See:
- Heat in Place calculation: Muffler, P., and Raffaele Cataldi. "Methods for regional assessment of geothermal resources." Geothermics 7.2-4 (1978): 53-89.
- Garg, S.K. and J. Combs. 2011. A Reexamination of the USGS Volumetric "Heat in Place" Method. Stanford University, 36th Workshop on Geothermal Reservoir Engineering; SGP-TR-191, 5 pp.
HIP-RA-X is the successor version to HIP-RA.