted, while the curves experience the strong effect of

false conductive layer, which however quickly attenuates with distance. Over the
graben the

and

⊥ − ⊥ −

and curves are drastically distorted by the strong S − effect

⊥ − (the descending branches of the curves are displaced from the locally normal

− − ¨ n -curve almost by two decades), while the and curves are distorted rather

slightly.
The tipper curves are shown in Fig. 7.47. They are much like those in the three-
segment model (Fig. 7.23).
Figure 7.48 shows the field profiles, which pass across the graben in the

y − direction.The electric and magnetic fields are normalized to the normal fields

E N E N H N

y x , y ,

observed at | y | →∞ . The field profiles are not too different from