Only 100 km down, the gravitational acceleration would not be reduced by very much. Assuming the Earth is spherical and has uniform density, the probe would only be below ~3% of the mass and would be less than 2% closer in distance. So the acceleration felt by the probe would only vary by a few percent.

At 100km, the gravitational acceleration is actually higher than at the surface.

http://www.wolframalpha.com/input/?i=gravitational+accelerat...

In fact, it increases until 3500km below the surface (2800km radius).

http://en.wikipedia.org/wiki/Gravity_of_Earth#mediaviewer/Fi...