Gravity: Spectral Techniques
            Spectral gravity techniques examine the relationship between gravity anomalies and topography in the wavenumber domain.  The basic idea is to examine how the lithosphere responds to topographic loads of different wavelengths.  For example, building a skyscraper will not cause a depression of the Moho; instead, the load will be flexurally compensated.  How large a “skyscraper” or mountain belt can be accommodated by flexure of the lithosphere with the necessity of Airy or Pratt isostatic compensation is a function of the strength of the lithosphere (i.e. its flexural rigidity, D,  or elastic thickness, Te). 
This elastic thickness can be viewed as a proxy for lithospheric thickness.  It is argued [Artemieva, 2006] that Te represents the depth to the 550 C isotherm in lithosphere older than ~200 Ma, citing a critical decrease in strength of olivine above this temperature.  As a side note, younger, feldspathic lithosphere may have critical temperature of ~350 C.  Thus in a one-dimensional conductive regime, there will be a linear relationship between elastic and overall lithospheric thickness.
In the following discussion it will be useful to bear a few equations in mind.
Wavenumber: k = 2π/λ   where λ is the wavelength
Elastic thickness: Te=12(1-ν2)D/E  where v=Poisson’s ratio; E=Young’s Modulus