P. Schmoldt, PhD - MTNet - DIAS

A.2.3. Jones Catechism
A.2. Auxiliary information regarding inversion processes
The term Jones Catechism, named after Alan G. Jones, describes a sequence of inversion
steps following the approach to “bring in structure slowly”. The Catechism pursues the
plan to reproduce the observed response by a minimum numbers of features with moderate
conductivity values (cf. Occam’s razor). This approach is motivated by the circumstance
that MT inversion models are non-unique and that the MT method is most sensitive to
the conductance of the subsurface, rather than to the conductivity of a specific region.
This means that localised features with high conductivity can yield, to a certain degree,
similar responses than a more extensive feature with lower conductivity (cf. Sec. 5).
In MT inversion, the Jones Catechism is implemented by starting the inversion process
with the phase of the TM mode, the parameter most sensitive to lateral boundaries and
unaffected by static shift, before subsequently introducing the phase of the TE mode and
apparent resistivities. The order of the inversion steps is suited to fit the regularisation of
nonlinear conjugate gradient (NLCG) inversion (Sec. 6.3), implemented among other in
the WinGLink software package [WinGLink, 2005].
During inversion of the PICASSO Phase I dataset the Jones Catechism is realised by
initially inverting only for the TM mode with a moderate error floor for the phase and for
a maximum error floor for the apparent resistivity, making the latter virtually irrelevant.
Once the RMS misfit approaches a minimum value, TE mode phase data are added (again
by assigning a maximum error floor to the apparent resistivity data). This is followed by
further introducing first the apparent resistivity of the TM mode and thereafter of the TE
mode by lowering the respective error floors. Once all modes are introduced, error floors
for the parameters are subsequently reduced to a minimum, wherein values are a trade-off
between error floor and RMS misfit of the model (cf. Sec. 10).
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