Modelling of infinite large seabed (with COMSOL 4.3)

Hi

I'm conducting thermal calculations on a cable in the seabed (with COMSOL 4.3) using the Heat transfer module.

I am modelling the seabed as a big square wherein the cable is placed. The upper boundary of the square (the seabed) is the interface with the seawater. This boundary should have a constant temperature (or convection).
The other boundaries of the square should be infinite.

I've tried to use the Infinte Element domain, but I am not exactly sure on how to use it?

Can anyone help me?

Thanks,
Daniel