Fumarole A steam vent and surface thermal feature that is commonly associated with steaming ground and mud pots in hydrothermally and volcanically active areas.

Geochemical Tracer An environmentally benign chemical that is injected to trace fluid movement through the geothermal reservoir and that can be easily detected in produced hot fluid.

Geothermal Gradient The increase in temperature with depth below the surface, usually reported in units of °C/km.