= Flexural gravity admittance
{title2=$Z(k)=2\pi G\rho_c[1-e^{-kt}/(1+\ell_f^4k^4)]$}
For a surface load above a uniform-density <crust> over a denser substrate, a simple <lithospheric flexure> model gives the indicated <gravity-topography admittance>, with $\ell_f^4=D/[g(\rho_m-\rho_c)]$. The surface mass contributes positive gravity; downward displacement of the <density> interface contributes a compensating negative signal attenuated by $e^{-kt}$. Hence $Z\to0$ at long <wavelength> and $Z\to2\pi G\rho_c$ at short <wavelength>. Here $t$ is the interface depth, not automatically the <effective elastic thickness>.
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