AVO analysis and cross-plotting have become established techniques for risk reduction in hydrocarbon exploration and production. More recently these concepts have been applied to repeat seismic surveys to assess production related changes in hydrocarbon reservoirs. Here these methods are further extended to provide amplitude variation with offset and azimuth (AVOA) attributes and cross-plotting tools for monitoring changes in reservoir stress and fluid properties using 4D-4C data. New theory for interpretation of AVOA cross-plots and definition of background trends is presented in terms of anisotropy and fracture parameters These equations allow analysis of time-lapse variations in stress or saturation with the possibility of distinguishing changes in matrix properties from those associated with fractures or low aspect-ratio porosity. The insight gained is used to interpret AVOA data from the Teal South 4D-4C experiment.