Abstract. Many metallic mineral deposits have sufficient physical property contrasts, particularly density,
to be detectable using seismic methods. These deposits are sometimes significant for our society and
economic growth and can help to accelerate the energy transition towards decarbonization. However,
their exploration at depth requires high-resolution and sensitive methods. Following a series of
2D seismic trials, a sparse, narrow source–receiver azimuth, 3D seismic survey was conducted in
the Blötberget mine, in central Sweden, covering an area of approximately 6 km2 for
deep-targeting iron oxide deposits and their host rock structures. The survey benefited from a
collaborative work by putting together 1266 seismic recorders and a 32 t vibrator,
generating 1056 shot points in a fixed geometry setup. Shots were fired at every 10 m
where possible, and receivers were placed at every 10–20 m. Notable quality data were acquired
despite the area being dominated by swampy places as well as by built-up roads and historical
tailings. The data processing had to overcome these challenges for the static
corrections and strong surface waves in particular. A tailored for hardrock setting and processing workflow was
developed for handling such a dataset, where the use of mixed 2D and 3D refraction static
corrections was relevant. The resulting seismic volume is rich in terms of reflectivity, with clear
southeast-dipping reflections originating from the iron oxide deposits extending vertically and
laterally at least 300 m beyond what was known from available boreholes. As a result, we
estimate potential additional resources from the 3D reflection seismic experiment on the order of
10 Mt to be worth drilling for detailed assessments. The mineralization is crosscut by at least
two major sets of northwest-dipping reflections interpreted to dominantly be normal faults and to be
responsible for much of the lowland in the Blötberget area. Moreover, these
post-mineralization faults likely control the current 3D geometry of the deposits. Curved and
submerged reflections interpreted from folds or later intrusions are also observed, showing the
geological complexity of the study area. The seismic survey also delineates the near-surface
expression of a historical tailing as a by-product of refraction static corrections, demonstrating
why 3D seismic data are so valuable for both mineral exploration and mine planning applications.
The Lancaster Field: progress in opening the UK's fractured basement play