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 as

$g_h=g\left(1-\frac{2h}{R_e}\right)$

$\text{ where }{R}_e\text{ is radius of earth. }$

The acceleration due to gravity at a depth  d  is given as 

$g_d=g\left(1-\frac{d}{R_e}\right)$

$\text{ Given, }{g}_h=g_d$

$\therefore g\left(1-\frac{2h}{R_e}\right)=g\left(1-\frac{d}{R_e}\right)$

$\therefore d=2h=2\times 1=2 \mathrm{km}$               $(\because h=1 \mathrm{km})$