We have investigated seismic waveform tomography to haracterize fractures in petroleum reservoirs. Seismic reflection data are used in a frequency-domain inversion to reconstruct subsurface attenuation images. The images show fracture distributions, from which fracture density is estimated. Fractures smaller than or equal to a half-wavelength of seismic act as single scatterers, producing images of strong attenuation ellipses and from which fracture density can be estimated. When fracture size approaches one wavelength, fracture orientation affects the attenuation image. Horizontal fractures act as individual reflectors and produce strong tomographic attenuation images from which fracture density can be estimated. The strength of the attenuation image decreases when the fracture angle relative to horizontal increases; vertical fractures produce the weakest attenuation image. Consequently, the accuracy of fracture density measurements decreases with increased fracture angle unless waveform tomography includes different seismic modes acquired from several directions.