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To meet the United States Government nuclear explosion monitoring requirements with high confidence, the Air Force Technical Applications Center needs new and improved capabilities for analyzing regional seismic teleseismic, and infrasound event data. Recently, the National Nuclear Security Administration has decided to move toward 3D modeling to improve knowledge of the compressional and shear velocity structure and enable us to reduce uncertainty and more accurately detect, locate, and identify small (body wave magnitude mb4) seismic events. For seismically active areas, inaccurate models can be corrected using the kriging methodology and therefore, it is possible to detect, locate, and identify large events even with limited resolution models. This is not necessarily the case for smaller events, however, and it is even more of a challenge for aseismic regions. Furthermore, interest on near-regional to local monitoring demands that we address the Earth's heterogeneities and 3D complexities. Motivated by the shortcomings of existing single-parameter inversion methods in accurate prediction of other geophysical parameters, this research was mainly focused on the development and refinement of advanced multivariate inversion techniques to generate a realistic, comprehensive, and high-resolution 3D model of the seismic structure of the crust and upper mantle that satisfies multiple independent geophysical datasets. We present 3D seismic velocity models of the crust and upper mantle beneath three different regions (northwest China; the East Africa Rift System; and Utah) resulting from the simultaneous and joint use of seismic body wave arrival times surface wave dispersion measurements, and gravity data.