I have created couple of graphs in R

    d5 <- scan('d500.txt')
    p1 <- acf(d5,lag.max = 36 ,type = c("covariance"),plot = TRUE)


[![enter image description here][1]][1]


Then the next one is correlation

    p1 <- acf(d5,lag.max = 36 ,type = c("correlation"),plot = TRUE)

[![enter image description here][2]][2]

And pacf

    p1 <- pacf(d5,lag.max = 36 ,plot = TRUE)

[![enter image description here][3]][3]

If I take a one difference operation

    d51 <- diff(d5)
    p1 <- acf(d51,lag.max = 36 ,type = c("correlation"),plot = TRUE)
[![enter image description here][4]][4]


And then again acf
[![enter image description here][5]][5]


and pacf

[![enter image description here][6]][6]


  [1]: https://i.sstatic.net/c0eMc.png
  [2]: https://i.sstatic.net/VXNEL.png
  [3]: https://i.sstatic.net/OmvAk.png
  [4]: https://i.sstatic.net/D5cen.png
  [5]: https://i.sstatic.net/k0FXk.png
  [6]: https://i.sstatic.net/IqeOz.png

How to interpret this?What does sinusoidal acf(cov) shows?