Introduction to this special section: Passive seismic and microseismic—Part 2

The increase in the United States' production of unconventional gas from about 5% in 1995 to around 60% in 2009 is because of hydraulic fracture stimulation of horizontal wells, an advance in engineering technology equivalent to the introduction of 3D and 4D in reflection seismic (Figure 1). However, the process of inducing failure by raising fluid pressure in the reservoir is hardly a new technological breakthrough. The first well-documented account of this is probably of a technique employed by the Romans in 77 CE to undermine and instantly remove vast quantities of mountainside to extract gold from the buried mother lode at Las Medulas in northwest Spain.