A method of determining a vertically transverse isotropy, VTI, anisotropy along a horizontal section of a wellbore drilled into a formation, the method comprising: placing a sonic logging tool downhole in the horizontal section of the wellbore drilled into the formation; receiving seismic waves at the sonic logging tool placed downhole in the horizontal section; measuring compressional slowness (DTCO) of the seismic waves using the sonic logging tool placed downhole in the horizontal section and determining an elastic constant C11 of the horizontal section using the compressional slowness (DTCO) and density log data measured in the horizontal section; measuring vertically polarized shear slowness (DTSV) of the seismic waves using the sonic logging tool placed downhole in the horizontal section and determining an elastic constant C44 of the horizontal section using the vertically polarized shear slowness (DTSV) and the density log data measured in the horizontal section; measuring horizontally polarized shear slowness (DTSH) of the seismic waves using the sonic logging tool placed downhole in the horizontal section and determining an elastic constant C66 of the horizontal section using the horizontally polarized shear slowness (DTSH) and the density log data measured in the horizontal section; determining a vertical compressional slowness of the horizontal section corresponding to an elastic constant C33 of the horizontal section using a model with a condition, wherein the model is built using second sonic log data and second density log data of a vertical wellbore drilled into the formation, and wherein the second sonic log data of the vertical wellbore comprises shear slowness measured in the vertical wellbore and compressional slowness (DTCO) measured in the vertical wellbore, and wherein the condition is that the shear slowness of the vertical wellbore is equal to the vertically polarized shear slowness (DTSV) of the horizontal section; and determining a VTI anisotropy along the horizontal section using the elastic constants C11, C44, C66, and C33 of the horizontal section. placing a sonic logging tool downhole in the horizontal section of the wellbore drilled into the formation; receiving seismic waves at the sonic logging tool placed downhole in the horizontal section; measuring compressional slowness (DTCO) of the seismic waves using the sonic logging tool placed downhole in the horizontal section and determining an elastic constant C11 of the horizontal section using the compressional slowness (DTCO) and density log data measured in the horizontal section; measuring vertically polarized shear slowness (DTSV) of the seismic waves using the sonic logging tool placed downhole in the horizontal section and determining an elastic constant C44 of the horizontal section using the vertically polarized shear slowness (DTSV) and the density log data measured in the horizontal section; measuring horizontally polarized shear slowness (DTSH) of the seismic waves using the sonic logging tool placed downhole in the horizontal section and determining an elastic constant C66 of the horizontal section using the horizontally polarized shear slowness (DTSH) and the density log data measured in the horizontal section; determining a vertical compressional slowness of the horizontal section corresponding to an elastic constant C33 of the horizontal section using a model with a condition, wherein the model is built using second sonic log data and second density log data of a vertical wellbore drilled into the formation, and wherein the second sonic log data of the vertical wellbore comprises shear slowness measured in the vertical wellbore and compressional slowness (DTCO) measured in the vertical wellbore, and wherein the condition is that the shear slowness of the vertical wellbore is equal to the vertically polarized shear slowness (DTSV) of the horizontal section; and determining a VTI anisotropy along the horizontal section using the elastic constants C11, C44, C66, and C33 of the horizontal section.