Pumping tests on a major water supply well create a cone of depression with a to

Question

Pumping tests on a major water supply well create a cone of depression with a top surface that extends more than 1000 m from the well. The area is underlain with totally saturated sand possessing a 40% porosity and a density of 2.60 g/cm^3. The specific yield of the pumping is estimated at 30% (30% of the fluid occupying the 40% void space). Can the removal of water be detected by a gravity measurement with an accuracy of 0.01 mGal? For modeling purposes assume that the cone of depression can be represented sufficiently by a horizontal cylinder with a radius of 300 m and a center at a depth of 300 m.

Explanation / Answer

if we consider the cone of depression as cylinder the gravity can be calculated by

4/3pi *R3*dp

R=300 m

where dp= density contrast between host and anomalous mass i.e (density of rock- density of water)

=2.60gm-1.0 gm/cm3=1.6g/cm3=0.016g/m3

=4/3*3.14*300*300*300*0.016

=113,040,000*0.016

=1,808,640g

The removal of water can be detected with an accuracy of 0.01 mGal

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