There is no answer as the problem is ill-conditioned.

We have relative error |log_(b(1 + delta))(a) - log_b(a)| / |log_b(a)| = |log(b) / (log(b) + log(1 + delta)) - 1|. By Taylor expansion log(1 + delta) ≈ delta, so the relative error is |log(b) / (log(b) + delta) - 1| = |delta / (log(b) + delta)|. For b ≈ 1, the relative error is approximately 1 as well.