The acceleration due to gravity at a height 1 km above the earth is the same as at a depth d below the surface of earth. Then : 

d = 2 km

Above the surface :

g' = g (1 - \(\frac{2h}{R_e}\))

\(\Delta g\) = g \(\frac{2h}{R_e}\)

Below earth surface :

g' = g (1 - \(\frac{d}{R_e}\))

\(\Delta g\) = g \(\frac{d}{R_e}\)

⇒ d = 2h = 2 x 1 km = 2 km