I think they drill more holes at shallow depth to confirm continuety of vein structure.

If they drill a series for 3 holes from each set up at 30, 45, an 60 degrees and these 3 hole sets of 75M, 150M and 250M or roughly 500M per set.

Thus you would intersect @ Roughly 40-50M, 100M and 150 M depth.. 3 intersects will allow you to configure dip angle, grade , width... You need this to determine most efficent targeting for deeper holes.