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

The acceleration due to gravity at a height  h  is given asgh=g1-2hRe where Re is radius of earth. The acceleration due to gravity at a depth  d  is given as gd=g1-dRe Given, gh=gd∴g1-2hRe=g1-dRe∴  d=2h=2×1=2 km               (∵h=1 km)