Theoretical geophysics was one of the earliest disciplines patronized at CSIR-NGRI. Such an emphasis is quite unusual for many contemporary Earth Science research institutes. Here the aim is to use the laws of continuum physics by converting the ideas into mathematical equations and apply their solutions to understand geophysical processes. All geophysical fields are governed by second order partial differential equations such as potential diffusion and wave equations. Potential and wave equations are useful to understand the structure of the earth while diffusion equation help in facing its evolution. To understand geometry and constitution of the complex objects analog simulation and numerical modeling are adopted. Besides the above equation centric studies, processing of large international datasets is routinely undertaken to understand periodicity of important earth processes, where information theory played a deterministic role.