From Wikipedia: https://en.wikipedia.org/wiki/Relief#Bas-relief_or_low_relief
A low relief or bas-relief is a projecting image with a shallow overall depth, for example used on coins, on which all images are in low relief. In the lowest reliefs the relative depth of the elements shown is completely distorted, and if seen from the side the image makes no sense, but from the front the small variations in depth register as a three-dimensional image.
Example image from the same page:
This can obviously be done with sculpting, but I feel like there should be an easier/faster way to make such an effect (for example a coin, medallion, religious icon, etc.) from an existing 3d scene rather than recreating from scratch by sculpting. I'm not sure how, but I think rendering the scene in a specific way via the Compositing Nodes and then applying that rendered image as Displacement modifier on a grid mesh might give such an effect.
Here are some models from Blendswap someone trying to find a solution can use for testing: http://www.blendswap.com/blends/search?keywords=statue&is_fan_art=1&blend_license=&render_engine=&sort=downloads&direction=desc