8160eae2____GeoServer_2_7_x_User_Manual__Description
[Parameter] AbsolutePath [Mandatory] Y [Description] 
It controls whether or not the path stored inside the “location” attribute represents an absolute path or a path relative to the location of the shapefile index. Notice that a relative index ensure much more portability of the mosaic itself. Default value for this parameter is False, which means relative paths.
Using the ImageMosaic plugin — GeoServer 2.7.x User Manual