See the FAQ: How to do a minimal reproducible example reprex for beginners to be able to receive answers that do not have to employ imagined data.
set.seed(137)
m <- matrix(rexp(200, rate=.1), ncol=20)
which(m[,1:20] < 1, arr.ind = TRUE)
#> row col
#> [1,] 2 3
#> [2,] 1 4
#> [3,] 2 4
#> [4,] 3 6
#> [5,] 9 9
#> [6,] 3 11
#> [7,] 7 13
#> [8,] 6 15
#> [9,] 9 15
#> [10,] 1 16
#> [11,] 10 16