Using seismically constrained magnetotelluric inversion to recover velocity structure in the shallow lithosphere

Abstract:

Typical surface wave tomography can recover well the velocity structure of the upper mantle in the depth range between 70-200km.
For a successful inversion, we have to constrain the crustal structure and assess the impact on the resulting models. In addition,
we often observe potentially interesting features in the uppermost lithosphere which are poorly resolved and thus their interpretation
has to be approached with great care.

We are currently developing a seismically constrained magnetotelluric (MT) inversion approach with the aim of
better recovering the lithospheric properties (and thus seismic velocities) in these problematic areas. We perform a 3D MT inversion constrained by a fixed seismic velocity model from surface wave tomography.
In order to avoid strong bias, we only utilize information on structural boundaries to combine these two methods.
Within the region that is well resolved by both methods, we can then extract a velocity-conductivity relationship. By translating the conductivities
retrieved from MT into velocities in areas where the velocity model is poorly resolved, we can generate an updated velocity model and test what impact
the updated velocities have on the predicted data.

We test this new approach using a MT dataset acquired in central Botswana over the Okwa terrane and the adjacent Kaapvaal and Zimbabwe Cratons together
with a tomographic models for the region. Here, both datasets have previously been used to constrain lithospheric structure and show some similarities.
We carefully asses the validity of our results by comparing with observations and petrophysical predictions for the conductivity-velocity relationship.