Mapping and distribution with depth of alteration in rocks is critical in engineering planning because it has a fundamental impact on the geotechnical properties of the materials. Lateral heterogeneity on a weathered rock massif makes boreholes inadequate for its complete characterization. Geophysical methods increase spatial sampling along the study area and can be related to geotechnical parameters, so subsoil conditions can be better understood.In order to determine its geotechnical qualities and variability along two different profiles, we attempt to characterize a granite massif in north‐west Spain by the integration of results from seismic refraction, multichannel analysis of surface waves (MASW) and electrical resistivity tomography methods (ERT). The study area, the so‐called Carlés granite, shows all the weathering grades from sandy soil to fresh rock. A reference borehole where samples were taken and laboratory measurements were made, serves as a direct check for the results of one of the profiles, the other being interpreted without any direct information. This approach has permitted the evaluation of the advantages and limitations of each geophysical method and created an accurate geotechnical model of the massif, correlating physical and geotechnical parameters such as rock quality designation, weathering grade, or standard penetration test.The field seismic velocities have been compared with the ultrasonic measurements at the laboratory, permitting an evaluation of the field and laboratory elastic constants. The trend in the values of these parameters agrees with the field and laboratory test for the shallow parts of the massif. However, unrealistic elastic constants have been obtained for fresh rock based on the results of the field experiments. This is related to an apparent underestimation of the velocity of seismic S‐waves for the deepest layers. This fact suggests that the methodology followed throughout this work is able to provide a full geotechnical model of an altered rock massif for the first tens of metres, discriminating between different weathered levels. It is also useful and reliable when inferring elastic constants for depths of up to 20 m. However, its validity becomes doubtful with depth, so care must be taken when calculating elastic moduli and trying to extrapolate directly to a rock massif.

Intraplate seismicity in NW Spain, an otherwise stable continental area, is dominated by low magnitude events and occurs both in swarms or dispersed along faults. A detailed study of one of the most active fault segments, the Ventaniella Fault, has produced an accurate image of foci distribution and revealed new insights on the origin of this lingering activity. The improved location of earthquakes by a temporary seismic network has allowed to better constrain the geometry of the seismogenic segments of the fault at depth. Between 2015 and 2017, a portable seismic array of 10 seismographs recorded 45 low magnitude earthquakes (<2) at depths between 9-18 km. These hypocenters define a tubular trend plunging to the northwest. The linear seismicity pattern is interpreted as the result of the intersection at depth of two main fault planes: a NW-SE fault reactivated in the Alpine Orogeny and the frontal thrust of the Cantabrian Mountains running E-W. The clustering of earthquakes along this particular line of intersecting faults coincides spatially with the presence at depth of an important lateral gradient in crustal thickness, related to the termination of the crustal root beneath the Cantabrian Mountains.The mechanical constraints in the continental crust imposed by the arrangement of crustal scale faults and the gradient in crustal thickness may have reactivated seismically old faults in a context of a stable continental area.

SUMMARY This study presents the first detailed analysis of ambient noise tomography in an area of the continental upper crust in the Cantabrian Mountains (NW Spain), where a confluence of crustal scale faults occurs at depth. Ambient noise data from two different seismic networks have been analysed. In one side, a 10-short-period station network was set recording continuously for 19 months. A second set of data from 13 broad-band stations was used to extend at depth the models. The phase cross-correlation processing technique was used to compute in total more than 34 000 cross-correlations from 123 station pairs. The empirical Green's functions were obtained by applying the time–frequency, phase-weighted stacking methodology and provided the emergence of Rayleigh waves. After measuring group velocities, Rayleigh-wave group velocity tomographic maps were computed at different periods and then they were inverted in order to calculate S-wave velocities as a function of depth, reaching the first 12 km of the crust. The results show that shallow velocity patterns are dominated by geological features that can be observed at the surface, particularly bedding and/or lithology and fracturing associated with faults. In contrast, velocity patterns below 4 km depth seem to be segmented by large structures, which show a velocity reduction along fault zones. The best example is the visualization in the tomography of the frontal thrust of the Cantabrian Mountains at depth, which places higher velocity Palaeozoic rocks over Cenozoic sediments of the foreland Duero basin. One of the major findings in the tomographic images is the reduction of seismic velocities above the area in the crust where one seismicity cluster is nucleated within the otherwise quiet seismic area of the range. The noise tomography reveals itself as a valuable technique to identify shear zones associated with crustal scale fractures and hence, lower strain areas favourable to seismicity.

Nowadays, most planned large infrastructure projects increasingly use multi‐technique geophysical methods integrated with geotechnical surveys to assess detection and exploration of karst‐related systems. This article focuses on the case of a large sinkhole found during the geophysical surveys that were carried out along Riyadh’s new Metro Line 3 (Saudi Arabia). This line is the longest (41 km) of the six that are currently under construction in the framework of the largest public rail infrastructure project, the Riyadh Metro Project (176 km). A multi‐technique geophysical survey combining seismic (1040 m of seismic refraction tomography and four downhole tests), electrical (1035 m of electrical resistivity tomography), and electromagnetic methods (1040 m of ground penetration radar) was conducted to shed light on the subsurface geology along the section of this case study. The combination of the geophysical methods led to early identification of a subsurface area of lower resistivity and seismic velocity than the background values of the carbonate bedrock. It also revealed smaller fractures that could lead to future sinkhole formation. A subsequent dense investigatory borehole grid (14 boreholes, five probeholes, 38 standard penetration tests, nine TV‐logging, and five pressuremeter tests) further confirmed the presence of a buried sinkhole. This paper shows the results of each individual geophysical method as well as the final geotechnical interpretation based on the combination of geophysical methods with borehole drilling. It concludes that the use of a single method for karst assessment, whether a geophysical method or borehole drilling, does not allow a sufficiently detailed geotechnical profile of the ground. This case study provides basic guidance on the most suitable and accurate techniques to detect similar karstic features across Riyadh.

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