A form of Kirchhoff's wave equation is presented which is useful to the geophysicist doing an amplitude interpretation of seismic reflection data. A simple rearrangement of Kirchhoff's retarded potential equation allows the reflection process to be evaluated as a convolution of the derivative of the source wavelet with a term called the 'wavefront sweep velocity'. The wavefront sweep velocity is a measure of the rate at which the incident wavefront covers the reflecting boundary.By comparing wavefront sweep velocities for geologic models with different curvature, one obtains an intuitive feeling for the relation of diffraction and reflection amplitudes to boundary curvature. Also, from this convolutional form of the wave equation, the geometrical optics solution for reflection amplitude is easily obtained. But more important, from the wavefront sweep velocity approach, a graphical method evolves which allows the geophysicist to use compass and ruler to estimate the effects of curvature and diffraction on seismic amplitude.