Mutual reactance between cables

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
$\frac{\mu_{0} \omega \ln{\left(2 \right)}}{2 \pi}$valid for flat formations
$\frac{\mu_{0} \omega \ln{\left(\frac{L_{earth}}{a_{m}} \right)}}{2 \pi}$equivalent mutual reactance with earth return
$\mu_{0}$
$\omega$