def isochone_ratio(e, rd, r_hyp):
    """
    Get the isochone ratio as described in Spudich et al. (2013) PEER
    report, page 88.

    :param e:
        a float defining the E-path length, which is the distance from
        Pd(direction) point to hypocentre. In km.
    :param rd:
        float, distance from the site to the direct point.
    :param r_hyp:
        float, the hypocentre distance.
    :returns:
        c_prime, a float defining the isochone ratio
    """
    if e == 0.0:
        c_prime = 0.8
    elif e > 0.0:
        c_prime = 1.0 / (1.0 / 0.8 - (r_hyp - rd) / e)
    return c_prime
