I would examine the ht-index from different perspectives such as degrees of street connectivity, building heights, park size etc.. The higher the ht-index, the more biophilic 

http://arxiv.org/abs/1305.0883

Some key qualities in the link and a scale in the PDF file.

Hope that helps

http://biophiliccities.org/what-are-biophilic-cities/

Hello both Andries and Bin.  I am also interested to know to which extent the index could define grades (or percentages of Biophilia, so that we could easily propose urban interventions needed in some areas or have the flexibility to alter some measures to obtain better outcomes in urban design. I suppose that Bin has published his work somewhere.  Is it possible to read it or find it?  I shall be grateful, if we keep in touch.

Herewith the source: Jiang B. and Yin J. (2014), Ht-index for quantifying the fractal or scaling structure of geographic features, Annals of the Association of American Geographers, 104(3), 530–541.

You could also refer to this paper, mainly on visualization:

Jiang B. (2015), Head/tail breaks for visualization of city structure and dynamics, Cities, 43, 69-77, Preprint: http://arxiv.org/abs/1501.03046

Many thanks Bin.  I shall read with great attention.

Thanks for your interest!

This small tool might be of your use: https://github.com/digmaa/HeadTailBreaks

Hi Andries,

I found some interesting papers that may be helpful too. However, it is hard to find an existing translation of the concept of biophilia into concrete indicators indeed. These papers may help you develop some:

http://eprints.qut.edu.au/69039/

http://www.mdpi.com/2071-1050/5/8/3328/htm

These two dissertations especially look into attitudes of people towards nature, not urban planning and design, and policy, but may be helpful anyway:

http://digitalscholarship.unlv.edu/thesesdissertations/1855/

http://gradworks.umi.com/15/57/1557883.html

Well, I'm late to this thread, but for what its worth I am working on this exact topic. I have been using GIS to map urban biophlic elements in Wellington. I expect more papers to be published about this work soon, but for now here is a conference paper. Conference Paper What makes a city 'biophilic'? Observations and experiences ...