I found the solution
TinSurface tinSurface = acTrans.GetObject(SurfaceId, OpenMode.ForRead) as TinSurface;
// Get the DEMFileName Property from drawing
for (int b = 0; b < tinSurface.DEMFilesDefinition.Count; b++)
{
SurfaceOperationAddDEMFile operation = tinSurface.DEMFilesDefinition[b];
string DEMfilenamePath = operation.DEMFileName.ToString();
// the RemoveAt will act at this level
ed.WriteMessage(DEMfilenamePath);
}