Simulation of CMOS passive components with no explicit ground in Momentum

Dear experts

Consider a case in which we have an inductor (or whatever passive component) that has no explicit ground. That is, for increasing the Q factor, ground beneath the inductor is removed. Also consider that we don't want to excite it in a differential mode.
Now if I want to simulate this inductor in Momentum simulator of ADS, what should I do about ground references of the two ports of the inductor?
If there is no explicit ground, Momentum will suppose that GND is in infinity. Is this setting correct?

If you change the bottom layer "cover" to "stripline", I think it will work.In this case, an additional layer "Air" will be replaced with metal layer.

The fact is I didn't define cover for my differential inductor, since ports were differential.
But for simulating inductor with single-ended ports, I need to have a ground reference which will not be defined if I set my design with removing cover.
As this cover, in case of CMOS will be something like 500um beneath the inductor, I think it will not have effect on the results.
Besides, there is an example in ADS, named "SPIRAL", in which beneath the silicon substrate is assumed to be a perfect conductor.
If I just make my settings like this example, that is, defining a cover beneath the silicon, will I get reasonable results?