Consider a scalar travel t,ime field, constructed by assigning to each point in space, the time at which a wave front passes. The gradient of this scalar field is called the s[ownesa. It has the units of reciprocal velocity and the direction of most rapid increase in the travel time field. Phase velocity is defined as the reciprocal of slowness. Phase velocity is normal to the wavefront, while ray velocity is not usually so. The ray and phase velocities at any point on a wavefront are related by analytical expressions.