Embedded, or buried, microstrip is similar to the surface version, however the signal line is embedded between two dielectrics and located a known distance from the reference plane.
Embedded microstrip with two dielectric layers, one below and one above the trace
In this structure the two dielectrics are arranged one below and one above the trace (designated 1B1A). Embedding the signal line can lower the impedance by as much as 20% compared to an equivalent surface microstrip construction.