This invention relates to radioactivity well logging and more particularly to a method and system for locating a coal-bearing zone in the earth and identifying its quality based primarily upon BTU and ash contents. 
The coal mining industry has primarily used coring and core assaying in locating coal-bearing zones, in defining the thickness of such zones, and in determining coal quality from BTU and ash contents.  Coal consists primarily of carbon, hydrogen, ash, and sulfur.  The density or specific gravity of coal is strongly related to its ash content.  Ash represents that portion of coal that is left over after combustion and consists primarily of oxides of silicon, aluminum, and iron. 
The carbon component and the BTU per pound of the coal are found to be linearly dependent.  In the HANDBOOK OF CHEMISTRY AND PHYSICS, 35th Edition, by Chemical Rubber Publishing Co., Cleveland, Ohio, on pages 1758 and 1759, there is listed in table format the BTU per pound content versus percent carbon in coal samples taken from numerous locations.  A least square straight line fit to this data indicates that the following relationship exists for a correlation coefficient of 0.986; EQU BTU/lb .times.  10.sup.-3 = 17.815(wt.  fraction of carbon) + 0.0046(1)
Consequently, the BTU content of a coal-bearing zone can be directly determined from a measure of the carbon content of the coal-bearing zone.