Page 8 - Geochemical Remote Sensing of The Sub-Surface

VII

PREFACE

Once, all of the Earth resources needed to meet the needs of society were either
clearly evident at the surface (river-bed placers) or had characteristic visible surface
manifestations (gossans, oil seeps). Growth in demand for metals and fossil fuels
prompted prospecting and exploration on a scale that has ensured that the endowment of
such (near-)surface deposits has been discovered and evaluated in all but the most
inaccessible places on Earth. The principle of using subtler non-visible clues in
prospecting and exploration was taking shape in the early decades of the 20 th century. Its
practical value was demonstrated with the introduction of new instrumental techniques
(especially in chemical analysis) that were able to furnish the appropriate data. This
marks the origins of what we now call mineral exploration geochemistry. It had gained
widespread acceptance by mid-century and went on to account for countless new
discoveries in a period of unprecedented growth in metal demand. Once again, however,
we face the problem of exploration-technique exhaustion. Most deposits amenable to
discovery by drainage geochemistry and soil geochemistry may well have been
discovered. Innovations in analytical chemistry and geographic information systems
improve data quality and data interpretability, but these represent refinements of an
established technique rather than a new technique.
As early as the opening decades of the 20 th century the petroleum industry was
searching for subsurface resources that had no conventional expression at surface. The
minerals industry found itself in a similar position in the closing decades. So far, for
prospecting, both industries have relied mainly on well-constructed geological models
and remote sensing of the subsurface of target areas by geophysical techniques, most
obviously seismic surveys in petroleum exploration, conductivity and gravity surveys in
mineral exploration. Alongside these, however, are thoroughly-researched and field-
tested techniques for detecting, near the surface, geochemical expressions of subsurface
petroleum reservoirs and mineral deposits.
Gases play an important role in this geochemical remote sensing of the subsurface.
Some are indicators of major or trace components of the subsurface resource: light
hydrocarbons leak from petroleum reservoirs; sulphur gases are generated by sulphide
mineral oxidation; and volatile mercury is released by sulphide oxidation. Others with an
indirect link to the resource act as pathfinders: radiodecay of uranium generates radon
and helium; sulphide oxidation consumes oxygen and generates sulphuric acid, which