1. A prestack separating method for a seismic wave to determine true wave vector directions comprising:receiving, by a three-component receiver, at a surface above a multi-layer underground medium, at a certain time window, a P-wave, a S1-wave and a S2-wave of the seismic wave, wherein the P-wave was reflected from a first interface of the multi-layer underground medium, the S1-wave and the S2-wave were reflected from a second interface of the multi-layer underground medium, wherein the first interface is deeper than the second interface relative to the surface

projecting, by the three-component receiver, the P-wave, the S1-wave and the S2-wave into a Z-R-T coordinate system, such that all the projections appear on the three-component receiver at the surface, wherein the Z-R-T coordinate system comprises a Z coordinate, a R coordinate, and a T coordinate, wherein projecting the P-wave, the S1-wave, and the S2-wave into the Z-R-T coordinate system generates a projection matrix, wherein the Z coordinate is a vertical component, the R coordinate is a component of a source-to-receiver azimuth and the T coordinate is a component orthogonal to a direction of the R coordinate, and wherein the projection matrix satisfies equation:

wherein P is a vector of the P-wave, S1 is a vector of the S1-wave, and S2 is a vector of the S2-wave, eZ is a base vector of a direction of the Z coordinate, eR is a base vector of the direction of the R coordinate, and eT is a base vector of a direction of the T coordinate, PZ is an amplitude of the P-wave on the Z coordinate, S1Z is an amplitude of the S1-wave on the Z coordinate, and S2Z is an amplitude of the S2-wave on the Z coordinate, PR is an amplitude of the P-wave on the R coordinate, S1R is an amplitude of the S1-wave on the R coordinate, S2R is an amplitude of the S2-wave on the R coordinate, PT is an amplitude of the P-wave on the T coordinate, S1T is an amplitude of the S1-wave on the T coordinate, and S2T is an amplitude of the S2-wave on the T coordinate;

forming, by the three-component receiver, a composite vector from the P vector, the S1 vector, and the S2 vector, wherein the composite vector satisfies equation:

A(composite)=Z(composite)+R(composite)+T(composite),wherein A(composite) is the composite vector, Z(composite) is a vector of the composite vector in the direction of the Z coordinate, R(composite) is a vector of the composite vector in the direction of the R coordinate, and T(composite) is a vector of the composite vector in the direction of the T coordinate wherein Z(composite), R(composite) and T(composite) satisfy equation:

transforming, by the three-component receiver, the composite vector to an anisotropic wave vector matrix, comprising an anisotropic wave, wherein the anisotropic wave vector matrix satisfies equation:

wherein eP is a base vector of the P-wave, eS1 vector is a base vector of the S1-wave, and eS2 vector is a base vector of the S2-wave,

wherein cos(eP,eZ) is a cosine of an angle between the eP vector and the eZ vector, cos(eP,eR) is a cosine of an angle between the eP vector and the eR vector, cos(eP,eT) is a cosine of an angle between the eP vector and the eT vector, cos(eS1,eZ) is a cosine of an angle between the eS1 vector and the eZ vector, cos(eS1,eR) is a cosine of an angle between the eS1 vector and the eR vector, cos(eS1,eT) is a cosine of an angle between the eS1 vector and the eT vector, cos(eS2,eZ) is a cosine of an angle between the eS2 vector and the eZ vector, cos(eS2,eR) is a cosine of an angle between the eS2 vector and the eR vector, and cos(eS2,eT) is a cosine of an angle between the eS2 vector and the eT vector; and

performing, by the three-component receiver, a rotation transformation of an affine coordinate system on the anisotropic wave vector matrix to generate a wave separation matrix, wherein the wave separation matrix satisfies equation:

wherein P(pure) is a vector of the P-wave of the seismic wave, S1(pure) is a vector of the S1-wave of the seismic wave, and S2(pure) is a vector of the S2-wave of the seismic wave, wherein directions of the P(pure) vector, the S1(pure) vector, and the S2(pure) vector are the true wave vector directions,

wherein the true wave vector directions of the S1-wave of the seismic wave and the S2-wave of the seismic wave are separated from the true wave direction of the P-wave of the seismic wave, and wherein the true wave vector directions of the S1-wave of the seismic wave and the S2-wave of the seismic wave are utilized to predict fracture parameters of the multi-layer underground medium.