The standard height for assessing fields for compliance with exposure limits is 1 m above ground.
As you get closer to the ground, you get closer to the conductors, so the field from an underground cable goes up. Because a cable may only be 1 m below ground (as opposed to an overhead line which may be 12 m above ground) this effect is more pronounced than for the overhead line. This graph shows the effect for a cable buried 1 m below ground:
The effect is most extreme for the shallowest cables, which are 400 kV and 275 kV “trough” cables, typically 0.3 m below ground. The effect for one of these cables is shown here:
Note: these are just example cable installations. Each specific cable installation would need its own calculation, but the general pattern would be similar.