Here is my results using the filter>edge detect>sobel, duplicate two new layers for your image and on each new layer run the sobel filter but on one layer only check the horizontal box and on the other layer check the vertical box, set the top layer to grain merge then merge the two layers together. Destaurate the layer then duplicate and change mode to grain merge, keep duplicate the grain merge layer until a suitable depth is achieved, merge all layers except original colour layer, Finally move original colour layer to top of stack and also set layer mode to grain merge
![Image](http://www.gimpchat.com/files/2144_Relief%2001.png)