1. The "angular" wavenumber, times the wavenumber. Where a distinction is made, the wavenumber k is the number of waves per unit distance, the reciprocal of wavelength:
where =wavelength, f=frequency, and V=velocity. Thus is to wavenumber in the spatial sense as angular frequency is to frequency f in the time sense. Confusingly, both and k are often called wavenumber, i.e., some authors use instead of the above.
2. Seismic usage often implies apparent wavenumber , being the apparent wavelength and Va the apparent velocity. If this definition is used, ka varies with the angle between the raypath and the line of measure (the line of the spread, usually).