55 2. Surface geophysical methods Subsurface investigations inherently carry uncertainty, especially when working in complex or poorly understood geological environments. A feasibility study reduces this uncertainty by identifying the limitations and strengths of each method in the specific context of the site. This helps reduce the risk of failed surveys or inaccurate interpretations, which can lead to expensive rework or the failure of large-scale projects, such as geothermal energy exploration or construction planning. A feasibility study helps identify the most suitable methods based on the geological conditions of the site. For example, if there is little contrast in the seismic velocities of rock layers but a strong difference in electrical resistivity, electrical methods may be more effective than seismic methods. This preliminary evaluation ensures that only the most appropriate and effective methods are deployed in the main survey. Full-scale geophysical surveys can be costly, especially when multiple methods are involved. A feasibility study allows for testing the effectiveness of the methods on a smaller scale, reducing the risk of investing in techniques that may not produce useful results. Also, geophysicists can tailor their approach to mitigate potential problems, such as improving signal-to-noise ratios or adjusting survey geometry. Before delving into the specifics of each geophysical method, it is essential to review the characteristics of rocks, including their mineral composition, porosity, and other physical properties. These characteristics significantly influence the outcomes of geophysical surveys. For instance, the mineral content of a rock determines its magnetic and electrical properties, while porosity affects its ability to store and transmit fluids, impacting seismic and resistivity measurements. Additionally, the overall rock properties, such as density and elasticity, play a crucial role in how seismic signals propagate. A thorough review of these rock properties is necessary to accurately interpret geophysical data and effectively apply these methods in geothermal exploration and other subsurface investigations. Following a concise review of the physical and pore space properties of rocks, we provide an overview of surface geophysical methods and illustrate their application with selected field examples. Since different geophysical methods provide complementary information, integrating their results often provides the best subsurface model. 2.1 Physical properties of rocks and pore space properties This discussion emphasizes the characterization of natural rocks, which are inherently heterogeneous and composed of diverse minerals with varying physical properties, shapes, sizes, and spatial arrangements. These complexities, along with boundary effects, interactions, and thermodynamic conditions, fundamentally influence the physical properties of rocks.
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