docs: def velocities_to_moduli(rho, v_phi, v_s):
      """
      convert velocities to moduli
      mainly to support Burnman operations
  
      :param rho: density in kg/m^3
      :param v_phi: bulk sound speed in m/s
      :param v_s: shear velocity in m/s
      :return: K_s and G
      """
      return v_phi * v_phi * rho, v_s * v_s * rho