## Publication 18

##
On the identification of a transition zone in electrical conductivity
between the lithosphere and asthenosphere: a plea for more precise phase
data

###
Cavaliere, T. And Jones A.G.

### Abstract

The magnetotelluric analytical solutions for an earth containing
transitional layers in which the electrical conductivity is a function
of depth has been considered previously for certain types of restricted
models.
However, an analytical formulation for an arbitrary n-layered earth containing
both constand conductivity layers and transitional layers has not been
published.
Herein is presented a general matrix technique for such a problem in which
the solution matrix is built up from n-1 layer connection matrices.
The solution matrix is extremely sparse for n large and can be solved
by O(n) operations rather than the usual O(n^3).

This theory is applied to generate the theoretical surface response of two
specific models of the lithosphere and asthenosphere.
THe first model has a lithosphere/asthenosphere boundary at 80 km, and is
representative of "young" oceanic crust and upper mantle.
The other model is representative of a continental crust and upper mantle
structure with an asthenosphere below 160 km depth.

For both models, techniques of linear inverse theory are applied to ascertain
if a transitional zone between the lithosphere and asthenosphere could be
resolved by surface measurements.
It is shown that the impedance phase data is far more important for resolving
this model parameter than is the apparent resistivity data.
Accordingly, the need for more precise phase information is stressed.

### Source

*J. Geophys.*, **55**, 23-30, 1984.

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