Self-dual codes better than the Gilbert-Varshamov bound

We show that every self-orthogonal code over Fq of length n can be extended to a self-dual code, if there exists self-dual codes of length n. Using a family of Galois towers of algebraic function fields we show that over any nonprime field Fq, with q64, except possibly q=125, there are infinite families of self-dual codes, which are asymptotically better than the asymptotic Gilbert-Varshamov bound.