USBM wettability index
The U.S. Bureau of Mines (USBM), developed by Donaldson et al. in 1969,  is a method to measure wettability of petroleum reservoir rocks. In this method, the areas under the forced displacement Capillary pressure curves of oil and water drive processes are denoted as  and  to calculate the USBM index. USBM index is positive for water-wet rocks, and negative for oil-wet systems. Bounded USBM (or USBM*) 
The USBM index is theoretically unbounded and can vary from negative infinity to positive infinity. Since other wettability indices such as Amott-Harvey, Lak wettability index and modified Lak are bounded in the range of -1 to 1, Abouzar Mirzaei-Paiaman highlighted the bounded form of USBM (called USBM*) as a replacement of the traditional USBM as 

USBM* varies from -1 to 1 for strongly oil-wet and strongly water-wet rocks, respectively. See also
 Wetting
 Amott test
 Lak wettability index

References 

Petroleum geology
Surface science
Fluid mechanics