The Hat Creek coal deposit: A geophysical case history

Abstract

The Hat Creek Valley encloses one of the world's thickest known coal-deposits. The coal response to magnetics, electromagnetics, induced polarization, various downhole logging techniques and gravity surveys are examined. The magnetic method was used for field mapping of geological units and was particularly useful in detecting volcanic rocks and burned zones. Electromagnetic methods detected conductive fault zones with varying degrees of success, while an induced polarization survey gave poor results because of the scarcity and habit of polarizable minerals in the deposit. Downhole logging techniques were used to correlate geologic units between drillholes. The gravity method directly detected the low density coal and yielded one of the world's largest recorded gravity anomalies over a body of economic importance. Analysis of the gravity data indicates a faulted coal deposit of variable width and depth, extending continuously for 19 km. Coal resources are estimated at 1.5–2.2 · 10 10 tonne (17–24 billion ton).