182 Geophysics in Geothermal Exploration From a geophysical perspective, the characterization and surveillance of geothermal targets usually focuses on the electrical and electromagnetic properties of the subsurface (e.g. Muñoz, 2014). However, as in any geophysical study, multi-physics approaches facilitate interpretation. Thus, both objectives, exploration and surveillance, can benefit from geophysical seismic methods which help to better understand the geological configuration of the subsurface, locating fractured zones and interfaces, identify hydrothermal fluids presence and circulation pathways. Seismic methods are usually a good complementary to electrical and electromagnetic information to understand the distribution and evolution of petro-physical parameters that are key indicators in geothermal contexts. However, the present state of the geothermal energy market prevents investing as much financial efforts in geophysical exploration strategies as for the hydrocarbons market for instance, hence typically discarding the use of technologies such as 3D active seismic (for exploration) or 4D active seismic (for surveillance). As a consequence, cost-effective strategies must be deployed to accompany this movement. Passive seismic methods are part of the solution. By opposition to active seismic methods, passive approaches do not require the costly deployment of logistics associated with the use of an active seismic source. Instead, they are based on the analysis of the ambient seismic signal, which can be cost-effectively recorded using seismic sensors in passive mode, and which, if properly processed and interpreted, can provide useful information about the spatial distribution and temporal evolution of the subsurface seismic properties. Passive seismic methods emergence – The progression of seismic acquisition technologies In the 1880s, modern earthquake detection began with the invention of the seismograph, an instrument capable of capturing ground motion produced by seismic waves. British scientist John Milne, often regarded as the founder of modern seismology, created a horizontal-pendulum seismograph sensitive enough to record distant earthquakes. The technical principle was straightforward: as the ground moved, the seismograph’s heavy mass remained stationary due to its inertia, allowing the relative motion of the ground and the mass to be traced. These early seismographs gave scientists a new way to measure the strength and duration of seismic waves, leading to the classification of different wave types and laying the foundation for the Richter scale in the 1930s, a scale that quantifies earthquake magnitude based on wave amplitude. Initially, earthquake recordings were analog, relying on ink pens to trace waveforms onto paper rolls. This method had limitations: recordings had to be reset after each event, and the analog traces could be difficult to interpret, especially for large, overlapping seismic events. In the 1960s, seismology began a significant shift as stations worldwide established standardized networks like the World-Wide Standardized Seismograph Network (WWSSN), which allowed comparison and cross-validation
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