38 Geophysics in Geothermal Exploration allowing us to drill deep into the Earth and access high-temperature geothermal reservoirs. Today, geothermal power plants can produce electricity by tapping into hot water, two-phase or steam reservoirs, while enhanced geothermal systems (EGS) create artificial reservoirs by injecting water into low permeability rocks to generate hot geothermal fluids. Additionally, ground-source heat pumps make it possible to use stable temperatures just below the Earth’s surface for efficient heating and cooling in residential and commercial buildings. These advancements allow us to make use of geothermal energy far beyond natural manifestations, making it a sustainable and reliable source of heat and power. However, it’s essential to recognize that geothermal energy is not a one-size-fits-all resource. We can classify geothermal systems based on the intended usage, the fluid or geological context involved, and even the energy production design. Classifying by Usage • Direct Use of Hot Water: This is one of the oldest and most straightforward uses of geothermal energy, in which naturally heated water (30–80 °C) from geothermal springs or wells is used for heating buildings, agricultural greenhouses, aquaculture ponds, and industrial processes. • Electricity Generation: Higher temperatures, typically above 150 °C, are required to produce electricity. In these systems, steam from geothermal reservoirs drives turbines connected to generators. These are commonly used in areas with high geothermal activity, like volcanic regions. • Geothermal Heat Pumps (GHPs): GHPs leverage stable ground temperatures (10–16 °C) found a few meters below the surface to provide efficient heating and cooling for buildings. This technology is widely applicable and doesn’t require high temperatures. Classifying by Geological Settings • Shallow Geothermal Systems: This involves tapping into the moderate temperatures found at shallow depths, typically up to a few hundred meters, to power geothermal heat pumps. • Sedimentary Basin Systems: In regions with porous/fractured/karstified sedimentary layers, geothermal reservoirs of hot water can be found at moderate depths, often used for direct heating or low-temperature electricity production. • Volcanic Systems: High-temperature geothermal reservoirs in volcanic regions are ideal for electricity generation. Countries like Indonesia and New Zealand are renowned for tapping volcanic geothermal resources for power. • Rift and Fault Zones: In areas where tectonic plates pull apart or fracture, crust is thinner and heat flow is higher than usual promoting geothermal reservoir development in conjunction with volcanic activity. • Fractured Granite and Crystalline Rock: Some geothermal resources are found in fractured hard rock, where engineered geothermal systems (EGS) create or enhance pathways for water to circulate and absorb heat.
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