A water well drilling rig depth must fit exploration goals and aquifer depth.
Too deep or shallow harms data.
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Shallow drilling (<50m): Checks shallow groundwater and surface-water links. A farm area drilled 20-30m, finding main irrigation sand layers. But it misses deep aquifers, unfit for total resource assessment.
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Medium drilling (50-300m): Covers shallow and upper confined water. Good for small cities’ water supply surveys. A county drilled 200m, finding two aquifers—data for dual-source water plans.
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Deep drilling (>300m): Explores deep confined water, geothermal water. A dry area drilled 500m, finding sandstone aquifers with 15m³/h yield. Costs 3-5x more; needs careful planning.

Base depth on geophysics, geology and goals. One area drilled 120m (target <100m), covering aquifers at reasonable cost. The water well drilling rig thus balances data and expense.