In recent years, gas hydrates are playing an important role from a scientific and a resource-exploitation point of view. The increase in demand of energy requires unconventional forms of energy, and gas hy-drates are considered as one of these potential future energy sources. Therefore, technology has been developed to measure properties of gas hydrate bearing formations after and during drilling. In this context, numerical simulation of coupled waves in borehole drilling can contribute to improve infor-mation about the gas hydrate reservoir. We model the relationship between bottom simulating reflector and wave propagation along the drillstring. In particular, the bottom simulating reflector is associated with a change of wave propagation velocity due to alternated formation properties. In case of presence of free gas in an overpressured condition, the velocity of the coupled wave in the outer fluid decreases because the formation stiffness decreases. Synthetic data provides an excellent means in studying what happens when a borehole crosses a gas hydrate zone. Moreover, numerical simulation of coupled waves, adopted jointly with the most conventional methods, can contribute to add a small piece of knowledge for better understanding of the hydrate and free gas physical properties.