How to remove artifacts from pdf images generated using "surf"

Hello,

I need to generate a high quality pdf 2D image to import in a powerpoint presentation. The image needs to be displayed with a log scale on its y-axis, so I cannot use image or imagesc - instead I'm using surf with view(0,90). The issue is that surf generates artifacts when converted to pdf (light grey lines and grid on top of the image); imagesc doesn't but cannot accommodate an axis in log scale.

I know the pdf artifacts can be removed in Acrobat by selecting the appropriate options, however they're always back when importing the same pdf file in powerpoint.

I'd like to be able to either: i) go back to imagesc instead of surf (but I don't know how to do that since I need the log-scale on the y-axis); or, ii) have powerpoint not display the artifacts generated by surf.

I'm using the "export_fig" function from the Matlab Central File Exchange (and R2012b).

Any idea how to fix this, or find a workaround?...

Thanks in advance for your help,

Pierre