Here are two approaches to adding noise to data:

1.
Use the formula described in the manual  i.e. contaminate
the exact data with Gaussian noise (GN) of standard deviation (sd) equalling
5% of the chargeability value plus additional GN with sd equal to a constant
(for example 0.1 pfe).

2.
Similar to the above, however instead of
using 5% of the chargeability value for the sd use 5% of a power law
function of the Vp i.e. 0.5Vp^-0.3.  This is derived (the
constants are arbitrary) by plotting chargeability error against Vp.

The errors for both cases are then the sd of the noise (as
specified in the manual),  however you should add an additional error
floor to this value to get a suitable model.  You may not have to add
error floor for the resistivity inversions.