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Carbonatites are rare, carbonate-rich magmatic rocks that make up a minute portion of the crust only, yet they are of great relevance for our understanding of crustal and mantle processes. Although they occur in all continents and from Archaean to present, the deeper plumbing system of carbonatite ring-complexes is usually poorly constrained. Here, we show that carbonatite ring-complexes can be explained by caldera-style volcanism. Our geophysical investigation of the Alnö carbonatite ring-complex in central Sweden identifies a solidified saucer-shaped magma chamber at ~3 km depth that links to surface exposures through a ring fault system. Caldera subsidence during final stages of activity caused carbonatite eruptions north of the main complex, providing the crucial element to connect plutonic and eruptive features of carbonatite magmatism. The way carbonatite magmas are stored, transported and erupt at the surface is thus comparable to known emplacement styles from silicic calderas.

We have developed a multicomponent broadband seismic landstreamer system based on digital sensors and particularly suitable for noisy environments and areas in which high-resolution images of the subsurface are desired. We have evaluated results, interpretations, and approaches using the streamer in the planning of an approximately 3-km-long underground tunnel in the city of Varberg in the southwestern Sweden. Prospective targets were imaging of the shallow (<20 m) bedrock surface and weak zones, such as fracture and shear zones. Over the course of three weeks, 25 profiles were acquired with a total length of approximately 7.5 km using a source and receiver spacing of 2–4 m. A novel approach of the data acquisition was to integrate the landstreamer with wireless sensors in areas in which the accessibility was restricted by roads and also to increase the source-receiver distances (offsets). Although the area was highly noisy, the seismic data, in conjunction with available boreholes, successfully led to delineation of the bedrock surface, its undulations, and areas of poor rock quality. To overcome challenges due to geologic complexities and crooked-line data acquisition, 3D tomographic inversion of first breaks was carried out. Comparisons of the results with the existing boreholes indicated that in most places, the bedrock surface was well resolved by the method, which supported the indication of weak zones in the bedrock, represented by low-velocity structures in the tomographic results. We also evaluated the effect of poor geodetic surveying, particularly regarding elevation data, which adulterated the tomography results toward undulating bedrock surfaces or zones of low velocities.