calculate volume of sinks in ArcGIS scene 9.3

Dear GIS'ler,

I have a xyz data set (3D data) in shp format. The data represents a
small plot of 1.50 X 2.50 sqr meter. The plot ist not plane - the hole
plot is about 20 cm sloped. On my plot there are an awful lot of very
small sinks at different elevation (z)levels.
Now I wonder how I can calculate the volume of all sinks on my plot.
So far I converted the shp data into raster data. Now I want to use
the cut/fill procedure to calculate the volume of all sinks. My
problem is to generate the after raster for that operation. What I
need are planes above all my sinks at the edge of each sink (pour
point), right!? What I did so far was to create contour lines and
convert them into raster data. The Problem is there are also contour
lines in the sinks and I only want them on the the edge of each sink.
To cut them by hand would be labour-intensive because of a very
heterogene structure.
Is that approach right to generate the after raster for the cut/fill
operation? Or is there another easier solution. Hope so.

Many thanks in advance.
Cheers,
Markus