Mutual reactance between conductors

The equation for the equivalent mutual reactance with earth return is from the Cigré guide 531 chapter 4.2.3.8.

Symbol
$X_m$
Unit
$\Omega$/m
Formulae
$\omega \frac{\mu_0}{2\pi} \ln2$valid for flat formations
$\omega \frac{\mu_0}{2\pi} \ln\left(\frac{L_{earth}}{a_m}\right)$equivalent mutual reactance with earth return