Temperature is an important factor for evaluating the seismic response of deep reservoirs. We have developed an amplitude-variation-with-offset approximation based on the Lord-Shulman thermoelasticity theory. The model predicts two compressional (P and T) waves (the second is a thermal mode) and a shear (S) wave. The T mode is due to the coupling between the elastic and heat equations. In the thermoelastic case, the approximation is more accurate than in the elastic case. Its accuracy is verified by comparison with the exact equations calculated in terms of potential functions. We examine two reservoir models with high temperatures and compute synthetic seismograms that illustrate the reliability of the approximation. Moreover, we consider real data to build a model and find that the approximate equation not only simplifies the calculations but also is accurate enough and can be used to evaluate the temperature-dependent elastic properties, providing a basis for further application of the thermoelasticity theory, such as geothermal exploration, thermal-enhanced oil recovery, and ultradeep oil and gas resources subject to high temperatures.