[vtkusers] How to clip an image according to a surface?

[james at whitehocott.plus.com]

Hi
I use paraview 3.8.1 to visualize geophysical data. My data are arranged
as an unstructured grid of cell data. This gives an image of resistivity
(the colour value) in 3 dimensions.
I also have another image giving the maximum depth to which I can trust
the previous data. This is a polydata dataset of point data, where the
points are simply the depths.

Is it possible to clip or threshold my original image (the unstructured
grid) according to the depths given in the polydata?  I.e cut away any
data in the original image that is below xx depth, where xx is given in
the polydata surface?

Thanks
James R