Borehole seismic images and magnetic measurements in boreholes

Purpose and goal
The purpose of this pilot study was to test a prototype borehole seismic system with sources and receivers downhole, along with vector magnetic measurements in the same borehole. These tests were to be performed in a maximum 50 m long and 21.8 cm wide sub-horisontal borehole within 200 m of an ore deposit. A successful test of the benefits of joint evaluation of the three-component (3C) borehole magnetic and directional seismic would justify miniaturising the system to fit into 76 mm boreholes, a size commonly used in mineral exploration.

Structure and implementation
A 45 m long and 218 mm diameter sub-horisontal borehole was drilled in the Sofia gallery in the Kiruna mine for testing purposes. Teams from the GFZ in Potsdam and Technical University in Braunschweig, Germany, successfully performed the seismic and magnetic measurements, respectively. The potential to develop a system suitable for measurements in near-vertical fluid-filled 76 mm boreholes is high, but several development steps are necessary.

Results
The seismic data show clear reflections along the borehole and also ones that intersect the Sofia gallery. These correlate well with magnetic anomalies. Reflections from distances of up to 70 m are also observed in the data. More analyses are required before the nature of the discontinuities generating them can be determined. The long wavelength total magnetic field anomaly in the borehole and tunnel is well modeled using an elliptical body model. Development of systems that can enter short 76 mm sub-horisontal boreholes is considered feasible as a next step.