Gassmann refers to a concept in geophysics and reservoir engineering, specifically the Gassmann equation. This equation describes how the elastic properties of a porous rock change when it is saturated with different fluids, such as oil or water. It helps in understanding how seismic waves travel through these rocks, which is crucial for oil and gas exploration. The Gassmann equation is named after D. Gassmann, who introduced it in 1951. It provides a mathematical framework for predicting the behavior of rocks under varying conditions, aiding geoscientists in assessing subsurface resources and improving the accuracy of seismic surveys.