Broadband seismic data acquisition and processing of iron oxide deposits in Blötberget, Sweden

Abstract

In June 2022, an innovative seismic survey was conducted in Blötberget, central Sweden, to evaluate the effectiveness of employing both a broadband seismic source and broadband receivers for mineral exploration in a challenging hardrock setting. The Blötberget mine hosts high-quality iron oxides, predominantly magnetite and hematite, sometimes enriched with apatite. These deposits comprise 10–50 m thick sheet-like horizons with a moderate eastward dip ($\sim$45°) along an NNE-trending zone. The survey employed a combination of co-located micro-electromechanical sensors, three-component recorders, surface and borehole distributed acoustic sensing, along with a 77-kN broadband seismic vibrator operating with 2–200 Hz linear sweeps. A tailored processing workflow was applied to preserve the broadband nature of the recorded data, and a one-dimensional velocity model was derived from the borehole distributed acoustic sensing data for migration and time-to-depth conversion purposes. Compared to the previous seismic surveys, the resulting seismic cross section reveals several well-defined reflections with improved resolution. Notably, a reflection intersecting the main deposits at a depth of approximately 1200 m exhibits a distinct polarity reversal relative to the reflection from the mineralization, providing further evidence for its interpretation as originating from a fault zone. Shallow reflections align with geological boundaries and partially coincide with weak magnetic anomalies. Additional reflections were revealed underneath the known mineralization on both sides of the fault zone and may suggest the presence of potential additional resources. The delineation of these reflections and the fault zone is critical for future mine planning and development in the region. This case study underscores the potential of broadband data in achieving high-resolution subsurface imaging in hardrock environment and its pivotal role in mineral resource assessment processes.