## 8 Seismic RQD  
Traditional RQD calculations are sensitive to the borehole orientation relative to fracture orientation, which can lead to significant overestimations or underestimations of rock quality. Seismic methods overcome this limitation by capturing the effects of fractures and anisotropy over a broader spatial scale, thus providing a more holistic representation of rock mass quality. In highly fractured or weak rock masses, traditional core recovery is often insufficient to obtain reliable RQD values. Seismic methods, by contrast, are unaffected by physical core recovery and can detect discontinuities and weak zones indirectly through their impact on wave velocities. Moreover, traditional RQD measurements are limited to specific borehole locations, offering no information about the continuity of rock mass properties between sampling points. Seismic surveys provide continuous profiles, capturing variations in rock quality over large areas and enabling a more comprehensive understanding of subsurface conditions. Seismic techniques to estimate RQD over larger areas involve analyzing seismic wave velocities, particularly P-wave velocities, which are influenced by the density and elasticity of the rock mass. Fractures and joints within the rock can slow down seismic waves; thus, by measuring these velocities, it is possible to infer rock quality. Seismic methods allow for the evaluation of rock quality without the need for extensive drilling, reducing environmental impact and cost. Also, seismic surveys can cover large areas, offering continuous data that help identify variations in rock quality between boreholes. However, seismic data may not detect very small fractures, potentially leading to an overestimation of rock quality. Moreover, the relationship between seismic velocities and RQD can be influenced by factors such as rock type, saturation, and stress conditions, necessitating careful analysis. Recent studies have explored the use of cooperative inversion techniques, combining seismic reflection data with borehole information to build more accurate 3D RQD models. This approach enhances the reliability of RQD estimations, particularly in complex geological settings [37]. Additionally, advancements in seismic data acquisition and processing have improved the detection of fractures and discontinuities, leading to more precise assessments of rock mass quality [38]. Advanced algorithms are used to interpret seismic data, identifying patterns and relationships that may not be apparent through traditional analysis. These techniques enable more precise correlations between seismic velocities and RQD, improving predictions in complex geological settings. Moreover, seismic RQD can be used in conjunction with DFN models to incorporate volumetric fracture intensity and other parameters, addressing the inherent anisotropy and heterogeneity of rock masses. This hybrid approach enhances the predictive capabilities of seismic methods for engineering applications. Figure 18: RQD and C values’ relationship, for RQD ranging from 10% to 90% [36]. Seismic techniques also reduce the environmental impact and logistical challenges associated with traditional RQD methods. By minimizing the need for extensive drilling, seismic surveys offer a more sustainable approach to rock mass characterization. Additionally, their ability to operate in remote and inaccessible terrains expands the scope of geotechnical investigations, making them indispensable in projects such as large-scale tunneling, mining, and infrastructure development. This article is not intended to present seismic measurements, which have a very wide body of literature.