Geometry or Terrain from scene.j3o

Hello

I wanna create LoD for my map and models. I’m using scene in .j3o format. I found code:
[java]LodGenerator lodGenerator = new LodGenerator(geometry);
lodGenerator.bakeLods(LodGenerator.TriangleReductionMethod.PROPORTIONAL, 0.1f,0.5f);[/java]
but i dont have idea how to get geometry of terrain from my scene