I've noticed that DF generates rock based on a 48x48 grid system, you can see it quite clearly with reveal.exe, each of these squares is filled with some type of 'layer' rock and secondary minerals such as ores and gems. Their are three general types, large clusters, veins and small clusters. If one is primarily interested in finding veins which hold the lions share of all valuable metals you want to maximize the chance you will 'strike' a vein while minimizing the amount of rock needed to make the strike. The wiki currently lists a 15x15 block technique as its highest labor efficiency at 12% of all rock mined. But by taking the grid pattern into account a superior exploratory mining technique can be designed.
Steps
1 - Orient your self to the center or near center of one of the 48x48 blocks, this can be done by counting manualy from the edge of the map or finding the center of a large cluster of rock as these are centered on the grid.
2 - Dig a vertical staircase strait down through the center of the block
3 - On any level ware you hit a large cluster type stone skip it as their will be no vein in that block
4 - On the other levels dig a + shaped cross outward from the staircase with each leg 12 tiles long for a total mined area of 48 tiles per z level.
This should work because the veins have a very high probability of intersecting this + shape due to their length, though it is possible that very short or 'bent' veins could elude you. The main benefit is a very high labor efficiency, if we consider the whole block of 2304 tiles is checked with 48 tiles mined out a mere 2% of the total rock (without factoring the considerable savings from skipping Large Cluster filled areas) and I'd estimate 75% of all veins will be struck. If future expansion is desired its easy to upgrade to denser forms of mining like 15x15 blocks or extending branches off the + shape to get at those few remaining hidden veins.
If anyone is interested in trying this pattern and then run map reveal and compare how many veins were found with how many were missed.