What is the primary geophysical method used to delineate the caprock integrity above a geothermal reservoir?

The primary geophysical method used to delineate caprock integrity above a geothermal reservoir is seismic reflection surveying. Seismic reflection involves generating seismic waves at the surface using a source such as a vibrator truck or explosives, and then recording the reflected waves using geophones, which are sensitive instruments that detect ground motion. These reflected waves provide information about subsurface geological structures and properties. Caprock, which is an impermeable layer of rock overlying a geothermal reservoir, prevents geothermal fluids from escaping to the surface. Its integrity is crucial for maintaining reservoir pressure and preventing geothermal resource loss. Seismic reflection surveys are used to image the subsurface and identify faults, fractures, or thinning in the caprock layer. A continuous, strong seismic reflection from the top of the caprock indicates its intact nature. Conversely, disrupted or absent reflections may suggest fracturing or faulting, which can compromise its sealing ability. By analyzing the amplitude, frequency, and travel time of the reflected seismic waves, geophysicists can infer the physical properties of the caprock, such as its density and seismic velocity. These properties are then used to assess its structural integrity and sealing capacity. For example, areas with low seismic velocity within the caprock may indicate zones of increased porosity or fracturing, suggesting potential leakage pathways. Post-stack seismic attributes, such as coherence and curvature, are often used to enhance the visibility of faults and fractures within the seismic data, providing a more detailed assessment of caprock integrity. Furthermore, seismic data can be integrated with well log data and geological information to create a comprehensive understanding of the subsurface and to refine the interpretation of the seismic results.