Knowledge of the S-wave velocity () and density () is essential for oil and gas reservoir detection and characterization. However, reliable recovery of both parameters, especially density, from the reflected PP-wave data is a difficult issue because this inverse problem is highly ill-conditioned. The reflected SV-SV wave is easier to process than the PS-wave, and it can provide better estimates of and than the PP-wave because it is more sensitive to these parameters than the PP-wave. I have developed a simultaneous inversion for and based on a modified approximation of the SV-SV wave reflection coefficient. The modified equation includes only two parameters (natural logarithms of and ) to be inverted, and it has high accuracy even at large incident angles and for strong impedance contrasts. I find that simultaneous inversion based on the modified approximation is well-posed when using data of small-to-moderate incident angle (20o–30o) and that the misfit function can be easily regularized. The new simultaneous inversion method is applied to an SV-SV wave prestack data set acquired from a 2D nine-component survey. The field data example demonstrates that the new method can recover stable and high-resolution density and S-wave velocity information, which can be used to investigate rock mineral composition, porosity, and fluid content.