Receivers and transmitters can be located anywhere in conducting parts of the model domain, but some care is needed to make sure MARE2DEM will output meaningful results. Seafloor EM transmitters and receivers should be located at least a little bit above the seafloor boundary; this is so that MARE2DEM knows they are in the seawater rather than the seafloor below. Likewise, land EM receivers and transmitters should be positioned a tad below the surface. A small vertical distance such as 0.1 m is sufficient; make sure you remember that z is positive down, so seafloor receivers should have a z position slightly less than the seafloor depth. You can also have borehole transmitters and receivers, but these have only received a small amount of testing so far.