Simplified Rayliegh velocity of the site.

        This follows the simplifications proposed by Urzua et al. (2017)

        Returns
        -------
        rayleigh_vel : float
            Equivalent shear-wave velocity.

def simplified_rayliegh_vel(self):
        """Simplified Rayliegh velocity of the site.

        This follows the simplifications proposed by Urzua et al. (2017)

        Returns
        -------
        rayleigh_vel : float
            Equivalent shear-wave velocity.
        """
        # FIXME: What if last layer has no thickness?
        thicks = np.array([l.thickness for l in self])
        depths_mid = np.array([l.depth_mid for l in self])
        shear_vels = np.array([l.shear_vel for l in self])

        mode_incr = depths_mid * thicks / shear_vels ** 2
        # Mode shape is computed as the sumation from the base of
        # the profile. Need to append a 0 for the roll performed in the next
        # step
        shape = np.r_[np.cumsum(mode_incr[::-1])[::-1], 0]

        freq_fund = np.sqrt(4 * np.sum(
            thicks * depths_mid ** 2 / shear_vels ** 2
        ) / np.sum(
            thicks *
            # Roll is used to offset the mode_shape so that the sum
            # can be calculated for two adjacent layers
            np.sum(np.c_[shape, np.roll(shape, -1)], axis=1)[:-1] ** 2))
        period_fun = 2 * np.pi / freq_fund
        rayleigh_vel = 4 * thicks.sum() / period_fun
        return rayleigh_vel