The use of ground- and down-hole electromagnetic techniques to localise deep-seated conductors

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Sandstrom, Bertil; Pantze, Robert
Holding Date 08 September 2008

Geophysics has always played an important role in exploration work carried out by Boliden Mineral. In particular, various electromagnetic techniques have been extensively used. Since 1980 a three-component (3D) frequency domain down-hole EM system has been routinely used in exploration for sulphide mineralisation. That system was later developed into a ground EM system. Both systems utilise the same transmitter and operate at four different frequencies. Transmitting an alternating current through a loop source at surface creates an electromagnetic field. The receivers measure the amplitude and phase of the electromagnetic field in three different directions for each frequency. The measured field is the combined effect of the primary surface field and the response from the subsurface (the secondary field).
The down-hole EM receiver consists of a probe with three receiver coils. During the survey the probe is attached to the drill rods. The ground EM receiver consists of a box carried by a field technician on his back.
Older ground EM surveys were designed primarily to localise conductors close to surface or at moderate depths. A typical survey area was 1000 m x 1000 m surrounded by a transmitter loop of 1200 m x 1200 m.
Over the past few years ground EM survey design, data treatment and interpretation routines have undergone strong development. Interpretation of EM surveys gives information about the presence, location, geometry and conductivity of conductive bodies. Large transmitter loops, typically 2500 m x 1500 m, combined with long survey profiles and the use of lower frequencies have made it possible to locate deep seated conductors.
In the Kristineberg area, interpretation of a ground EM survey showed that a good conductor was located 900-1100 m below surface. Borehole KRC2990 from surface was aimed to explain the cause of the anomaly. At 1050 m below surface the conductor was intersected. This intersection contained rich massive sulphide mineralization, which is now named L-zone.
In the Garpenberg mine a down-hole EM survey was performed in bore hole GAE728 during an exploration campaign in 1997. Close to the bottom of the hole an off-hole conductor was indicated at the 1400 m level. The conductor was later checked with borehole GAE853. This hole was the discovery hole of the Lappberget deposit, which has recently come into production.