Using the magnetic fields module in Cable HV software you can easily perform magnetic field density level studies for high voltage power cable applications.
The relative permeability of the medium to which the cables are installed has an effect on the magnetic field intensity. Increases in relative permeability increases magnetic field intensity. The permeability of soil is only slightly higher than air.
The below table is from reference: Scott, J. H. (1983). Electrical and magnetic properties of rock and soil (No. 83-915). US Geological Survey.