Obtaining a number of (x, y) points that correspond to specific values of concentration that are chosen at random

I need to generate a matrix of points given that they meet the condition that at these (x,y) points concentration is greater than 10. Note that I first run a code that gives me concentration at each location c(x,y,t), and now from the results of the first run I need Matlab to "randomly" pick (x,y) points with the above condition. Also, note the dimensions of the results from the first Matlab run (which the random sampling should be based on): concentration changes with location and time and is 52x61x61, x is 1x61, y is 1x52, and time is 1x61.

For example, for a randomly chosen concentration with a value of 50, what is x and y at which this value is observed? I need to do this for 12 different points; 12 different values of c(x,y,t).

I hope my question makes sense. Thanks.