An apparatus and a method for estimating interval anellipticity parameter by inversing effective anellipticity parameter in the depth domain using a least-squares method. One embodiment of interval anellipticity parameter estimator includes: 1) an interface configured to receive seismic data and borehole information; 2) a depth convertor configured to obtain a function of depth of effective anisotropy parameter based on said borehole information; 3) an inverse transformer configured to set up said function of depth of effective anisotropy parameter as a least-squares fitting problem based on said P-wave data; and 4) an iterative solver configured to use iterative methods to solve said least-squares fitting problem and to obtain an anisotropy model containing interval anellipticity parameter.