We suppose that x[n] and y[k] are a DFT pairs, i.e.
where sum_n means the sum for n=0,1,N-1(similar for sum_k),
and dt is the sampling interval, df is the sampling frequency,
N is the sampling number.
To obey Parseval's theorem, we get
and df=1/(N*dt), also consider the amplitude symmetry of the real series,
we should multiply a coefficent to y[k]:
but in SAC software, this value is N*dt/2,
so do you have any idea?